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Cell Biology

Cell Biology – About.com Education

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Cell biology is the subdiscipline of biology that studies the basic unit of life, the cell. It deals with all aspects of the cell including cell anatomy, cell division (mitosis and meiosis), and cell processes includingcell respiration, and cell death. Cell biology does not stand alone as a discipline but is closely related to other areas of biology such as genetics, molecular biology, and biochemistry.

Based on one of the basic principles of biology, the cell theory, the study of cells would not have been possible without the invention of the microscope. With the advanced microscopes of today, such as the Scanning Electron Microscope and Transmission Electron Microscope, cell biologists are able to obtain detailed images of the smallest of cell structures and organelles.

Significant Events in Cell Biology

There have been several significant events throughout history that have led to the development of the field of cell biology as it exists today. Below are a few of these major events:

Careers in Cell Biology

Study in the field of cell biology can lead to various career paths. Many cell biologists are research scientists who work in industrial or academic laboratories. Other opportunities include:

All living organisms are composed of cells. Some organisms are comprised of cells that number in the trillions. There are two primary types of cells: eukaryotic and prokaryotic cells. Eukaryotic cells have a defined nucleus, while the prokaryotic nucleus is not defined or contained within a membrane. While all organisms are composed of cells, these cells differ among organisms. Some of these differing characteristics include cell structure, size, shape, and organelle content. For example, animal cells, bacterial cells, and plant cells have similarities, but they are also noticeably different. Cells have different methods of reproduction. Some of these methods include: binary fission, mitosis, and meiosis. Cells house an organisms genetic material (DNA), which provides instructions for all cellular activity.

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Cell Biology - About.com Education

Biochemistry

Biochemistry | Article about biochemistry by The Free Dictionary

science concerned chiefly with the chemistry of biological processes; it attempts to utilize the tools and concepts of chemistry, particularly organic and physical chemistry, for elucidation of the living system. The science has been variously referred to as physiological chemistry and as biological chemistry. Molecular biology, a term first used in 1950, is used to describe the area of research, closely related to and often overlapping biochemistry, conducted by biologists whose approach to and interest in biology are principally at the molecular level of organization. The related field of

brings to biology the techniques and attitudes of the physicist. Cell biology is concerned with the organization and functioning of the individual cell and depends greatly on biochemical techniques. As the study of life forms demonstrated similar or even identical processes occurring in widely divergent species, it has taken the biochemist to unravel the underlying chemical basis for these phenomena. Biochemists study such things as the structures and physical properties of biological molecules, including the proteins, the carbohydrates, the lipids, and the nucleic acids; the mechanisms of enzyme action; the chemical regulation of metabolism; the molecular basis of genetic expression; the chemistry of vitamins; chemoluminescence; biological oxidation; and energy utilization in the cell. The study of the chemistry of the immune response offers the possibility of treatment and cure for such diseases as

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See L. Stryer, Biochemistry (3d ed. 1988); C. K. Mathews and K. E. van Holde, Biochemistry (1990); G. Zubay, Biochemistry (3d ed. 1993).

the study of the chemical compounds, reactions, etc., occurring in living organisms http://restools.sdsc.edu/ http://www.geocities.com/peterroberts.geo/biology.htm#bioch

The study of the substances and chemical processes which occur in living organisms. It includes the identification and quantitative determination of the substances, studies of their structure, determining how they are synthesized and degraded in organisms, and elucidating their role in the operation of the organism.

biological chemistry, the science dealing with the composition of organisms; the structure, properties, and localization of compounds observed in organisms; the pathways and laws governing the formation of these compounds; and the sequence and mechanisms of transformations and their biological and physiological roles. Biochemistry is subdivided into the biochemistry of microorganisms, of plants, of animals, and of man. This subdivision is arbitrary, since there is much in common in the composition of the various objects of study and in the biochemical processes taking place in them. For this reason, the research carried out on microorganisms complements and enriches research on plant or animal tissues and cells. Although the different branches of biochemical research are intimately connected, it is accepted practice to divide biochemistry into static biochemistry, concerned predominantly with the analysis of the composition of organisms; dynamic biochemistry, concerned with the transformation of substances; and functional biochemistry, which elucidates the chemical processes that underlie various manifestations of the life functions. The last branch of research is sometimes referred to by the special name physiological chemistry.

The totality of chemical reactions taking place in an organism, from the acquisition of materials which enter the organism from without (assimilation) and their breakdown (dissimilation) to the formation of the end products that are secreted, constitutes the essence and content of metabolismthe main and constant criterion of all living things. Understandably, the study of metabolism in all its details is one of the major tasks of biochemistry. Biochemical research embraces a very wide range of questions: there is no branch of theoretical or applied biology, chemistry, or medicine which is not linked with it. Thus, contemporary biochemistry unites many related scientific disciplines that became independent in the middle of the 20th century.

The accumulation of biochemical information and establishment of biochemistry in the 16th to 19th centuries. Biochemistry took shape as an independent science at the end of the 19th century; however, its origins reach far back into the past. From the first half of the 16th century until the second half of the 17th century, iatrochemists (chemistphysicians) made their contribution to the development of chemistry and medicine: the German physician and natural scientist P. Paracelsus, the Dutch scholars J. B. van Heimont and F. Sylvius, and others studied the digestive juices, bile, and the processes of fermentation. Sylvius, a famous physician, attributed particularly great importance to the correct balance of acids and alkalies in the human organism; he believed that many if not all diseases were caused by a disturbance of this balance. Many of the positions espoused by the iatrochemists were naive and entirely mistaken; however, it must not be forgotten that chemistry did not yet exist at that time. The most generally accepted theory governing science at that time was the so-called phlogiston theory. Nevertheless, equilibrium experiments were carried out on man with exact records of body mass and secretion by the Italian scientist S. Santorio at the beginning of the 17th century. These experiments led to the description of perspiratio insensibilisthe loss of mass owing to insensible perspiration.

The great discoveries in the areas of physics and chemistry in the 18th and beginning of the 19th centuries (the discovery of many simple substances and compounds, the formulation of the gas laws, the discovery of the laws of conservation of matter and energy) laid the scientific foundation of general chemistry. After the discovery of oxygen as a component of air, the Dutch botanist J. Ingenhousz was able to describe the continual formation of CO2 by plants and the release of oxygen by the green parts of the plant stimulated by sunlight. Ingenhousz experiments marked the beginning of the study of plant respiration and the processes of photosynthesis, which are still being explored in detail.

At the end of the first quarter of the 19th century, only a very small number of organic substances were known. In the textbook of the German chemist L. Gmelin published in 1822, only 80 organic compounds are named. At that time the tasks and possibilities of organic chemistry were still unclear. The Swedish scientist J. Berzelius thought that organic bodies were divided into two clearly differentiated classesplants and animals; he also thought that the essence of living matter derived from something other than its inorganic elements. This something else, which he called life force, lies entirely beyond the realm of inorganic elements. Berzelius expressed doubt that man will ever be able to produce organic substances artificially and confirm such analysis by synthesis (1827). The untenability of these views, which were typical of vitalism, was demonstrated very shortly. As early as 1828, the German chemist F. Whler, a student of Berzelius, produced urea by synthetic means. Urea had been described in the 18th century by the French scientist H. Rouelle as one of the component parts of urine in mammals. Soon there followed the synthesis of other natural organic compounds and of artificial compounds unknown in nature. Thus, the wall separating organic from inorganic compounds was broken down.

Beginning with the second half of the 19th century, organic chemistry increasingly became synthetic chemistry, within which efforts were directed at the preparation of new carbon compounds, especially those having industrial use. The study of the composition of plant and animal specimens was not yet included. Knowledge in this area was obtained by chance as a by-product of work by chemists, botanists, plant and animal physiologists, pathologists, and physicians whose interests included chemical research. Thus, in 1814, the Russian chemist K. S. Kirkhgof described the conversion of starch into sugar under the effect of extract of sprouted barley seedsthe action of amylase. By the middle of the 19th century, other enzymes were described: salivary amylase, which breaks down polysaccharides; and pepsin in gastric juice and trypsin in the pancreatic fluid, which break down protein. Berzelius introduced the concept of catalysts into chemistry and included all enzymes known at that time in this category. In 1835 the French chemist M. Chevreul described creatine in muscle tissue; shortly thereafter, the structurally related creatinine was discovered in urine. The German chemist J. von Liebig established the presence of lactic acid in the skeletal muscles and the accumulation of this substance during work. In 1839 he established that food was composed of protein, fats, and carbohydrates, which are the main components of animal and plant organisms. In the mid-19th century the structure of fat was established and its synthesis was carried out by the French chemist P. Berthelot; the synthesis of carbohydrates was accomplished by the Russian scientist A. M. Butlerov, who also proposed a theory of the structure of organic compounds that retains its importance even today. The systematic study of proteins was begun by the Dutch physician and chemist G. J. Mulder in the 1830s and has continued intensively ever since. At the same time, in connection with the description of yeast cells (C. Cagniard de La Tour in France and T. Schwann in Germany, 183638), scientists began actively studying the process of the metabolism of sugar and formation of alcohol, which had long since attracted attention. Among those who studied fermentation were J. von Liebig and the French scientist L. Pasteur. Pasteur came to the conclusion that fermentation was a biological process that required the participation of living yeast cells. Liebig, on the other hand, regarded the metabolism of sugar as a complicated chemical reaction. This dispute was resolved when the Russian chemist M. M. Manassein (1871) and, with even more clarity, the German scientist E. Buchner (1897), proved the ability of the fluid extracted from yeast cells to induce alcoholic fermentation. Thus, the correctness of the chemical theory of enzyme action formulated by Liebig in 1870 was confirmed; the basic principles of this theory have retained their importance to this day.

A significant quantity of information accumulated regarding the chemical composition of plant and animal organisms and the chemical reactions taking place in them; at the same time, attempts were made to systematize and organize this information in treatises. The earliest of these were the textbooks of J. Simon (1842) and of Liebig (1847), published in Germany; and the textbook of physiological chemistry by A. I. Khodnev, issued in Russia (1847).

The origin and development of contemporary trends in biochemistry. At the end of the 19th century and during the 20th century, the development of biochemistry took on a markedly specialized character which reflects the problems and the objects of study. Plant biochemistry developed predominantly in subdepartments of botany and of plant physiology. The biochemistry of microorganisms is also closely related to plant biochemistry. Biochemists of all countries have studied proteins, carbohydrates, lipids, and vitamins (the component parts of plants, animals, and microorganisms) in the most varied specimens.Glycosides, tanning agents, essential oils, alkaloids, antibiotics, and other so-called secondary products can be regarded as characteristic of plants and microorganisms. Among the above mentioned compounds, many glycosides were synthesized by enzymes by the French chemist E. Bourquelot and his coworkers (191118). The classic work of the German chemist R. Willstatter (191015) played an exceptional role in deciphering the structure of the anthocyaninsthe glycosides that make up the pigments of flowers and fruits. The German chemist A. Hofmann (18901900) studied the group of alkaloids (nitrogenous heterocyclic substances of fundamental character). Later, other outstanding researchers studied the alkaloids (R. Willsttter, the Russian chemists A. P. Orekhov and A. A. Shmuk, and many others). Leading chemists and biochemistsPerkin, Jr. (Great Britain), H. Euler (Sweden), and othersalso successfully studied the essential oils and the terpenes.

An outstanding role in the development of plant biochemistry in Russia (at the end of the 19th century and during the first half of the 20th century) was played by Professor A. S. Famintsyn of the University of St. Petersburg and his students D. I. Ivanovskii (who discovered viruses) and I. P. Borodin (who studied the oxidation processes in plant organisms and their relation to protein transformation).

The work of S. P. Kostychev (professor at the University of St. Petersburglater, Leningrad State University) on anaerobic carbohydrate metabolism and plant respiration enriched chemical physiology by the discovery of new intermediates in fermentation and by the formulation of original views on the nature of oxidation processes, protein metabolism, and nitrogen fixation by plants. M. S. Tsvet, professor at the University of Warsaw, made a significant contribution with his column chromatography method, which is still used today. The Moscow school of physiologists and plant biochemists was represented by K. A. Timiriazev, who studied photosynthesis and the chemistry of chlorophyll. His studentsV. I. Palladin, who worked on biological oxidation; D. P. Prianishnikov, who studied nitrogen metabolism in plants; V. S. Butkevich, who enriched theoretical biochemistry with his research on protein and protein metabolism in plants; and A. R. Kizel, who studied arginine and urea metabolism in plants and structural elements in cell protoplasmwere the founders of the great schools and original directions in contemporary general and evolutionary biochemistry, and also of physiology and plant biochemistry, which developed fruitfully in the last 25 years of the 20th century. In the 20th century, researchers in the biochemistry of microorganisms and plants solved many common problems involving natural compounds (including macromolecules), their structures and paths of formation and breakdown, and the properties of enzymes participating in these processes. It should be noted that microorganisms gradually became the favorite specimens for various enzymological studies and for the solution of problems in biochemical genetics.

All this research created a firm foundation for the solution of many specific problems, including industrial problems. Among the latter were the production of new antibiotics, the development of methods for purifying them, and the search for conditions favorable to the microbiological synthesis not only of antibiotics but also of other biologically active compoundsvitamins, critical amino acids, nucleotides, and so on.

TECHNICAL AND INDUSTRIAL BIOCHEMISTRY. the requirements of the national economyproblems of profitable production of raw materials and their practical and rational storage, correct processing, and effective use; problems of raising the yield of cultivated plants; questions of viticulture and the technology of wine-making; and the requirements of the food industryhave led to the creation of a new branch of biochemistry: technical and industrial biochemistry. In the USSR, this area is represented most strongly by the A. N. Bakh Institute of Biochemistry (A. I. Oparin, V. L. Kretovich, L. V. Metlitskii, R. M. Feniksova, and others) and the Institute of Plant Physiology of the Academy of Sciences of the USSR (A. L. Kursanov and his coworkers and students). I. P. Ivanov (All-Union Institute of Plant-Growing), V. L. Kretovich, M. I. Kniaginichev, their coworkers, and many others have greatly contributed to the study of the biochemistry of grain crops. The work carried out at the A. N. Bakh Institute on the Biochemistry of Catechins has played an important role in the development of production of tea and tanning agents.

ANIMAL AND HUMAN BIOCHEMISTRY (MEDICAL AND PHYSIOLOGICAL CHEMISTRY). The development of animal and human biochemistry has been greatly furthered by the numerous groups of physiologists, chemists, pathologists, and medical doctors working in different countries. In France, in the laboratory of the physiologist C. Bernard, glycogen was discovered in the liver of mammals (1857) and the pathways of its formation and the mechanisms regulating its breakdown were studied; also in France, L. Corvisart (1856) discovered the enzyme trypsin in pancreatic juice. In Germany, F. Hoppe-Seyler, A. Kossel, E. Fischer, E. Ab-derhalden, O. Hammarsten, and others made detailed studies of simple and complex proteins, their structure and properties, and the substances formed by artificial degradation upon heating with acids and bases or by the action of enzymes. In England, F. Hopkins, the founder of the Cambridge school of biochemists, investigated the amino acid composition of proteins, discovered tryptophan and glutathione, and studied the role of amino acids and vitamins in nutrition.

Russian scientists working in the departments of higher academic institutions and in specialized institutes made an important contribution to the development of biochemistry at the turn of the 20th century. In the Military Medical Academy, A. la. Danilevskii and his coworkers studied problems of protein chemistry, methods for isolating and purifying enzymes, mechanisms of enzyme action, and the conditions for reversibility of enzyme reactions. At the Institute for Experimental Medicine, M. V. Nentskii carried out research on the chemistry of porphyrins and the biosynthesis of urea, and also on bacterial enzymes which are responsible for the breakdown of amino acids. The collaboration of the laboratories of A. la. Danilevskii and M. V. Nentskii with the laboratory of I. P. Pavlov in research on digestion and the formation of urea in the liver was especially fruitful. At Moscow University, V. S. Gulevich conducted detailed and successful research into extractive (nonprotein) substances present in muscle and discovered many new nitrogencontaining compounds of unique structure (carnosine, carnitine, and others). The detailed study of the various enzyme reactions which take place in the parenchymatous organsmainly in the liverand which govern the normal course of transport processes has been and remains the object of much research. In the second half of the 19th century and during the 20th century, much attention has been devoted to the biochemical study of excitable tissue, predominantly of the brain and muscle. In the USSR, A. V. Palladin, G. E. Vladimirov, E. M. Kreps, and their students and coworkers have worked on these problems. By the middle of the 20th century, neurochemistry had become one of the independent branches of biochemistry. The biochemistry of the blood was studied comprehensively. The respiratory function of the blood (that is, the binding and release of carbon dioxide and oxygen by the blood) was studied in the laboratory of C. Ludwig in Vienna in the mid-19th century and later in greater detail in various countries. The data obtained led to the analysis of the structure and properties of hemoglobin in its normal and pathological states, the detailed study of the reaction between hemoglobin and oxygen, and the elucidation of the laws governing the acidbase balance.

Biochemistry achieved great success in the study of vitamins, hormones, and mineral substances, and especially of trace elements, their distribution in various organisms, their physiological roles, the mechanisms of their action, and their regulating influence on enzyme reactions and transport processes. Of great importance is the question of the relation between structure and function, which characterizes the problems of biochemical pharmacology in dealing with medicinal preparations; and the study of the primary mechanism of their action, which involves intervention in the enzyme reactions that form the basis of the metabolic processes. In the mid-20th century, biochemical research carried out in clinics and devoted to the study of the biochemical features of the organism and the chemical makeup of blood, urine, and other fluids and tissues of the patient acquired an independent status. This area, which received extensive development, is the basis of clinical biochemistry.

VITAMINOLOGY. In 1880, in G. A. Bunges laboratory, a young Russian physician named N. I. Lunin first described the supplementary nutrient factors found in milk. Similar observations were made by the Dutch physician C. Eijkman, who in 1896 described the presence of a vital factor in rice bran. In 1912, the Polish researcher C. Funk isolated the active component in crystalline form and called it a vitamin. Work in this area was greatly expanded, and gradually many other vitamins were discovered. Today, vitaminology is one of the most important branches of biochemistry and of nutrition.

BIOCHEMISTRY OF HORMONES. Research on the analysis of the chemical structure of the products of glands of internal secretion (hormones), the pathways of their formation in the organism, their modes of action, and the possibility of synthesizing them in the laboratory constitutes one of the most important areas of biochemical research. The biochemistry of steroid hormones is part of the general problem of the biochemistry of sterines. The successes achieved in this area are largely a result of the use of initial and intermediate compounds labeled with carbon (14C). A close relationship has been established between a wide range of research on protein substances and the specialized study of the structure and function of hormones of proteinlike character. The study of the hormone activity of a given preparation is impossible without a thorough analysis of the biochemical mechanisms governing its activity. Thus, data concerning the chemistry and biochemistry of hormones contribute equally to our knowledge of endocrinology and of biochemistry.

ENZYMOLOGY. The study of enzymes is an entirely independent area of biochemistry. In this field, the problem of the structure of enzymatic proteins is closely interwoven with physicochemical problems of chemical kinetics and catalysis. In the second half of the 20th century, much new information has been added to our conception of enzyme structure and of their presence in the natural state in the form of complexes. The analysis of enzyme structure in conjunction with the activity exhibited by enzymes under various conditions has led to the understanding of the role of individual amino acids (mainly cysteine, lysine, histidine, tyrosine, and serine) in the formation of the active sites of enzymes. The structure of many coenzymes has been determined along with their significance for enzyme activity and also the relation between coenzymes and vitamins. R. Willsttter, L. Michaelis, G. Embden, and O. Meyerhof (Germany), J. Sumner and J. Northrop (Usa), H. Euler (Sweden), and A. N. Bakh (USSR) all made important contributions to the development of enzymology during the first half of the 20th century. Those who actively continued their research, set up schools, and opened up new areas include O. Warburg (West Berlin) and F. Lynen (Federal Republic of Germany), R. Peters and H. Krebs (Great Britain), H. Theorell (Sweden), F. Lipmann and D. Koshland (USA), F. Sorm (Czechoslovakia), F. Straub (Hungary), and T. Baranowski and J. Heller (Poland). In the USSR, the field of research is represented by V. A. Engelgardt and M. N. Liubimova, who established the enzyme activity of muscle protein and, in particular, the adenosine triphosphate activity of myosin and the process of oxidative phosphorylation; A. E. Braunshtein, who, in collaboration with M. G. Kritsman, discovered the process of the transfer of an amino group; A. I. Oparin and A. L. Kursanov, who studied the role of cell structure in the manifestation of enzyme activity; and S. R. Mardashev, who successfully studied the decarboxylation of amino acids. Research on large complexes of enzymes is being conducted in the laboratories of L. Reed (USA), M. Koike (Japan), D. Sanadi (USA), F. Lynen (West Germany), S. E. Severin (USSR), and others. The Soviet scientist V. A. Belitser greatly furthered our understanding of the efficiency of the role played by respirationdiscovered by V. A. Engelgardtin the formation of energyrich compounds; G. E. Vladimirov specified the quantity of energy (10 calories, or 42 joules) liberated by the hydrolysis of ATP. Studies in this area were isolated at first, but in the 1950s and later, work was greatly expanded, largely owing to research by D. Green, B. Chance, A. Lehninger, and E. Racker (USA), and E. Slater (Netherlands). In the USSR, this problem has been studied in the biochemistry sub-departments at Moscow State University and Leningrad State University, and also in independent laboratories (S. A. Neifakh, V. P. Skulachev, and others). In addition, contemporary research has demonstrated the marked influence of the salt content of the surroundings and of individual ions on enzymatic processes and the important role of trace elements in the realization of enzyme activity.

EVOLUTIONARY AND COMPARATIVE BIOCHEMISTRY. Studies of the chemistry of animals, plants, and microorganisms have shown that, in spite of the universality of basic biochemical structures and processes in all living organisms, there are specific differences determined by the level of ontogenetic and phylogenetic development of the specimen under examination. The accumulation of facts has provided the foundation for comparative biochemistry, whose object is to find the laws governing the biochemical evolution of organisms. In this connection, the problem of the origin of life on earth has great theoretical importance. Several important hypotheses of A. I. Oparins theory on the origin of life have received experimental confirmation in work done at the Bakh Institute, in the Subdepartment of Plant Biochemistry at Moscow State University, and in many foreign laboratories (for example, J. Oro and S. W. Fox in the USA).

HISTOCHEMISTRY AND CYTOCHEMISTRY. With the development of the techniques of morphological research, and especially with the introduction of the electron microscopewhich revealed many formerly unknown structures in the cell nucleus and protoplasminto laboratory work, new tasks presented themselves to biochemistry. On the borderline between morphological and biochemical research new areas of study have grown up. These include histochemistry and cytochemistry, which study the localization and transformation of substances in cells and tissues using biochemical and morphological methods.

BIOORGANIC CHEMISTRY. the detailed investigation of the structure of biopolymerssimple and complex proteins, nucleic acids, polysaccharides, and lipidsand the analysis of the effects of biologically active small molecular natural compounds (coenzymes, nucleotides, vitamins, and so on) led to the necessity of studying the relationship between the structure of a substance and its biological function. The formulation of this problem brought about a proliferation of research carried out on the border between biological and organic chemistry. This research area received the name of bioorganic chemistry.

MOLECULAR BIOLOGY. the development of methods for separating subcellular structures (ultracentrifugation) and for obtaining separate fractions containing the cell nuclei, mitochondria, ribosomes, and so on made possible the detailed study of the composition and biological functions of the separated components. The application of the methods of electrophoresis in conjunction with chromatography made possible the detailed characterization of macro-molecular compounds. The parallel development of analytic determination permitted the analysis of very small quantitites of mate-erial. This advance was linked to the introduction of physical (mainly optical) methods of analysis into biology and biochemistry (fluorometry, spectrophotometry in various regions of the spectrum, mass spectrometry, nuclear magnetic resonance, electron paramagnetic resonance, and gas and liquid chromatography), with the use of radioactive isotopes; sensitive automatic analyzers of amino acids, peptides, and nucleotides; polarimetry; macromolecular electrophoresis; and other methods. These developments led to the appearance of yet another independent branch of biochemistry, closely related to biophysics and physical chemistry, called molecular biology.

MOLECULAR GENETICS. Molecular genetics, in spite of some of its specific objectives, can be considered a part of molecular biology. Thus, for example, the analysis of the mechanism governing the occurrence of many hereditary malfunctions in the metabolism and actions of an organism has made possible the clarification of the role of the cessation or modification of the biosynthesis of those protein substances which have enzymic, immunological, or other biological activity. In this connection, the study of disruptions in the metabolism of carbohydrates and amino acids (for example, phenylalanine, tyrosine, and tryptophan) and the formation of pathological forms of hemoglobin and other biological compounds are relevant.

The development of new research methods between 1950 and 1970 has produced great advances in biochemistry. Foremost is the elucidation of protein structure and the determination of the sequential arrangement of amino acids within proteins. The first sequential arrangement of amino acids in the proteinlike hormone insulin was worked out by the English biochemist F. Sanger; later, the structure of the enzyme ribonuclease was determined by C. Hirs, S. Moore, and W. Stein (USA), who devised the method of automatic analysis of amino acids which became standard in biochemical laboratories. The same enzyme, ribonuclease, obtained from various sources was studied by C. Anfinsen (USA), F. Egami (Japan), and others. F. Sorm, B. Keil, and their coworkers (Czechoslovakia), B. Hartley (Great Britain), and others established the sequential arrangement of amino acids in many proteolytic enzymes. A major achievement of the 1960s was the chemical synthesis of hormonesthe adrenocorticotropic hormone, a molecule containing 23 amino acids (the natural hormone has 39 amino acids), and insulin, a molecule made up of 51 amino acidsand of the enzyme ribonuclease (124 amino acids).

In the USSR, work on problems of structure and synthesis of biologically active substances is being pursued at the Institute for the Chemistry of Natural Compounds (director, M. M. Shemiakin), at the Institute of Biological and Medical Chemistry (director, V. N. Orekhovich), and at other institutes and university departments.

The English scientists M. Perutz and J. Kendrew and their coworkers used X-ray analysis with great success in the determination of the structure of myoglobin and hemoglobin. In 1956 and 1957 the entire structure of lysozyme was worked out by the English biochemist D. Phillips and others. Equally important successes were achieved in the analysis of complex proteins, nucleoproteins, nucleic acids, and nucleotides. The triumphal accomplishment of biochemistry, molecular biology, and genetics was the research which established the role of nucleic acids in the biosynthesis of proteins and the predetermining influence of nucleic acids on the structure and properties of proteins synthesized within cells. This work elucidated the biochemical basis of the transmission of traits by inheritance from one generation to another. It is also difficult to overestimate the importance of the research which determined the sequence of nucleotides in transfer RNA (ribonucleic acid) and the elaboration of methods for the organic synthesis of polynucleotides. The work of the following investigators has been especially fruitful in the aforementioned areas: J. Buchanan, E. Chargaff, J. Davidson, D. Davis, A. Kornberg, S. Ochoa, J. Watson, and M. Wilkins (USA); F. Crick and F. Sanger (Great Britain); F. Jacob and J. Monod (France); and A. N. Belozerskii, A. S. Spirin, V. A. Engelgardt, and A. A. Baev (USSR).

Scientific institutions, societies, and periodicals.. The questions addressed to biochemistry by related scientific disciplinesmedicine and all its branches, agriculture (plant-growing and livestockraising), the food industry, theoretical and applied biology, soil science, hydrobiology, and oceanologyare continually increasing in scope. Each special field of biochemistry, in the USSR and abroad, utilizes a network of specialized institutes and laboratories. In the USSR, scientific work in biochemistry is conducted in central scientific research institutes within the various systems: in the Academy of Sciences of the USSRthe A. N. Bakh Institute of Biochemistry, the Institute of Evolutionary Physiology and Biochemistry, the Institute of Plant Physiology, the Institute of Molecular Biology, the Institute of the Chemistry of Natural Compounds; in academies of the various republicsthe Institute of Biochemistry of the Ukrainian SSR, the Armenian SSR, the Uzbek SSR, and the Lithuanian SSR; in branch academiesthe Institute of Biological and Medical Chemistry of the Academy of Medical Sciences of the USSR, the Biochemistry Department of the Institute of Experimental Medicine of the Academy of Medicine of the USSR, the Institute of Experimental Endocrinology and Hormone Chemistry of the Academy of Medical Sciences of the USSR, and the Institute of Nutrition of the Academy of Medical Sciences of the USSR; and in the institutes of the All-Union Academy of Agricultural Sciences and of many ministries (ministries of health, agriculture, food industry, and so on). Research in biochemistry is conducted in the bioorganic chemistry laboratory at Moscow State University and in many university subdepartments of biochemistry. Problems of biochemistry are studied in the central and branch institutes devoted to areas of botany, physiology, and pathology and in institutes of experimental and clinical medicine, the food industry, physical culture, and many other institutes. Most specialists in biochemistry, both in the USSR and abroad, are trained in universities, where the faculties of chemistry and biology contain specialized departments. Biochemists with a more limited background are trained in medical, technical, agricultural, and other institutions.

In the majority of countries, there are scientific biochemical societies united under the Federation of European Biochemical Societies and the International Union of Biochemistry. These organizations hold symposia and conferences, and also congressesyearly in the case of the Federation of European Biochemical Societies (the first took place in 1964), and once every three years in the case of the International Union of Biochemistry (the first was held in 1949; the congresses became especially popular and well attended beginning with the fifth, which was held in Moscow in 1961). In the USSR, the All-Union Biochemical Society, with numerous sections in the republics and cities, was organized in 1958. It has approximately 6,500 members. Actually, the number of biochemists in the USSR is much greater.

The quantity of periodical literature in which biochemical work is published is very great and continues to increase every year. Among the foreign and international journals, the best known are Journal of Biological Chemistry (Baltimore, 1905), Biochemistry (Washington, D.C., 1964), Archives of Biochemistry and Biophysics (New York, 1942), Biochemical Journal (London, 1906), Phytochemistry (Oxford-New York, 1962), Molecular Biology (international journal published in England), Bulletin de la Socit de Chimie Biologique (Paris, 1914), Enzymologia (The Hague, 1936), Giornale di Biochimica (Rome, 1955), Acta Biologica et Medica Germanica (Leipzig, 1959), Hoppe Seylers Zeitschrift fr physiologische Chemie (Berlin, 1877), and Journal of Biochemistry (Tokyo, 1922). Popular yearbooks include Annual Review of Biochemistry (Stanford, 1932), Advances in Enzymology and Related Subjects of Biochemistry (New York, 1945), Advances in Protein Chemistry (New York, 1945), Advances in Enzyme Regulation (Oxford, 1963), and Advances in Molecular Biology. In the USSR, experimental work in biochemistry is published in the journals Biokhimiia (Moscow, 1936), Zhurnal evoliutsionnoi biokhimii i fiziologii (Moscow, 1965), Molekuliarnaia biologiia (Moscow, 1967), Voprosy meditsinskoi khimii (Moscow, 1955), Ukrainskii biokhimicheskii zhurnal (Kiev, 1926), Prikladnaia biokhimiia i mikrobiologiia (Moscow, 1965), Doklady AN SSSR (Moscow, 1933), Biulleten eksperimentalnoi biologii i meditsiny (Moscow, 1936), Izvestiia AN SSSR: Seriia biologii i meditsiny (Moscow, 1936), Izvestiia AN SSSR: Seriia khimicheskaia (Moscow, 1936), and Nauchnye doklady vysshei shkoly: Seriia biologicheskie nauki (Moscow, 1958).

General biochemical studies are published in the journal Uspekhi sovremennoi biologii (Moscow, 1932), the yearbook Uspekhi biologicheskoi khimii (vols. 18, 195067) published by the All-Union Biochemical Society, the journals Uspekhi khimii (Moscow, 1932) and Referativnyi zhurnal: Khimiia: Biologicheskaia khimiia (Moscow, 1955), and the journal of the Mendeleev All-Union Society. Publications of biochemical institutes appear frequently.

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Biochemistry | Article about biochemistry by The Free Dictionary

Biochemistry

Biochemistry – definition of biochemistry by The Free Dictionary

.

1. The study of the chemical substances and vital processes occurring in living organisms; biological chemistry; physiological chemistry.

2. The chemical composition of a particular living system or biological substance: viral biochemistry.

biochemical (--kl) adj. & n.

biochemically adv.

biochemist n.

1. (Biochemistry) the study of the chemical compounds, reactions, etc, occurring in living organisms

n.

the scientific study of the chemical substances and processes of living matter.

[188085]

bi`ochemical (- kl) adj., n.

bi`ochemic, adj.

bi`ochemically, adv.

bi`ochemist, n.

The scientific study of the chemical composition of living matter and of the chemical processes that go on in living organisms.

the study of the chemical processes that take place in living organisms. biochemist, n. biochemical, adj.

Study of the chemistry of life processes.

ThesaurusAntonymsRelated WordsSynonymsLegend:

Translations

n. bioqumica, ciencia que estudia los organismos vivos.

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Biochemistry - definition of biochemistry by The Free Dictionary

Biochemistry

Biochemistry | Define Biochemistry at Dictionary.com

Historical Examples

Manure spreaders and tree sprayers, reflective of advances in biochemistry, also survived.

"You know what you can do with your physiology and biochemistry," Bowman said succinctly.

From odorless garlic to tofu smelling of pork chops, everything is within the possibility of biochemistry.

biochemistry came into being and, with Liebig as foster-parent, grew into modern Physiology.

But about Bruckian anatomy, physiology or biochemistry, the little emissary would tell them nothing.

Several calves were born, and seemed to be doing well; the biochemistry of Tanith and Khepera were safely alike.

Completing work on his Master's in biochemistry at Cambridge when the Spanish show started.

They actually did have a remarkable grasp of physiology and biochemistry, and constantly sought to learn more.

But, as I've said, with no false modesty, I'm no slouch in my field of biochemistry.

British Dictionary definitions for biochemistry Expand

/bakmstr/

the study of the chemical compounds, reactions, etc, occurring in living organisms

Derived Forms

biochemical, adjectivebiochemically, adverbbiochemist, noun

Word Origin and History for biochemistry Expand

biochemistry in Medicine Expand

biochemistry biochemistry (b'-km'-str) n.

The study of the chemical substances and vital processes occurring in living organisms.

The chemical composition of a particular living system or biological substance.

biochemistry in Science Expand

biochemistry in Culture Expand

The study of the structure and interactions of the complex organic molecules found in living systems.

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Biochemistry | Define Biochemistry at Dictionary.com

Biochemistry

Department of Biochemistry | UW-Madison

Congenital sideroblastic anemia due to mutations in the mitochondrial HSP70 homologue HSPA9.

Schmitz-Abe K, Ciesielski SJ, Schmidt PJ, Campagna DR, Rahimov F, Schilke BA, Cuijpers M, Rieneck K, Lausen B, Linenberger ML, Sendamarai AK, Guo C, Hofmann I, Newburger PE, Matthews D, Shimamura A, Snijders PJ, Towne MC, Niemeyer CM, Dziegiel MH,...

Merchant S, Bednarek SY, Birchler JA, Coupland G, Eckardt NA, Genschik P, Greenberg J, Kieber JJ, Kliebenstein DJ, Pogson BJ, Smyth D

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Department of Biochemistry | UW-Madison

Physiology

Physiology – definition of physiology by The Free Dictionary

physiology - the branch of the biological sciences dealing with the functioning of organisms accommodation - (physiology) the automatic adjustment in focal length of the natural lens of the eye adaptation - (physiology) the responsive adjustment of a sense organ (as the eye) to varying conditions (as of light) abduction - (physiology) moving of a body part away from the central axis of the body adduction - (physiology) moving of a body part toward the central axis of the body control - (physiology) regulation or maintenance of a function or action or reflex etc; "the timing and control of his movements were unimpaired"; "he had lost control of his sphincters" antagonistic muscle - (physiology) a muscle that opposes the action of another; "the biceps and triceps are antagonistic muscles" humour, humor - (Middle Ages) one of the four fluids in the body whose balance was believed to determine your emotional and physical state; "the humors are blood and phlegm and yellow and black bile" neurophysiology - the branch of neuroscience that studies the physiology of the nervous system hemodynamics - the branch of physiology that studies the circulation of the blood and the forces involved kinesiology - the branch of physiology that studies the mechanics and anatomy in relation to human movement myology - the branch of physiology that studies muscles irradiation - (physiology) the spread of sensory neural impulses in the cortex cell death, necrobiosis - (physiology) the normal degeneration and death of living cells (as in various epithelial cells) acid-base balance, acid-base equilibrium - (physiology) the normal equilibrium between acids and alkalis in the body; "with a normal acid-base balance in the body the blood is slightly alkaline" autoregulation - (physiology) processes that maintain a generally constant physiological state in a cell or organism inhibition - (physiology) the process whereby nerves can retard or prevent the functioning of an organ or part; "the inhibition of the heart by the vagus nerve" nutrition - (physiology) the organic process of nourishing or being nourished; the processes by which an organism assimilates food and uses it for growth and maintenance relaxation - (physiology) the gradual lengthening of inactive muscle or muscle fibers stimulation - (physiology) the effect of a stimulus (on nerves or organs etc.) summation - (physiology) the process whereby multiple stimuli can produce a response (in a muscle or nerve or other part) that one stimulus alone does not produce homeostasis - (physiology) metabolic equilibrium actively maintained by several complex biological mechanisms that operate via the autonomic nervous system to offset disrupting changes innervate - stimulate to action; "innervate a muscle or a nerve" irritate - excite to some characteristic action or condition, such as motion, contraction, or nervous impulse, by the application of a stimulus; "irritate the glands of a leaf" abducent, abducting - especially of muscles; drawing away from the midline of the body or from an adjacent part adducent, adducting, adductive - especially of muscles; bringing together or drawing toward the midline of the body or toward an adjacent part afferent - of nerves and nerve impulses; conveying sensory information from the sense organs to the CNS; "afferent nerves"; "afferent impulses" efferent, motorial - of nerves and nerve impulses; conveying information away from the CNS; "efferent nerves and impulses" isometric - of or involving muscular contraction in which tension increases while length remains constant isotonic - of or involving muscular contraction in which tension is constant while length changes voluntary - controlled by individual volition; "voluntary motions"; "voluntary muscles" involuntary - controlled by the autonomic nervous system; without conscious control; "involuntary muscles"; "gave an involuntary start" pressor - increasing (or tending to increase) blood pressure; "pressor reflexes" tonic - of or relating to or producing normal tone or tonus in muscles or tissue; "a tonic reflex"; "tonic muscle contraction" sympathetic - of or relating to the sympathetic nervous system; "sympathetic neurons"; "sympathetic stimulation"

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Human Behavior

Human sexuality – Wikipedia, the free encyclopedia

This article is about human sexual anatomy, sexuality and perceptions. For information specifically about sexual activities, see Human sexual activity.

Human sexuality is the capacity of humans to have erotic experiences and responses. A person's sexual orientation can influence their sexual interest and attraction for another person.[1] Sexuality may be experienced and expressed in a variety of ways; including thoughts, fantasies, desires, beliefs, attitudes, values, behaviors, practices, roles, and relationships.[2] These may manifest themselves in biological, physical, emotional, social, or spiritual aspects. The biological and physical aspects of sexuality largely concern the human reproductive functions, including the human sexual response cycle and the basic biological drive that exists in all species.[3] Physical and emotional aspects of sexuality include bonds between individuals that is expressed through profound feelings or physical manifestations of love, trust, and care. Social aspects deal with the effects of human society on one's sexuality, while spirituality concerns an individual's spiritual connection with others. Sexuality also impacts and is impacted upon by cultural, political, legal, philosophical, moral, ethical, and religious aspects of life.

Sexual activity is a vital principle of human living that connects the desires, pleasures, and energy of the body with a knowledge of human intimacy. This results in erotic love, intimate friendship, human mating, and procreation. Interest in sexual activity typically increases when an individual reaches puberty.[4] Opinions differ on the origins of an individual's sexual orientation and sexual behavior. Some argue that sexuality is determined by genetics; some believe it is molded by the environment, and others argue that both of these factors interact to form the individual's sexual orientation.[1] This pertains to the nature versus nurture debate. In the former, one assumes that the features of a person innately correspond to their natural inheritance, exemplified by drives and instincts; the latter refers to the assumption that the features of a person continue to change throughout their development and nurturing, exemplified by ego ideals and formative identifications.

Genetic studies work on the premise that a difference in alleles corresponds to a variation in traits among people.[5] In the study of human chromosomes in human sexuality, research has shown that "ten percent of the population has chromosomal variations that do not fit neatly into the XX-female and XY-male set of categories".[6]

Evolutionary perspectives on human coupling, reproduction and reproduction strategies, and social learning theory provide further views of sexuality.[7] Socio-cultural aspects of sexuality include historical developments and religious beliefs. Examples include Jewish views on sexual pleasure within marriage and some views of other religions on avoidance of sexual pleasures.[3] Some cultures have been described as sexually repressive. The study of sexuality also includes human identity within social groups, sexually transmitted infections (STIs/STDs), and birth control methods.

Certain characteristics are believed to be innate in humans; these characteristics may be modified by the physical and social environment in which people interact.[8] Human sexuality is driven by genetics and mental activity. The sexual drive affects the development of personal identity and social activities.[9][10] An individual's normative, social, cultural, educational, and environmental characteristics moderate the sexual drive.[9] Two well-known theorists have taken opposing positions in the nature-versus-nurture debate. Sigmund Freud believed sexual drives are instinctive. Freud was a firm supporter of the nature argument; he viewed sexuality as the central source of human personality. John Locke believed in the nurture argument. Locke used his theory of the mind as a "tabula rasa" or blank slate: the environment is where one develops one's sexual drives.[11]

Freud's theory assumed that behavior is rooted in biology. He proposed that instincts are the principal motivating forces in the mental realm. He said there are a large number of instincts but they are reduced into two broad groups; Eros (the life instinct), which comprises the self-preserving and erotic instincts, and Thanatos (the death instinct), which comprises instincts invoking aggression, self-destruction, and cruelty.[12] Freud gave sexual drives a centrality in human life, actions, and behaviors that had not been accepted before his proposal. His instinct theory said humans are driven from birth by the desire to acquire and enhance bodily pleasures, thus supporting the nature debate. Freud redefined the term "sexuality" to make it cover any form of pleasure that can be derived from the human body,[12] and said the pre-genital zones are primitive areas of preliminary enjoyment preceding sexual intercourse and orgasm.[13] He also said pleasure lowers tension while displeasure raises it, influencing the sexual drive in humans. His developmentalist perspective was governed by inner forces, especially biological drives and maturation, and his view that humans are biologically inclined to seek sexual gratification demonstrates the nature side of the debate.[11]

Locke (16321704) rejected the assumption that there are innate differences among people and said people are strongly influenced by their social environments, especially by education.[11] He believed it is accurate to view a child's mind as a tabula rasa or blank slate; whatever goes into the mind originates in the surrounding environment.[11] As the person develops, they discover their identities. Locke proposed following a child from its birth and observing the changes that time makes; he said one will find that as the mind, through sensory information, becomes furnished with ideas, it becomes more awake and aware. He said that after some time, the child's mind begins to know the most familiar objects. As the child's brain develops, he or she begins to know the people and social surroundings of daily life, and can then distinguish the known from the unknown. This view supports the nurture side of the debate.[14]

Human sexual behavior is different from that of most other animal species; it seems to be affected by several factors. For example, while most non-human species are driven to partake in sexual behavior when reproduction is possible, humans are not sexually active only to reproduce.[15] The environment, culture, and social setting play major roles in the perception, attitudes, and behaviors of sexuality. Sexual behavior is also affected by the inability to detect sexual stimuli, incorrect labeling, or misattribution. This may in turn impede an individual's sexual performance.[15]

Like other mammals, humans are dioecious, primarily composed of male or female sexes,[16] with a small proportion (around 1%) of intersex individuals, for whom sexual classification may not be as clear.[17] The biological aspects of humans' sexuality deal with the reproductive system, the sexual response cycle, and the factors that affect these aspects. They also deal with the influence of biological factors on other aspects of sexuality, such as organic and neurological responses,[18] heredity, hormonal issues, gender issues, and sexual dysfunction.[19]

Males and females are anatomically similar; this extends to some degree to the development of the reproductive system. As adults, they have different reproductive mechanisms that enable them to perform sexual acts and to reproduce. Men and women react to sexual stimuli in a similar fashion with minor differences. Women have a monthly reproductive cycle, whereas the male sperm production cycle is more continuous.[3]

The hypothalamus is the most important part of the brain for sexual functioning. This is a small area at the base of the brain consisting of several groups of nerve cell bodies that receives input from the limbic system. Studies have shown that within lab animals, destruction of certain areas of the hypothalamus causes the elimination of sexual behavior.[citation needed] The hypothalamus is important because of its relationship to the pituitary gland, which lies beneath it. The pituitary gland secretes hormones that are produced in the hypothalamus and itself. The four important sexual hormones are oxytocin, prolactin, follicle-stimulating hormone, and luteinizing hormone.[3] Oxytocin is also known as the "love hormone"; it is released in both sexes during sexual intercourse when an orgasm is achieved. It is believed that oxytocin is involved with maintaining close relationships.[20][21] The hormone is also released in women when they give birth or are breastfeeding.[22] Both prolactic and oxytocin stimulate milk production in women. Follicle-stimulating hormone (FHS) is responsible for ovulation in women by triggering egg maturity; in men it stimulates sperm production.[23] Luteinizing hormone (LH) triggers ovulation, which is the release of a mature egg.[3]

The mons veneris, also known as the Mound of Venus, is a soft layer of fatty tissue overlaying the pubic bone.[24] Following puberty, this area grows in size. It has many nerve endings and is sensitive to stimulation.[3]

The labia minora and labia majora are collectively known as the lips. The labia majora are two elongated folds of skin extending from the mons to the perineum. Its outer surface becomes covered with hair after puberty. In between the labia majora are the labia minora, two hairless folds of skin that meet above the clitoris to form the clitoral hood, which is highly sensitive to touch. The labia minora become engorged with blood during sexual stimulation, causing them to swell and turn red.[3] The labia minora are composed of connective tissues that are richly supplied with blood vessels which cause the pinkish appearance. Near the anus, the labia minora merge with the labia majora.[25] In a sexually unstimulated state, the labia minora protects the vaginal and urethral opening by covering them.[26] At the base of the labia minora are the Bartholin's glands, which add a few drops of an alkaline fluid to the vagina via ducts; this fluid helps to counteract the acidity of the outer vagina since sperm cannot live in an acidic environment.[3]

The clitoris is developed from the same embryonic tissue as the penis; it or its glans alone consists of as many (or more in some cases) nerve endings as the human penis or glans penis, making it extremely sensitive to touch.[27][28][29] The clitoral glans, which is a small, elongated erectile structure, has only one known functionsexual sensations. It is the main source of orgasm in women.[30][31][32][33] Thick secretions called smegma collect in the clitoris.[3]

The vaginal opening and the urethral opening are only visible when the labia minora are parted. These opening have many nerve endings that make them sensitive to touch. They are surrounded by a ring of sphincter muscles called the bulbocavernosus muscle. Underneath this muscle and on opposite sides of the vaginal opening are the vestibular bulbs, which help the vagina grip the penis by swelling with blood during arousal. Within the vaginal opening is the hymen, a thin membrane that partially covers the opening in many virgins. Rupture of the hymen has been historically considered the loss of one's virginity, though by modern standards, loss of virginity is considered to be the first sexual intercourse. The hymen can be ruptured by activities other than sexual intercourse. The urethral opening connects to the bladder with the urethra; it expels urine from the bladder. This is located below the clitoris and above the vaginal opening.[3]

The breasts are external organs used for sexual pleasure in some cultures. Western culture is one of the few in which they are considered erotic.[3] The breasts are the subcutaneous tissues on the front thorax of the female body.[25] Breasts are modified sweat glands made up of fibrous tissues and fat that provide support and contain nerves, blood vessels and lymphatic vessels.[25] Their purpose is to provide milk to a developing infant. Breasts develop during puberty in response to an increase in estrogen. Each adult breast consists of 15 to 20 milk-producing mammary glands, irregularly shaped lobes that include alveolar glands and a lactiferous duct leading to the nipple. The lobes are separated by dense connective tissues that support the glands and attach them to the tissues on the underlying pectoral muscles.[25] Other connective tissue, which forms dense strands called suspensory ligaments, extends inward from the skin of the breast to the pectoral tissue to support the weight of the breast.[25] Heredity and the quantity of fatty tissue determine the size of the breasts.[3]

The female internal reproductive organs are the vagina, uterus, Fallopian tubes, and ovaries. The vagina is a sheath-like canal that extends from the vulva to the cervix. It receives the penis during intercourse and serves as a depository for sperm. The vagina is also the birth canal; it can expand to 10 centimetres (3.9in) during labor and delivery. The vagina is located between the bladder and the rectum. The vagina is normally collapsed, but during sexual arousal it opens, lengthens, and produces lubrication to allow the insertion of the penis. The vagina has three layered walls; it is a self-cleaning organ with natural bacteria that suppress the production of yeast.[3] The G-spot, named after the Ernst Grfenberg who first reported it in 1950, may be located in the front wall of the vagina and may cause orgasms. This area may vary in size and location between women; in some it may be absent. Various researchers dispute its structure or existence, or regard it as an extension of the clitoris.[34][35][36]

The uterus or womb is a hollow, muscular organ where a fertilized egg (ovum) will implant itself and grow into a fetus.[3] The uterus lies in the pelvic cavity between the bladder and the bowel, and above the vagina. It is usually positioned in a 90-degree angle tilting forward, although in about 20% of women it tilts backwards.[25] The uterus has three layers; the innermost layer is the endometrium, where the egg is implanted. During ovulation, this thickens for implantation. If implantation does not occur, it is sloughed off during menstruation. The cervix is the narrow end of the uterus. The broad part of the uterus is the fundus.[3]

During ovulation, the ovum travels down the Fallopian tubes to the uterus. These extend about four inches (10cm) from both sides of the uterus. Finger-like projections at the ends of the tubes brush the ovaries and receive the ovum once it is released. The ovum then travels for three to four days to the uterus.[3] After sexual intercourse, sperm swim up this funnel from the uterus. The lining of the tube and its secretions sustain the egg and the sperm, encouraging fertilization and nourishing the ovum until it reaches the uterus. If the ovum divides after fertilization, identical twins are produced. If separate eggs are fertilized by different sperm, the mother gives birth to non-identical or fraternal twins.[25]

The ovaries are the female gonads; they develop from the same embryonic tissue as the testicles. The ovaries are suspended by ligaments and are the source where ova are stored and developed before ovulation. The ovaries also produce female hormones progesterone and estrogen. Within the ovaries, each ovum is surrounded by other cells and contained within a capsule called a primary follicle. At puberty, one or more of these follicles are stimulated to mature on a monthly basis. Once matured, these are called Graafian follicles.[3] The female reproductive system does not produce the ova; about 60,000 ova are present at birth, only 400 of which will mature during the woman's lifetime.[25]

Ovulation is based on a monthly cycle; the 14th day is the most fertile. On days one to four, menstruation and production of estrogen and progesterone decreases, and the endometrium starts thinning. The endometrium is sloughed off for the next three to six days. Once menstruation ends, the cycle begins again with an FSH surge from the pituitary gland. Days five to thirteen are known as the pre-ovulatory stage. During this stage, the pituitary gland secretes follicle-stimulating hormone (FSH). A negative feedback loop is enacted when estrogen is secreted to inhibit the release of FSH. Estrogen thickens the endometrium of the uterus. A surge of Luteinizing Hormone (LH) triggers ovulation. On day 14, the LH surge causes a Graafian follicle to surface the ovary. The follicle ruptures and the ripe ovum is expelled into the abdominal cavity. The fallopian tubes pick up the ovum with the fimbria. The cervical mucus changes to aid the movement of sperm. On days 15 to 28the post-ovulatory stage, the Graafian folliclenow called the corpus luteumsecretes estrogen. Production of progesterone increases, inhibiting LH release. The endometrium thickens to prepare for implantation, and the ovum travels down the Fallopian tubes to the uterus. If the ovum is not fertilized and does not implant, menstruation begins.[3]

Males also have both internal and external genitalia that are responsible for procreation and sexual intercourse. Production of spermatozoa (sperm) is also cyclic, but unlike the female ovulation cycle, the sperm production cycle is constantly producing millions of sperm daily.[3]

The male genitalia are the penis and the scrotum. The penis provides a passageway for sperm and urine. An average-sized flaccid penis is about 334 inches (9.5cm) in length and 115 inches (3.0cm) in diameter. When erect, the average penis is between 412 inches (11cm) to 6 inches (15cm) in length and 112 inches (3.8cm) in diameter. The penis's internal structures consist of the shaft, glans, and the root.[3]

The shaft of the penis consists of three cylindrical bodies of spongy tissue filled with blood vessels along its length. Two of these bodies lie side-by-side in the upper portion of the penis called corpora cavernosa. The third, called the corpus spongiosum, is a tube that lies centrally beneath the others and expands at the end to form the tip of the penis (glans).[37]

The raised rim at the border of the shaft and glans is called the corona. The urethra runs through the shaft, providing an exit for sperm and urine. The root consists of the expanded ends of the cavernous bodies, which fan out to form the crura and attach to the pubic bone and the expanded end of the spongy body (bulb). The root is surrounded by two muscles; the bulbocavernosus muscle and the ischiocavernosus muscle, which aid urination and ejaculation. The penis has a foreskin that usually covers the glans; in many cultures, this is removed at birth in a procedure called circumcision.[3] In the scrotum, the testicles are held away from the body, one possible reason for this is so sperm can be produced in an environment slightly lower than normal body temperature.[38][39]

Male internal reproductive structures are the testicles, the duct system, the prostate and seminal vesicles, and the Cowper's gland.[3]

The testicles are the male gonads where sperm and male hormones are produced. Millions of sperm are produced daily in several hundred seminiferous tubules. Cells called the Leydig cells lie between the tubules; these produce hormones called androgens; these consist of testosterone and inhibin. The testicles are held by the spermatic cord, which is a tubelike structure containing blood vessels, nerves, the vas deferens, and a muscle that helps to raise and lower the testicles in response to temperature changes and sexual arousal, in which the testicles are drawn closer to the body.[3]

Sperm are transported through a four-part duct system. The first part of this system is the epididymis. The testicles converge to form the seminiferous tubules, coiled tubes at the top and back of each testicle. The second part of the duct system is the vas deferens, a muscular tube that begins at the lower end of the epididymis.[3] The vas deferens passes upward along the side of the testicles to become part of the spermatic cord.[37] The expanded end is the ampulla, which stores sperm before ejaculation. The third part of the duct system is the ejaculatory ducts, which are 1-inch (2.5cm)-long paired tubes that pass through the prostate gland, where semen is produced.[3] The prostate gland is a solid, chestnut-shaped organ that surrounds the first part of the urethra, which carries urine and semen.[3][37]

The prostate gland and the seminal vesicles produce seminal fluid that is mixed with sperm to create semen.[3] The prostate gland lies under the bladder and in front of the rectum. It consists of two main zones: the inner zone that produces secretions to keep the lining of the male urethra moist and the outer zone that produces seminal fluids to facilitate the passage of semen.[37] The seminal vesicles secrete fructose for sperm activation and mobilization, prostaglandins to cause uterine contractions that aid movement through the uterus, and bases that help neutralize the acidity of the vagina. The Cowper's glands, or bulbourethral glands, are two pea sized structures beneath the prostate.

The sexual response cycle is a model that describes the physiological responses that occur during sexual activity. This model was created by William Masters and Virginia Johnson. According to Masters and Johnson, the human sexual response cycle consists of four phases; excitement, plateau, orgasm, and resolution. During the excitement phase, one attains the intrinsic motivation to have sex. The plateau phase is the precursor to orgasm, which may be mostly biological for men and mostly psychological for women. Orgasm is the release of tension, and the resolution period is the unaroused state before the cycle begins again.[3]

The male sexual response cycle starts in the excitement phase; two centers in the spine are responsible for erections. Vasoconstriction in the penis begins, the heart rate increases, the scrotum thickens, the spermatic cord shortens, and the testicles become engorged with blood. In the plateau phase, the penis increases in diameter, the testicles become more engorged, and the Cowper's glands secrete pre-seminal fluid. The orgasm phase, during which rhythmic contractions occur every 0.8 seconds[verification needed], consists of two phases; the emission phase, in which contractions of the vas deferens, prostate, and seminal vesicles encourage ejaculation, which is the second phase of orgasm. Ejaculation is called the expulsion phase; it cannot be reached without an orgasm. In the resolution phase, the male is now in an unaroused state consisting of a refactory (rest) period before the cycle can begin. This rest period may increase with age.[3]

The female sexual response begins with the excitement phase, which can last from several minutes to several hours. Characteristics of this phase include increased heart and respiratory rate, and an elevation of blood pressure. Flushed skin or blotches of redness may occur on the chest and back; breasts increase slightly in size and nipples may become hardened and erect. The onset of vasocongestion results in swelling of the clitoris, labia minora, and vagina. The muscle that surrounds the vaginal opening tightens and the uterus elevates and grows in size. The vaginal walls begin to produce a lubricating liquid. The second phase, called the plateau phase, is characterized primarily by the intensification of the changes begun during the excitement phase. The plateau phase extends to the brink of orgasm, which initiates the resolution stage; the reversal of the changes begun during the excitement phase. During the orgasm stage the heart rate, blood pressure, muscle tension, and breathing rates peak. The pelvic muscle near the vagina, the anal sphincter, and the uterus contract. Muscle contractions in the vaginal area create a high level of pleasure, though all orgasms are centered in the clitoris.[3][40][41][42]

Sexual disorders, according to the DSM-IV-TR, are disturbances in sexual desire and psycho-physiological changes that characterize the sexual response cycle and cause marked distress, and interpersonal difficulty. There are four major categories of sexual problems: desire disorders, arousal disorders, orgasmic disorders, and sexual pain disorders.[3]

Sexuality in humans generates profound emotional and psychological responses. Some theorists identify sexuality as the central source of human personality.[43] Psychological studies of sexuality focus on psychological influences that affect sexual behavior and experiences.[19] Early psychological analyses were carried out by Sigmund Freud, who believed in a psychoanalytic approach. He also proposed the concepts of psychosexual development and the Oedipus complex, among other theories.[44]

Gender identity is a person's sense of self-identification as female, male, both, neither, or somewhere in between. The social construction of gender has been discussed by many scholars, including Judith Butler. More recent research has focused upon the influence of feminist theory and courtship.[45][46]

Sexual behavior and intimate relationships are strongly influenced by a person's sexual orientation.[47] Sexual orientation refers to the degree of emotional and physical attraction to members of the opposite sex, same sex, or both sexes.[47] Heterosexual people are attracted to the members of the opposite sex. Homosexual people are attracted to people of the same sex. Those who are bisexual are attracted to both men and women.

Before the High Middle Ages, homosexual acts appear to have been ignored or tolerated by the Christian church.[48] During the 12th century, hostility toward homosexuality began to spread throughout religious and secular institutions. By the end of the 19th century, it was viewed as a pathology.[48]Havelock Ellis and Sigmund Freud adopted more accepting stances; Ellis said homosexuality was inborn and therefore not immoral, not a disease, and that many homosexuals made significant contributions to society.[48] Freud wrote that all human beings as capable of becoming either heterosexual or homosexual; neither orientation was assumed to be innate.[49] According to Freud, a person's orientation depended on the resolution of the Oedipus complex. He said male homosexuality resulted when a young boy had an authoritarian, rejecting mother and turned to his father for love and affection, and later to men in general. He said female homosexuality developed when a girl loved her mother and identified with her father, and became fixated at that stage.[49]

Freud and Ellis said homosexuality resulted from reversed gender roles. In the early 21st century, this view is reinforced by the media's portrayal of male homosexuals as effeminate and female homosexuals as masculine.[49] A person's conformity or non-conformity to gender stereotypes does not always predict sexual orientation. Society believes that if a man is masculine he is heterosexual, and if a man is feminine he is homosexual. There is no strong evidence that a homosexual or bisexual orientation must be associated with atypical gender roles. By the early 21st century, homosexuality was no longer considered to be a pathology. Many factors, including: genetic factors, anatomical factors, birth order, and hormones in the prenatal environment, have been linked to homosexuality.[49]

Other than the need to procreate, there are many other reasons people have sex. According to one study conducted on college students (Meston & Buss, 2007), the four main reasons for sexual activities are; physical attraction, as a means to an end, to increase emotional connection, and to alleviate insecurity.[50]

In the past[when?], children were often assumed not to have sexuality until later development. Sigmund Freud was one of the first researchers to take child sexuality seriously. His ideas, such as psychosexual development and the Oedipus conflict, have been much debated but acknowledging the existence of child sexuality was an important development.[51] Freud gave sexual drives an importance and centrality in human life, actions, and behavior; he said sexual drives exist and can be discerned in children from birth. He explains this in his theory of infantile sexuality, and says sexual energy (libido) is the most important motivating force in adult life. Freud wrote about the importance of interpersonal relationships to one's sexual and emotional development. From birth, the mother's connection to the infant affects the infant's later capacity for pleasure and attachment.[52] Freud described two currents of emotional life; an affectionate current, including our bonds with the important people in our lives; and a sensual current, including our wish to gratify sexual impulses. During adolescence, a young person tries to integrate these two emotional currents.[53]

Alfred Kinsey also examined child sexuality in his Kinsey Reports. Children are naturally curious about their bodies and sexual functions. For example, they wonder where babies come from, they notice the differences between males and females, and many engage in genital play, which is often mistaken for masturbation. Child sex play, also known as playing doctor, includes exhibiting or inspecting the genitals. Many children take part in some sex play, typically with siblings or friends.[51] Sex play with others usually decreases as children grow, but they may later possess romantic interest in their peers. Curiosity levels remain high during these years, but the main surge in sexual interest occurs in adolescence.[51]

Adult sexuality originates in childhood. However, like many other human capacities, sexuality is not fixed, but matures and develops. A common stereotype suggests that people tend to lose interest in and ability to engage in sexual acts once they reach late adulthood. This stereotype is reinforced by Western pop culture, which often ridicules older adults who try to engage in sexual activities. Men are shown suffering heart attacks from over-excitement, and women are depicted as grateful if anyone shows an interest in them. The term "dirty old man" is applied to older men who show an interest in sex beyond a level the speaker considered appropriate . The language for older women, by contrast, is sexless, and older women are portrayed as sexually unattractive and undesirable. Sexuality, however, is similar to most other aspects of aging. Age does not necessarily change the need or desire to be sexually expressive or active. If a couple has been in a long-term relationship, the frequency of sexual activity may decrease, but not necessarily their satisfaction with each other. Many couples find that the type of sexual expression may change, and that with age and the term of relationship there is increased intimacy and love. If sex and sexual intimacy are important aspects in one's life during young and middle adulthood, they will continue to be factors in older adulthood.

Human sexuality can be understood as part of the social life of humans, which is governed by implied rules of behavior and the status quo. This narrows the view to groups within a society.[19] The socio-cultural context of society, including the effects of politics and the mass media, influences and forms social norms. Before the early 21st century, people fought for their civil rights. The civil rights movements helped to bring about massive changes in social norms; examples include the sexual revolution and the rise of feminism.[54][55]

The link between constructed sexual meanings and racial ideologies has been studied. Sexual meanings are constructed to maintain racial-ethnic-national boundaries by denigration of "others" and regulation of sexual behavior within the group. According to Joane Nagel, "Both adherence to and deviation from such approved behaviors, define and reinforce racial, ethnic, and nationalist regimes".[56][57]

The age and manner in which children are informed of issues of sexuality is a matter of sex education. The school systems in almost all developed countries have some form of sex education, but the nature of the issues covered varies widely. In some countries, such as Australia and much of Europe, age-appropriate sex education often begins in pre-school, whereas other countries leave sex education to the pre-teenage and teenage years.[58] Sex education covers a range of topics, including the physical, mental, and social aspects of sexual behavior. Geographic location also plays a role in society's opinion of the appropriate age for children to learn about sexuality. According to TIME magazine and CNN, 74% of teenagers in the United States reported that their major sources of sexual information were their peers and the media, compared to 10% who named their parents or a sex education course.[3]

In some religions, sexual behavior is regarded as primarily spiritual. In others it is treated as primarily physical. Some hold that sexual behavior is only spiritual within certain kinds of relationships, when used for specific purposes, or when incorporated into religious ritual. In some religions there are no distinctions between the physical and the spiritual, whereas some religions view human sexuality as a way of completing the gap that exists between the spiritual and the physical.[59]

Many religious conservatives, especially those of Abrahamic religions and Christianity in particular, tend to view sexuality in terms of behavior (i.e. homosexuality or heterosexuality is what someone does) and certain sexualities such as bisexuality tend to be ignored as a result of this. These conservatives tend to promote celibacy for gay people and may also tend to believe that sexuality can be changed through conversion therapy[60] or prayer to become an ex-gay. They may also see homosexuality as a form of mental illness, something that ought to be criminalised, an immoral abomination, caused by ineffective parenting, and view same-sex marriage as a threat to society.[61]

On the other hand, most religious liberals define sexuality-related labels in terms of sexual attraction and self-identification.[60] They may also view same-sex activity as morally neutral and legally acceptable as opposite-sex activity, unrelated to mental illness, genetically or environmentally caused (but not as the result of bad parenting), and fixed. They also tend to be more in favor of same-sex marriage.[61]

According to Judaism, sex between man and woman within marriage is sacred and should be enjoyed; celibacy is considered sinful.[3]

The Roman Catholic Church teaches that sexuality is "noble and worthy"[62] but that it must be used in accordance with natural law. For this reason, all sexual activity must occur in the context of a marriage between a man and a woman, and must not be divorced from the possibility of conception. All forms of sex without the possibility of conception are considered intrinsically disordered and sinful, such as the use of contraceptives, masturbation, and homosexual acts.[63]

In Islam, sexual desire is considered to be a natural urge that should not be suppressed, although the concept of free sex is not accepted; these urges should be fulfilled responsibly. Marriage is considered to be a good deed; it does not hinder spiritual wayfaring. The term used for marriage within the Quran is nikah, which literally means sexual intercourse. Although Islamic sexuality is restrained via Islamic sexual jurisprudence, it emphasizes sexual pleasure within marriage. It is acceptable for a man to have more than one wife, but he must take care of those wives physically, mentally, emotionally, financially, and spiritually.[64] Muslims believe that sexual intercourse is an act of worship that fulfils emotional and physical needs, and that producing children is one way in which humans can contribute to God's creation, and Islam discourages celibacy once an individual is married. However, homosexuality is strictly forbidden in Islam, and some Muslim lawyers have suggested that gay people should be put to death.[65]

Hinduism emphasizes that sex is only appropriate between husband and wife, in which satisfying sexual urges through sexual pleasure is an important duty of marriage. Any sex before marriage is considered to interfere with intellectual development, especially between birth and the age of 25, which is said to be brahmacharya and this should be avoided. Kama (sensual pleasures) is one of the four purusharthas or aims of life (dharma, artha, kama, and moksha).[66] The Hindu Kama Sutra deals partially with sexual intercourse; it is not exclusively a sexual or religious work.[67][68][69]

Sikhism views chastity as important, as Sikhs believe that the divine spark of Waheguru is present inside every individual's body, therefore it is important for one to keep clean and pure. Sexual activity is limited to married couples, and extramarital sex is forbidden. Marriage is seen as a commitment to Waheguru and should be viewed as part of spiritual companionship, rather than just sexual intercourse, and monogamy is deeply emphasised in Sikhism. Any other way of living is discouraged, including celibacy and homosexuality. However, in comparison to other religions, the issue of sexuality in Sikhism is not considered one of paramount importance.[70]

Sexuality has been an important vital part of human existence throughout history.[72] All civilizations have managed sexuality through sexual standards, representations, and behavior.[72]

Before the rise of agriculture, groups of hunter/gatherers (H/G) and nomads inhabited the world. Within these groups, some implications of male dominance existed, but there were signs that women were active participants in sexuality, with bargaining power of their own. These hunter/gatherers had less restrictive sexual standards that emphasized sexual pleasure and enjoyment, but with definite rules and constraints. Some underlying continuities or key regulatory standards contended with the tension between recognition of pleasure, interest, and the need to procreate for the sake of social order and economic survival. H/G groups also placed high value on certain types of sexual symbolism. Two common tensions in H/G societies are expressed in their art, which emphasizes male sexuality and prowess, with equally common tendencies to blur gender lines in sexual matters. One example of these male-dominated portrayals is the Egyptian creation myth, in which the sun god Atum masturbates in the water, creating the Nile River. In Sumerian myth, the Gods' semen filled the Tigris.[72]

Once agricultural societies emerged, the sexual framework shifted in ways that persisted for many millennia in much of Asia, Africa, Europe, and parts of the Americas. One common characteristic new to these societies was the collective supervision of sexual behavior due to urbanization, and the growth of population and population density. Children would commonly witness parents having sex because many families shared the same sleeping quarters. Due to landownership, determination of children's paternity became important, and society and family life became patriarchal. These changes in sexual ideology were used to control female sexuality and to differentiate standards by gender. With these ideologies, sexual possessiveness and increases in jealousy emerged. With the domestication of animals, new opportunities for bestiality arose. Males mostly performed these types of sexual acts and many societies acquired firm rules against it. These acts also explain the many depictions of half-human, half-animal mythical creatures, and the sports of gods and goddesses with animals.[72] While retaining the precedents of earlier civilizations, each classical civilization established a somewhat distinctive approach to gender, artistic expression of sexual beauty, and to behaviors such as homosexuality. Some of these distinctions are portrayed in sex manuals, which were also common among civilizations in China, Greece, Rome, Persia, and India; each has its own sexual history.[72]

During the beginning of the industrial revolution of the 18th and 19th centuries, many changes in sexual standards occurred. New, dramatic, artificial birth control devices such as the condom and diaphragm were introduced. Doctors started claiming a new role in sexual matters, urging that their advice was crucial to sexual morality and health. New pornographic industries grew and Japan adopted its first laws against homosexuality. In western societies, the definition of homosexuality was constantly changing; western influence on other cultures became more prevalent. New contacts created serious issues around sexuality and sexual traditions. There were also major shifts in sexual behavior. During this period, puberty began occurring at younger ages, so a new focus on adolescence as a time of sexual confusion and danger emerged. There was a new focus on the purpose of marriage; it was increasing regarded as being for love rather than only for economics and reproduction.[72]

Alfred Kinsey initiated the modern era of sex research. He collected data from questionnaires given to his students at Indiana University, but then switched to personal interviews about sexual behaviors. Kinsey and his colleagues sampled 5,300 men and 5,940 women. He found that most people masturbated, that many engaged in oral sex, that women are capable of having multiple orgasms, and that many men had had some type of homosexual experience in their lifetimes. Many[who?] believe he was the major influence in changing 20th century attitudes about sex. Kinsey Institute for Research in Sex, Gender, and Reproduction at Indiana University continues to be a major center for the study of human sexuality.[3] Before William Masters, a physician, and Virginia Johnson, a behavioral scientist, the study of anatomy and physiological studies of sex was still limited to experiments with laboratory animals. Masters and Johnson started to directly observe and record the physical responses in humans that are engaged in sexual activity under laboratory settings. They observed 10,000 episodes of sexual acts between 312 men and 382 women. This led to methods of treating clinical problems and abnormalities. Masters and Johnson opened the first sex therapy clinic in 1965. In 1970, they described their therapeutic techniques in their book, Human Sexual Inadequacy.[3]

Reproductive and sexual rights encompass the concept of applying human rights to issues related to reproduction and sexuality.[73] This concept is a modern one, and remains controversial, especially outside the West, since it deals, directly and indirectly, with issues such as contraception, LGBT rights, abortion, sex education, freedom to choose a partner, freedom to decide whether to be sexually active or not, right to bodily integrity, freedom to decide whether or not, and when, to have children.[74][75][76] According to the Swedish government, "sexual rights include the right of all people to decide over their own bodies and sexuality" and "reproductive rights comprise the right of individuals to decide on the number of children they have and the intervals at which they are born."[77] Such rights are not accepted in all cultures, with practices such criminalization of consensual sexual activities (such as those related to homosexual acts and sexual acts outside marriage), acceptance of forced marriage and child marriage, failure to criminalize all non-consensual sexual encounters (such as marital rape), female genital mutilation, or restricted availability of contraception, being common around the world.[78][79]

In humans, sexual intercourse and sexual activity in general have been shown to have health benefits, such as an improved sense of smell,[80]stress and blood pressure reduction,[81][82] increased immunity,[83] and decreased risk of prostate cancer.[84][85][86] Sexual intimacy and orgasms increase levels of oxytocin, which helps people bond and build trust.[87][88][89] A long-term study of 3,500 people between ages 30 and 101 by clinical neuropsychologist David Weeks, MD, head of old-age psychology at the Royal Edinburgh Hospital in Scotland, said he found that "sex helps you look between four and seven years younger", according to impartial ratings of the subjects' photographs. Exclusive causation, however, is unclear, and the benefits may be indirectly related to sex and directly related to significant reductions in stress, greater contentment, and better sleep that sex promotes.[90][91][92]

Sexual intercourse can also be a disease vector.[93] There are 19 million new cases of sexually transmitted diseases (STD) every year in the U.S.,[94] and worldwide there are over 340 million STD infections each year.[95] More than half of these occur in adolescents and young adults aged 1524 years.[96] At least one in four U.S. teenage girls has a sexually transmitted disease.[94][97] In the U.S., about 30% of 1517-year olds have had sexual intercourse, but only about 80% of 1519-year olds report using condoms for their first sexual intercourse.[98] In one study, more than 75% of young women age 1825 years felt they were at low risk of acquiring an STD.[99]

People both consciously and subconsciously seek to attract others with whom they can form deep relationships. This may be for companionship, procreation, or an intimate relationship. This involves interactive processes whereby people find and attract potential partners and maintain a relationship. These processes, which involve attracting one or more partners and maintaining sexual interest, can include:

The law regulates human sexuality in several ways, including: the criminal status of certain sexual behaviors; granting individuals the privacy or autonomy of individuals to make their own sexual decisions; protections regarding equality and non-discrimination; the recognition and protection of certain individuals' rights; legislation regarding marriage and the family; the status of laws protecting individuals from violence, harassment and persecution.[102]

Issues regarding human sexuality and human sexual orientation became entrenched in law in the Western world by the latter half of the twentieth century, as part of the gay liberation movement encouraging LGBT individuals to "come out of the closet" and engaging with the legal system, primarily through courts. Therefore, many issues regarding human sexuality and the law are found in the opinions of the courts.[103]

While the issue of privacy has been useful to sexual rights claims, some scholars have criticized its usefulness, saying that this perspective is too narrow and restrictive. The law is often slow to intervene in certain forms of coercive behavior that can limit individuals' control over their own sexuality (such as female genital mutilation, forced marriages or lack of access to reproductive health care). Many of these injustices are often perpetuated wholly or in part by private individuals rather than state agents, and as a result, there is an ongoing debate about the extent of state responsibility to prevent harmful practices and to investigate such practices when they do occur.[102]

State intervention with regards to sexuality also occurs, and is considered acceptable by some, in certain instances (e.g. same-sex sexual activity or prostitution).[102]

Niall Richardson, Clarissa Smith and Angela Werndly (2013) Studying Sexualities: Theories, Representations, Cultures (London: Palgrave MacMillan)

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Human Behavior

Behavior – Wikipedia, the free encyclopedia

Behavior or behaviour (see spelling differences) is the range of actions and mannerisms made by individuals, organisms, systems, or artificial entities in conjunction with themselves or their environment, which includes the other systems or organisms around as well as the (inanimate) physical environment. It is the response of the system or organism to various stimuli or inputs, whether internal or external, conscious or subconscious, overt or covert, and voluntary or involuntary.[1]

Although there is some disagreement as to how to precisely define behavior in a biological context, one common interpretation based on a meta-analysis of scientific literature states that "behavior is the internally coordinated responses (actions or inactions) of whole living organisms (individuals or groups) to internal and/or external stimuli"[2]

A broader definition of behavior, applicable to plants and other organisms, is similar to the concept of phenotypic plasticity. It describes behavior as a response to an event or environment change during the course of the lifetime of an individual, differing from other physiological or biochemical changes that occurs much rapidly, and excluding changes that are result of development (ontogeny).[3][4]

Behaviors can be either innate or learned.

Behavior can be regarded as any action of an organism that changes its relationship to its environment. Behavior provides outputs from the organism to the environment.[5]

Human behavior is believed to be influenced by the endocrine system and the nervous system. It is most commonly believed that complexity in the behavior of an organism is correlated to the complexity of its nervous system. Generally, organisms with more complex nervous systems have a greater capacity to learn new responses and thus adjust their behavior.[citation needed]

Behavior outside of psychology includes physical property and chemical reactions.

In environmental modeling and especially in hydrology, a "behavioral model" means a model that is acceptably consistent with observed natural processes, i.e., that simulates well, for example, observed river discharge. It is a key concept of the so-called Generalized Likelihood Uncertainty Estimation (GLUE) methodology to quantify how uncertain environmental predictions are.

In management, behaviors are associated with desired or undesired focuses. Managers generally note what the desired outcome is, but behavioral patterns can take over. These patterns are the reference to how often the desired behavior actually occurs. Before a behavior actually occurs, antecedents focus on the stimuli that influence the behavior that is about to happen. After the behavior occurs, consequences fall into place. They can come in the form of rewards or punishments.

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Embryology

Langman’s Medical Embryology: 9781451191646: Medicine …

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Langman's Medical Embryology: 9781451191646: Medicine ...

Anatomy

Muscular System – Muscles of the Human Body

[Continued from above] . . . Muscular System Anatomy

Muscle Types There are three types of muscle tissue:Visceral, cardiac, and skeletal.

The cells of cardiac muscle tissue are striatedthat is, they appear to have light and dark stripes when viewed under a light microscope. The arrangement of protein fibers inside of the cells causes these light and dark bands. Striations indicate that a muscle cell is very strong, unlike visceral muscles.

The cells of cardiac muscle are branched X or Y shaped cells tightly connected together by special junctions called intercalated disks. Intercalated disks are made up of fingerlike projections from two neighboring cells that interlock and provide a strong bond between the cells. The branched structure and intercalated disks allow the muscle cells to resist high blood pressures and the strain of pumping blood throughout a lifetime. These features also help to spread electrochemical signals quickly from cell to cell so that the heart can beat as a unit.

Skeletal muscle cells form when many smaller progenitor cells lump themselves together to form long, straight, multinucleated fibers. Striated just like cardiac muscle, these skeletal muscle fibers are very strong. Skeletal muscle derives its name from the fact that these muscles always connect to the skeleton in at least one place.

Gross Anatomy of a Skeletal Muscle Most skeletal muscles are attached to two bones through tendons. Tendons are tough bands of dense regular connective tissue whose strong collagen fibers firmly attach muscles to bones. Tendons are under extreme stress when muscles pull on them, so they are very strong and are woven into the coverings of both muscles and bones.

Muscles move by shortening their length, pulling on tendons, and moving bones closer to each other. One of the bones is pulled towards the other bone, which remains stationary. The place on the stationary bone that is connected via tendons to the muscle is called the origin. The place on the moving bone that is connected to the muscle via tendons is called the insertion. The belly of the muscle is the fleshy part of the muscle in between the tendons that does the actual contraction.

Names of Skeletal Muscles Skeletal muscles are named based on many different factors, including their location, origin and insertion, number of origins, shape, size, direction, and function.

Groups Action in Skeletal Muscle Skeletal muscles rarely work by themselves to achieve movements in the body. More often they work in groups to produce precise movements. The muscle that produces any particular movement of the body is known as an agonist or prime mover. The agonist always pairs with an antagonist muscle that produces the opposite effect on the same bones. For example, the biceps brachii muscle flexes the arm at the elbow. As the antagonist for this motion, the triceps brachii muscle extends the arm at the elbow. When the triceps is extending the arm, the biceps would be considered the antagonist.

In addition to the agonist/antagonist pairing, other muscles work to support the movements of the agonist. Synergists are muscles that help to stabilize a movement and reduce extraneous movements. They are usually found in regions near the agonist and often connect to the same bones. Because skeletal muscles move the insertion closer to the immobile origin, fixator muscles assist in movement by holding the origin stable. If you lift something heavy with your arms, fixators in the trunk region hold your body upright and immobile so that you maintain your balance while lifting.

Skeletal Muscle Histology Skeletal muscle fibers differ dramatically from other tissues of the body due to their highly specialized functions. Many of the organelles that make up muscle fibers are unique to this type of cell.

The sarcolemma is the cell membrane of muscle fibers. The sarcolemma acts as a conductor for electrochemical signals that stimulate muscle cells. Connected to the sarcolemma are transverse tubules (T-tubules) that help carry these electrochemical signals into the middle of the muscle fiber. The sarcoplasmic reticulum serves as a storage facility for calcium ions (Ca2+) that are vital to muscle contraction. Mitochondria, the power houses of the cell, are abundant in muscle cells to break down sugars and provide energy in the form of ATP to active muscles. Most of the muscle fibers structure is made up of myofibrils, which are the contractile structures of the cell. Myofibrils are made up of many proteins fibers arranged into repeating subunits called sarcomeres. The sarcomere is the functional unit of muscle fibers. (See Macronutrients for more information about the roles of sugars and proteins.)

Sarcomere Structure Sarcomeres are made of two types of protein fibers: thick filaments and thin filaments.

Function of Muscle Tissue The main function of the muscular system is movement. Muscles are the only tissue in the body that has the ability to contract and therefore move the other parts of the body.

Related to the function of movement is the muscular systems second function: the maintenance of posture and body position. Muscles often contract to hold the body still or in a particular position rather than to cause movement. The muscles responsible for the bodys posture have the greatest endurance of all muscles in the bodythey hold up the body throughout the day without becoming tired.

Another function related to movement is the movement of substances inside the body. The cardiac and visceral muscles are primarily responsible for transporting substances like blood or food from one part of the body to another.

The final function of muscle tissue is the generation of body heat. As a result of the high metabolic rate of contracting muscle, our muscular system produces a great deal of waste heat. Many small muscle contractions within the body produce our natural body heat. When we exert ourselves more than normal, the extra muscle contractions lead to a rise in body temperature and eventually to sweating.

Skeletal Muscles as Levers Skeletal muscles work together with bones and joints to form lever systems. The muscle acts as the effort force; the joint acts as the fulcrum; the bone that the muscle moves acts as the lever; and the object being moved acts as the load.

There are three classes of levers, but the vast majority of the levers in the body are third class levers. A third class lever is a system in which the fulcrum is at the end of the lever and the effort is between the fulcrum and the load at the other end of the lever. The third class levers in the body serve to increase the distance moved by the load compared to the distance that the muscle contracts.

The tradeoff for this increase in distance is that the force required to move the load must be greater than the mass of the load. For example, the biceps brachia of the arm pulls on the radius of the forearm, causing flexion at the elbow joint in a third class lever system. A very slight change in the length of the biceps causes a much larger movement of the forearm and hand, but the force applied by the biceps must be higher than the load moved by the muscle.

Motor Units Nerve cells called motor neurons control the skeletal muscles. Each motor neuron controls several muscle cells in a group known as a motor unit. When a motor neuron receives a signal from the brain, it stimulates all of the muscles cells in its motor unit at the same time.

The size of motor units varies throughout the body, depending on the function of a muscle. Muscles that perform fine movementslike those of theeyes or fingershave very few muscle fibers in each motor unit to improve the precision of the brains control over these structures. Muscles that need a lot of strength to perform their functionlike leg or arm muscleshave many muscle cells in each motor unit. One of the ways that the body can control the strength of each muscle is by determining how many motor units to activate for a given function. This explains why the same muscles that are used to pick up a pencil are also used to pick up a bowling ball.

Contraction Cycle Muscles contract when stimulated by signals from their motor neurons. Motor neurons contact muscle cells at a point called the Neuromuscular Junction (NMJ). Motor neurons release neurotransmitter chemicals at the NMJ that bond to a special part of the sarcolemma known as the motor end plate. The motor end plate contains many ion channels that open in response to neurotransmitters and allow positive ions to enter the muscle fiber. The positive ions form an electrochemical gradient to form inside of the cell, which spreads throughout the sarcolemma and the T-tubules by opening even more ion channels.

When the positive ions reach the sarcoplasmic reticulum, Ca2+ ions are released and allowed to flow into the myofibrils. Ca2+ ions bind to troponin, which causes the troponin molecule to change shape and move nearby molecules of tropomyosin. Tropomyosin is moved away from myosin binding sites on actin molecules, allowing actin and myosin to bind together.

ATP molecules power myosin proteins in the thick filaments to bend and pull on actin molecules in the thin filaments. Myosin proteins act like oars on a boat, pulling the thin filaments closer to the center of a sarcomere. As the thin filaments are pulled together, the sarcomere shortens and contracts. Myofibrils of muscle fibers are made of many sarcomeres in a row, so that when all of the sarcomeres contract, the muscle cells shortens with a great force relative to its size.

Muscles continue contraction as long as they are stimulated by a neurotransmitter. When a motor neuron stops the release of the neurotransmitter, the process of contraction reverses itself. Calcium returns to the sarcoplasmic reticulum; troponin and tropomyosin return to their resting positions; and actin and myosin are prevented from binding. Sarcomeres return to their elongated resting state once the force of myosin pulling on actin has stopped.

Types of Muscle Contraction The strength of a muscles contraction can be controlled by two factors: the number of motor units involved in contraction and the amount of stimulus from the nervous system. A single nerve impulse of a motor neuron will cause a motor unit to contract briefly before relaxing. This small contraction is known as a twitch contraction. If the motor neuron provides several signals within a short period of time, the strength and duration of the muscle contraction increases. This phenomenon is known as temporal summation. If the motor neuron provides many nerve impulses in rapid succession, the muscle may enter the state of tetanus, or complete and lasting contraction. A muscle will remain in tetanus until the nerve signal rate slows or until the muscle becomes too fatigued to maintain the tetanus.

Not all muscle contractions produce movement. Isometric contractions are light contractions that increase the tension in the muscle without exerting enough force to move a body part. When people tense their bodies due to stress, they are performing an isometric contraction. Holding an object still and maintaining posture are also the result of isometric contractions. A contraction that does produce movement is an isotonic contraction. Isotonic contractions are required to develop muscle mass through weight lifting.

Muscle tone is a natural condition in which a skeletal muscle stays partially contracted at all times. Muscle tone provides a slight tension on the muscle to prevent damage to the muscle and joints from sudden movements, and also helps to maintain the bodys posture. All muscles maintain some amount of muscle tone at all times, unless the muscle has been disconnected from the central nervous system due to nerve damage.

Functional Types of Skeletal Muscle Fibers Skeletal muscle fibers can be divided into two types based on how they produce and use energy: Type I and Type II.

Muscle Metabolism and Fatigue Muscles get their energy from different sources depending on the situation that the muscle is working in. Muscles use aerobic respiration when we call on them to produce a low to moderate level of force. Aerobic respiration requires oxygen to produce about 36-38 ATP molecules from a molecule of glucose. Aerobic respiration is very efficient, and can continue as long as a muscle receives adequate amounts of oxygen and glucose to keep contracting. When we use muscles to produce a high level of force, they become so tightly contracted that oxygen carrying blood cannot enter the muscle. This condition causes the muscle to create energy using lactic acid fermentation, a form of anaerobic respiration. Anaerobic respiration is much less efficient than aerobic respirationonly 2 ATP are produced for each molecule of glucose. Muscles quickly tire as they burn through their energy reserves under anaerobic respiration.

To keep muscles working for a longer period of time, muscle fibers contain several important energy molecules. Myoglobin, a red pigment found in muscles, contains iron and stores oxygen in a manner similar to hemoglobin in the blood. The oxygen from myoglobin allows muscles to continue aerobic respiration in the absence of oxygen. Another chemical that helps to keep muscles working is creatine phosphate. Muscles use energy in the form of ATP, converting ATP to ADP to release its energy. Creatine phosphate donates its phosphate group to ADP to turn it back into ATP in order to provide extra energy to the muscle. Finally, muscle fibers contain energy-storing glycogen, a large macromolecule made of many linked glucoses. Active muscles break glucoses off of glycogen molecules to provide an internal fuel supply.

When muscles run out of energy during either aerobic or anaerobic respiration, the muscle quickly tires and loses its ability to contract. This condition is known as muscle fatigue. A fatigued muscle contains very little or no oxygen, glucose or ATP, but instead has many waste products from respiration, like lactic acid and ADP. The body must take in extra oxygen after exertion to replace the oxygen that was stored in myoglobin in the muscle fiber as well as to power the aerobic respiration that will rebuild the energy supplies inside of the cell. Oxygen debt (or recovery oxygen uptake) is the name for the extra oxygen that the body must take in to restore the muscle cells to their resting state. This explains why you feel out of breath for a few minutes after a strenuous activityyour body is trying to restore itself to its normal state.

Prepared by Tim Taylor, Anatomy and Physiology Instructor

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Muscular System - Muscles of the Human Body