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58-497: In physiology , psychology , or psychophysics , a limen or a liminal point is a sensory threshold of a physiological or psychological response. Such points delineate boundaries of perception; that is, a limen defines a sensory threshold beyond which a particular stimulus becomes perceivable, and below which it remains unperceivable. Liminal , as an adjective, means situated at a sensory threshold, hence barely perceptible. Subliminal means below perception. The absolute threshold

116-588: A living system . As a subdiscipline of biology , physiology focuses on how organisms , organ systems , individual organs , cells , and biomolecules carry out chemical and physical functions in a living system. According to the classes of organisms , the field can be divided into medical physiology , animal physiology , plant physiology , cell physiology , and comparative physiology . Central to physiological functioning are biophysical and biochemical processes, homeostatic control mechanisms, and communication between cells. Physiological state

174-649: A bell and visual stimuli. In the 19th century, physiological knowledge began to accumulate at a rapid rate, in particular with the 1838 appearance of the Cell theory of Matthias Schleiden and Theodor Schwann . It radically stated that organisms are made up of units called cells. Claude Bernard 's (1813–1878) further discoveries ultimately led to his concept of milieu interieur (internal environment), which would later be taken up and championed as " homeostasis " by American physiologist Walter B. Cannon in 1929. By homeostasis, Cannon meant "the maintenance of steady states in

232-458: A cell releases chemical energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which are redox reactions. Although cellular respiration is technically a combustion reaction , it is an unusual one because of the slow, controlled release of energy from the series of reactions. Nutrients that are commonly used by animal and plant cells in respiration include sugar , amino acids and fatty acids , and

290-497: A distinct subdiscipline. In 1920, August Krogh won the Nobel Prize for discovering how, in capillaries, blood flow is regulated. In 1954, Andrew Huxley and Hugh Huxley, alongside their research team, discovered the sliding filaments in skeletal muscle , known today as the sliding filament theory. Recently, there have been intense debates about the vitality of physiology as a discipline (Is it dead or alive?). If physiology

348-455: A medical field originates in classical Greece , at the time of Hippocrates (late 5th century BC). Outside of Western tradition, early forms of physiology or anatomy can be reconstructed as having been present at around the same time in China , India and elsewhere. Hippocrates incorporated the theory of humorism , which consisted of four basic substances: earth, water, air and fire. Each substance

406-484: A membrane. This potential is then used to drive ATP synthase and produce ATP from ADP and a phosphate group. Biology textbooks often state that 38 ATP molecules can be made per oxidized glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle , and about 34 from the electron transport system). However, this maximum yield is never quite reached because of losses due to leaky membranes as well as

464-458: A reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD attaches to hydrogen from lactate to form ATP. In yeast, the waste products are ethanol and carbon dioxide . This type of fermentation is known as alcoholic or ethanol fermentation . The ATP generated in this process is made by substrate-level phosphorylation , which does not require oxygen. Fermentation

522-430: A unified science of life based on the cell actions, later renamed in the 20th century as cell biology . In the 20th century, biologists became interested in how organisms other than human beings function, eventually spawning the fields of comparative physiology and ecophysiology . Major figures in these fields include Knut Schmidt-Nielsen and George Bartholomew . Most recently, evolutionary physiology has become

580-550: Is a subdiscipline of botany concerned with the functioning of plants. Closely related fields include plant morphology , plant ecology , phytochemistry , cell biology , genetics , biophysics , and molecular biology . Fundamental processes of plant physiology include photosynthesis , respiration , plant nutrition , tropisms , nastic movements , photoperiodism , photomorphogenesis , circadian rhythms , seed germination , dormancy , and stomata function and transpiration . Absorption of water by roots, production of food in

638-400: Is a vital process that occurs in the cells of all living organisms . Respiration can be either aerobic, requiring oxygen, or anaerobic; some organisms can switch between aerobic and anaerobic respiration. The reactions involved in respiration are catabolic reactions , which break large molecules into smaller ones, producing large amounts of energy (ATP). Respiration is one of the key ways

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696-427: Is achieved through communication that occurs in a variety of ways, both electrical and chemical. Changes in physiology can impact the mental functions of individuals. Examples of this would be the effects of certain medications or toxic levels of substances. Change in behavior as a result of these substances is often used to assess the health of individuals. Much of the foundation of knowledge in human physiology

754-777: Is active in the inner membrane it short circuits the coupling between the electron transport chain and ATP synthesis . The potential energy from the proton gradient is not used to make ATP but generates heat. This is particularly important in brown fat thermogenesis of newborn and hibernating mammals. According to some newer sources, the ATP yield during aerobic respiration is not 36–38, but only about 30–32 ATP molecules / 1 molecule of glucose , because: So finally we have, per molecule of glucose Altogether this gives 4 + 3 (or 5) + 20 + 3 = 30 (or 32) ATP per molecule of glucose These figures may still require further tweaking as new structural details become available. The above value of 3 H / ATP for

812-400: Is essential for diagnosing and treating health conditions and promoting overall wellbeing. It seeks to understand the mechanisms that work to keep the human body alive and functioning, through scientific enquiry into the nature of mechanical, physical, and biochemical functions of humans, their organs, and the cells of which they are composed. The principal level of focus of physiology is at

870-462: Is known for having a corresponding humor: black bile, phlegm, blood, and yellow bile, respectively. Hippocrates also noted some emotional connections to the four humors, on which Galen would later expand. The critical thinking of Aristotle and his emphasis on the relationship between structure and function marked the beginning of physiology in Ancient Greece . Like Hippocrates , Aristotle took to

928-520: Is less efficient at using the energy from glucose: only 2 ATP are produced per glucose, compared to the 38 ATP per glucose nominally produced by aerobic respiration. Glycolytic ATP, however, is produced more quickly. For prokaryotes to continue a rapid growth rate when they are shifted from an aerobic environment to an anaerobic environment, they must increase the rate of the glycolytic reactions. For multicellular organisms, during short bursts of strenuous activity, muscle cells use fermentation to supplement

986-476: Is modified to become α-ketoglutarate (5 carbons), succinyl-CoA , succinate , fumarate , malate and, finally, oxaloacetate . The net gain from one cycle is 3 NADH and 1 FADH 2 as hydrogen (proton plus electron) carrying compounds and 1 high-energy GTP , which may subsequently be used to produce ATP. Thus, the total yield from 1 glucose molecule (2 pyruvate molecules) is 6 NADH, 2 FADH 2 , and 2 ATP. In eukaryotes, oxidative phosphorylation occurs in

1044-442: Is not transferred to the mitochondrion and finally oxidized to the carbon dioxide (CO 2 ), but reduced to ethanol or lactic acid in the cytoplasm . Without oxygen, pyruvate ( pyruvic acid ) is not metabolized by cellular respiration but undergoes a process of fermentation . The pyruvate is not transported into the mitochondrion but remains in the cytoplasm, where it is converted to waste products that may be removed from

1102-580: Is perhaps less visible nowadays than during the golden age of the 19th century, it is in large part because the field has given birth to some of the most active domains of today's biological sciences, such as neuroscience , endocrinology , and immunology . Furthermore, physiology is still often seen as an integrative discipline, which can put together into a coherent framework data coming from various different domains. Initially, women were largely excluded from official involvement in any physiological society. The American Physiological Society , for example,

1160-540: Is the condition of normal function. In contrast, pathological state refers to abnormal conditions , including human diseases . The Nobel Prize in Physiology or Medicine is awarded by the Royal Swedish Academy of Sciences for exceptional scientific achievements in physiology related to the field of medicine . Because physiology focuses on the functions and mechanisms of living organisms at all levels, from

1218-435: Is the lowest amount of sensation detectable by a sense organ. This neuroscience article is a stub . You can help Misplaced Pages by expanding it . Physiology Physiology ( / ˌ f ɪ z i ˈ ɒ l ə dʒ i / ; from Ancient Greek φύσις ( phúsis )  'nature, origin' and -λογία ( -logía )  'study of') is the scientific study of functions and mechanisms in

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1276-447: Is the preferred method of pyruvate production in glycolysis , and requires pyruvate to the mitochondria in order to be oxidized by the citric acid cycle . The products of this process are carbon dioxide and water, and the energy transferred is used to make bonds between ADP and a third phosphate group to form ATP ( adenosine triphosphate ), by substrate-level phosphorylation , NADH and FADH 2 . The negative ΔG indicates that

1334-462: The pyruvate is oxidized. The overall reaction can be expressed this way: Starting with glucose, 1 ATP is used to donate a phosphate to glucose to produce glucose 6-phosphate . Glycogen can be converted into glucose 6-phosphate as well with the help of glycogen phosphorylase . During energy metabolism, glucose 6-phosphate becomes fructose 6-phosphate . An additional ATP is used to phosphorylate fructose 6-phosphate into fructose 1,6-bisphosphate by

1392-400: The 10 protons from oxidizing NADH would produce 2.72 ATP (instead of 2.5) and the 6 protons from oxidizing succinate or ubiquinol would produce 1.64 ATP (instead of 1.5). This is consistent with experimental results within the margin of error described in a recent review. The total ATP yield in ethanol or lactic acid fermentation is only 2 molecules coming from glycolysis , because pyruvate

1450-421: The ATP production from the slower aerobic respiration, so fermentation may be used by a cell even before the oxygen levels are depleted, as is the case in sports that do not require athletes to pace themselves, such as sprinting . Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor, such as oxygen, to produce large amounts of energy and drive

1508-472: The CO 2 generated annually by terrestrial ecosystems . Glycolysis is a metabolic pathway that takes place in the cytosol of cells in all living organisms. Glycolysis can be literally translated as "sugar splitting", and occurs regardless of oxygen's presence or absence. In aerobic conditions, the process converts one molecule of glucose into two molecules of pyruvate (pyruvic acid), generating energy in

1566-509: The Krebs cycle. However, if oxygen is not present, fermentation of the pyruvate molecule will occur. In the presence of oxygen, when acetyl-CoA is produced, the molecule then enters the citric acid cycle (Krebs cycle) inside the mitochondrial matrix, and is oxidized to CO 2 while at the same time reducing NAD to NADH . NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation. To fully oxidize

1624-466: The addition of two protons, water is formed. The table below describes the reactions involved when one glucose molecule is fully oxidized into carbon dioxide. It is assumed that all the reduced coenzymes are oxidized by the electron transport chain and used for oxidative phosphorylation. Although there is a theoretical yield of 38 ATP molecules per glucose during cellular respiration, such conditions are generally not realized because of losses such as

1682-418: The blood . Santorio Santorio in 1610s was the first to use a device to measure the pulse rate (the pulsilogium ), and a thermoscope to measure temperature. In 1791 Luigi Galvani described the role of electricity in the nerves of dissected frogs. In 1811, César Julien Jean Legallois studied respiration in animal dissection and lesions and found the center of respiration in the medulla oblongata . In

1740-421: The body and the physiological processes through which they are regulated." In other words, the body's ability to regulate its internal environment. William Beaumont was the first American to utilize the practical application of physiology. Nineteenth-century physiologists such as Michael Foster , Max Verworn , and Alfred Binet , based on Haeckel 's ideas, elaborated what came to be called "general physiology",

1798-463: The bulk production of ATP. Anaerobic respiration is used by microorganisms, either bacteria or archaea , in which neither oxygen (aerobic respiration) nor pyruvate derivatives (fermentation) is the final electron acceptor. Rather, an inorganic acceptor such as sulfate ( SO 2− 4 ), nitrate ( NO − 3 ), or sulfur (S) is used. Such organisms could be found in unusual places such as underwater caves or near hydrothermal vents at

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1856-410: The cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD so it can be re-used in glycolysis. In the absence of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD for glycolysis. This waste product varies depending on the organism. In skeletal muscles,

1914-460: The conversion of pyruvate to acetyl-CoA, one molecule of NADH and one molecule of CO 2 is formed. The citric acid cycle is also called the Krebs cycle or the tricarboxylic acid cycle . When oxygen is present, acetyl-CoA is produced from the pyruvate molecules created from glycolysis. Once acetyl-CoA is formed, aerobic or anaerobic respiration can occur. When oxygen is present, the mitochondria will undergo aerobic respiration which leads to

1972-424: The cost of moving pyruvate (from glycolysis), phosphate, and ADP (substrates for ATP synthesis) into the mitochondria. All are actively transported using carriers that utilize the stored energy in the proton electrochemical gradient . The outcome of these transport processes using the proton electrochemical gradient is that more than 3 H are needed to make 1 ATP. Obviously, this reduces the theoretical efficiency of

2030-473: The cost of moving pyruvate and ADP into the mitochondrial matrix, and current estimates range around 29 to 30 ATP per glucose. Aerobic metabolism is up to 15 times more efficient than anaerobic metabolism (which yields 2 molecules of ATP per 1 molecule of glucose). However, some anaerobic organisms, such as methanogens are able to continue with anaerobic respiration , yielding more ATP by using inorganic molecules other than oxygen as final electron acceptors in

2088-538: The death rate from surgery by a substantial amount. The Physiological Society was founded in London in 1876 as a dining club. The American Physiological Society (APS) is a nonprofit organization that was founded in 1887. The Society is, "devoted to fostering education, scientific research, and dissemination of information in the physiological sciences." In 1891, Ivan Pavlov performed research on "conditional responses" that involved dogs' saliva production in response to

2146-451: The election of women was celebrated in 2015 with the publication of the book "Women Physiologists: Centenary Celebrations And Beyond For The Physiological Society." ( ISBN   978-0-9933410-0-7 ) Prominent women physiologists include: Human physiology Animal physiology Plant physiology Fungal physiology Protistan physiology Algal physiology Bacterial physiology Respiration in plant Cellular respiration

2204-440: The electron transport chain. They share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. The post-glycolytic reactions take place in the mitochondria in eukaryotic cells , and in the cytoplasm in prokaryotic cells . Although plants are net consumers of carbon dioxide and producers of oxygen via photosynthesis , plant respiration accounts for about half of

2262-461: The equivalent of one glucose molecule, two acetyl-CoA must be metabolized by the Krebs cycle. Two low-energy waste products , H 2 O and CO 2 , are created during this cycle. The citric acid cycle is an 8-step process involving 18 different enzymes and co-enzymes. During the cycle, acetyl-CoA (2 carbons) + oxaloacetate (4 carbons) yields citrate (6 carbons), which is rearranged to a more reactive form called isocitrate (6 carbons). Isocitrate

2320-534: The form of two net molecules of ATP . Four molecules of ATP per glucose are actually produced, but two are consumed as part of the preparatory phase . The initial phosphorylation of glucose is required to increase the reactivity (decrease its stability) in order for the molecule to be cleaved into two pyruvate molecules by the enzyme aldolase . During the pay-off phase of glycolysis, four phosphate groups are transferred to four ADP by substrate-level phosphorylation to make four ATP, and two NADH are produced when

2378-413: The help of phosphofructokinase . Fructose 1,6-biphosphate then splits into two phosphorylated molecules with three carbon chains which later degrades into pyruvate. Pyruvate is oxidized to acetyl-CoA and CO 2 by the pyruvate dehydrogenase complex (PDC). The PDC contains multiple copies of three enzymes and is located in the mitochondria of eukaryotic cells and in the cytosol of prokaryotes. In

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2436-502: The humoral theory of disease, which also consisted of four primary qualities in life: hot, cold, wet and dry. Galen ( c.  130 –200 AD) was the first to use experiments to probe the functions of the body. Unlike Hippocrates, Galen argued that humoral imbalances can be located in specific organs, including the entire body. His modification of this theory better equipped doctors to make more precise diagnoses. Galen also played off of Hippocrates' idea that emotions were also tied to

2494-458: The humors, and added the notion of temperaments: sanguine corresponds with blood; phlegmatic is tied to phlegm; yellow bile is connected to choleric; and black bile corresponds with melancholy. Galen also saw the human body consisting of three connected systems: the brain and nerves, which are responsible for thoughts and sensations; the heart and arteries, which give life; and the liver and veins, which can be attributed to nutrition and growth. Galen

2552-410: The individual." In more differentiated organisms, the functional labor could be apportioned between different instruments or systems (called by him as appareils ). In 1858, Joseph Lister studied the cause of blood coagulation and inflammation that resulted after previous injuries and surgical wounds. He later discovered and implemented antiseptics in the operating room, and as a result, decreased

2610-402: The leaves, and growth of shoots towards light are examples of plant physiology. Human physiology is the study of how the human body's systems and functions work together to maintain a stable internal environment. It includes the study of the nervous, endocrine, cardiovascular, respiratory, digestive, and urinary systems, as well as cellular and exercise physiology. Understanding human physiology

2668-449: The level of organs and systems within systems. The endocrine and nervous systems play major roles in the reception and transmission of signals that integrate function in animals. Homeostasis is a major aspect with regard to such interactions within plants as well as animals. The biological basis of the study of physiology, integration refers to the overlap of many functions of the systems of the human body, as well as its accompanied form. It

2726-466: The mitochondrial cristae . It comprises the electron transport chain that establishes a proton gradient (chemiosmotic potential) across the boundary of the inner membrane by oxidizing the NADH produced from the Krebs cycle. ATP is synthesized by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP. The electrons are finally transferred to exogenous oxygen and, with

2784-455: The molecular and cellular level to the level of whole organisms and populations, its foundations span a range of key disciplines: There are many ways to categorize the subdisciplines of physiology: Although there are differences between animal , plant , and microbial cells, the basic physiological functions of cells can be divided into the processes of cell division , cell signaling , cell growth , and cell metabolism . Plant physiology

2842-588: The most common oxidizing agent is molecular oxygen (O 2 ). The chemical energy stored in ATP (the bond of its third phosphate group to the rest of the molecule can be broken allowing more stable products to form, thereby releasing energy for use by the cell) can then be used to drive processes requiring energy, including biosynthesis , locomotion or transportation of molecules across cell membranes . Aerobic respiration requires oxygen (O 2 ) in order to create ATP . Although carbohydrates , fats and proteins are consumed as reactants , aerobic respiration

2900-409: The notion of physiological division of labor, which allowed to "compare and study living things as if they were machines created by the industry of man." Inspired in the work of Adam Smith , Milne-Edwards wrote that the "body of all living beings, whether animal or plant, resembles a factory ... where the organs, comparable to workers, work incessantly to produce the phenomena that constitute the life of

2958-443: The reaction is exothermic ( exergonic ) and can occur spontaneously. The potential of NADH and FADH 2 is converted to more ATP through an electron transport chain with oxygen and protons (hydrogen ions) as the " terminal electron acceptors ". Most of the ATP produced by aerobic cellular respiration is made by oxidative phosphorylation . The energy released is used to create a chemiosmotic potential by pumping protons across

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3016-598: The same year, Charles Bell finished work on what would later become known as the Bell–Magendie law , which compared functional differences between dorsal and ventral roots of the spinal cord . In 1824, François Magendie described the sensory roots and produced the first evidence of the cerebellum's role in equilibration to complete the Bell–Magendie law. In the 1820s, the French physiologist Henri Milne-Edwards introduced

3074-503: The synthase assumes that the synthase translocates 9 protons, and produces 3 ATP, per rotation. The number of protons depends on the number of c subunits in the Fo c-ring , and it is now known that this is 10 in yeast Fo and 8 for vertebrates. Including one H for the transport reactions, this means that synthesis of one ATP requires 1 + 10/3 = 4.33 protons in yeast and 1 + 8/3 = 3.67 in vertebrates . This would imply that in human mitochondria

3132-410: The waste product is lactic acid . This type of fermentation is called lactic acid fermentation . In strenuous exercise, when energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation is catalyzed by lactate dehydrogenase in

3190-434: The whole process and the likely maximum is closer to 28–30 ATP molecules. In practice the efficiency may be even lower because the inner membrane of the mitochondria is slightly leaky to protons. Other factors may also dissipate the proton gradient creating an apparently leaky mitochondria. An uncoupling protein known as thermogenin is expressed in some cell types and is a channel that can transport protons. When this protein

3248-402: Was also the founder of experimental physiology. And for the next 1,400 years, Galenic physiology was a powerful and influential tool in medicine . Jean Fernel (1497–1558), a French physician, introduced the term "physiology". Galen, Ibn al-Nafis , Michael Servetus , Realdo Colombo , Amato Lusitano and William Harvey , are credited as making important discoveries in the circulation of

3306-923: Was founded in 1887 and included only men in its ranks. In 1902, the American Physiological Society elected Ida Hyde as the first female member of the society. Hyde, a representative of the American Association of University Women and a global advocate for gender equality in education, attempted to promote gender equality in every aspect of science and medicine. Soon thereafter, in 1913, J.S. Haldane proposed that women be allowed to formally join The Physiological Society , which had been founded in 1876. On 3 July 1915, six women were officially admitted: Florence Buchanan , Winifred Cullis , Ruth Skelton , Sarah C. M. Sowton , Constance Leetham Terry , and Enid M. Tribe . The centenary of

3364-410: Was provided by animal experimentation . Due to the frequent connection between form and function, physiology and anatomy are intrinsically linked and are studied in tandem as part of a medical curriculum. Involving evolutionary physiology and environmental physiology , comparative physiology considers the diversity of functional characteristics across organisms. The study of human physiology as

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