Endocytosis

Endocytosis is the process by which the plasma membrane folds inward to bring substances into the cell. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma membrane or cell membrane. The process opposite to endocytosis is exocytosis.

Through endocytosis animal cells engulf particulate material, such as cellular debris and microorganisms; macromolecules, such as proteins and complex sugars; and low-molecular-weight molecules, such as vitamins and simple sugars. Cells engage in at least three different types of endocytosis: 1) Phagocytosis (literally, cell-eating) is the process by which cells ingest solids, such as bacteria, viruses, or the remnants of cells which have undergone apoptosis; in phagocytosis the membrane invaginates enclosing the wanted particles in a pocket, then engulfs the object by pinching it off, and the object is sealed off into a large vacuole known as a phagosome. 2) Pinocytosis (literally, cell-drinking), which is how cells take in liquids. 3) Receptor-mediated endocytosis is a more specific active event where the cytoplasm membrane folds inward to form coated pits; in this case, proteins or other trigger particles lock into receptors/ ligands in the cell’s plasma membrane; it is then, and only then that the particles are engulfed.

Endocytosis (Video)

Exocytosis

Exocytosis is a process of cellular excretion in which substances contained in vesicles are discharged from the cell by fusion of the vesicular membrane with the outer cell membrane. Or, put it in other words, exocytosis is a cellular process in which cells eject waste products or chemical transmitters (such as hormones) from the interior of the cell. Exocytosis is similar in function to endocytosis but working in the opposite direction.

In multicellular organisms there are two types of exocytosis: 1) Ca2+ triggered non-constitutive and 2) non Ca2+ triggered constitutive. Exocytosis in neuronal chemical synapses is Ca2+ triggered and serves interneuronal signalling. Constitutive exocytosis is performed by all cells and serves the release of components of the extracellular matrix, or just delivery of newly-synthesized membrane proteins that are incorporated in the plasma membrane after the fusion of the transport vesicle. Exocytosis is the opposite of endocytosis.

There are five steps to exocytosis: 1) in this first step, the vesicle containing the waste product is transported through the cytoplasm towards the part of the cell from which it will be eliminated; 2) as the vesicle approaches the cell membrane, it is secured and pulled towards the part of the cell from which it will be eliminated; 3) in third step, the vesicle comes in contact with the cell membrane, where it begins to chemical and physically merge with the proteins in the cell membrane; 4) the fourth step involves the chemical preparations for the last step of exocytosis; 5) in the last step, the proteins forming the walls of the vesicle merge with the cell membrane and breach, pushing the vesicle contents (waste products or chemical transmitters) out of the cell. This step is the primary mechanism for the increase in size of the cell's plasma membrane.

Exocytosis (video)

G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) constitute a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal transduction pathways and, ultimately, cellular responses. G protein-coupled receptors are found only in eukaryotes, including yeast, choanoflagellates, and animals. The ligands that bind and activate the G protein-coupled receptors include light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters, and vary in size from small molecules to peptides to large proteins. G protein-coupled receptors are involved in many diseases, and are also the target of approximately 30% of all modern medicinal drugs.

The main function of the G protein-coupled receptors is to transduce extracellular stimuli into intracellular signals. They are among the largest and most diverse protein families in mammalian genomes. On the basis of homology with rhodopsin, they are predicted to contain seven membrane-spanning helices, an extracellular N-terminus and an intracellular C-terminus. This gives rise to their other names, the 7-TM receptors or the heptahelical receptors. GPCRs transduce extracellular stimuli to give intracellular signals through interaction of their intracellular domains with heterotrimeric G proteins, and the crystal structure of one member of this group, bovine rhodopsin, has recently been solved.

There are two principal signal transduction pathways involving the G protein-coupled receptors: the cAMP signal pathway and the Phosphatidylinositol signal pathway. When a ligand binds to the GPCR it causes a conformational change in the GPCR, which allows it to act as a guanine nucleotide exchange factor (GEF). The GPCR can then activate an associated G-protein by exchanging its bound GDP for a GTP. The G-protein's a subunit, together with the bound GTP, can then dissociate from the ß and ? subunits to further affect intracellular signaling proteins or target functional proteins directly depending on the a subunit type.

Tyrosine

Tyrosine is a white crystalline amino acid, CHNO, which is used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. Tyrosine is obtained from the hydrolysis of proteins such as casein and is a precursor of epinephrine, thyroxine, and melanin. It was first discovered in 1846 by German chemist Justus von Liebig in the protein casein from cheese.

Tyrosine supports and assists neurotransmitters in the brain. L-Tyrosine supplementation helps reduce stress, improves mental alertness and mood, acts as an appetite suppressant and has a positive affect on sex drive. Tyrosine, which can be synthesized in the body from phenylalanine, is found in many high protein food products such as soy products, chicken, turkey, fish, peanuts, almonds, avocados, bananas, milk, cheese, and yogurt.

Transcytosis

Also called vesicular transport, transcytosis is a form of intracellular vesicular traffic in which endocytosed macromolecules are transferred across the cell and released via exocytosis at the opposite plasma membrane domain. In other words, transcytosis is a mechanism for transcellular transport in which a cell encloses extracellular material in an invagination of the cell membrane to form a vesicle, then moves the vesicle across the cell to eject the material through the opposite cell membrane by the reverse process.

Vesicles are employed to intake the macromolecules on one side of the cell, draw them across the cell, and eject them on the other side. While transcytosis is most commonly observed in cells of an epithelium, the process is also present elsewhere. Blood capillaries are a well-known site for transcytosis, though it occurs in other cells, including neurons and intestinal cells.

Basal Metabolic Rate

Basal metabolic rate (BMR), and the resting metabolic rate (RMR), is the amount of energy expended while at rest in a neutrally temperate environment, in the post-absorptive state (meaning that the digestive system is inactive, which requires about twelve hours of fasting in humans). The release of energy in this state is sufficient only for the functioning of the vital organs, such as the heart, lungs, brain and the rest of the nervous system, liver, kidneys, sex organs, muscles and skin. BMR decreases with age and with the loss of lean body mass. Increasing muscle mass increases BMR. Aerobic fitness level, a product of cardiovascular exercise, while previously thought to have effect on BMR, has been shown in the 1990s not to correlate with BMR, when fat-free body mass was adjusted for.

Both basal metabolic rate and resting metabolic rate are usually expressed in terms of daily rates of energy expenditure. The early work of the scientists J. Arthur Harris and Francis G. Benedict showed that approximate values could be derived using body surface area (computed from height and weight), age, and sex, along with the oxygen and carbon dioxide measures taken from calorimetry. Studies also showed that by eliminating the sex differences that occur with the accumulation of adipose tissue by expressing metabolic rate per unit of "fat-free" or lean body weight, the values between sexes for basal metabolism are essentially the same. Exercise physiology textbooks have tables to show the conversion of height and body surface area as they relate to weight and basal metabolic values. The primary organ responsible for regulating metabolism is the hypothalamus. The hypothalamus is located on the brain stem and forms the floor and part of the lateral walls of the third ventricle of the cerebrum.

Metabolism

Metabolism is the set of chemical processes which occur in cells of living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories: 1) catabolism, which breaks down organic matter to harvest energy in cellular respiration; 2) anabolism, which uses energy to construct components of cells such as proteins and nucleic acids.

Metabolism comprises all the chemical reactions by which molecules taken into an organism are broken down to produce energy and by which energy is used to build up complex molecules. All metabolic reactions fall into one of two general categories: catabolic and anabolic reactions, or the processes of breaking down and building up, respectively. An example of metabolism from daily life takes place in the process of taking in and digesting nutrients.

Catabolism and anabolism share an important common sequence of reactions known collectively as the citric acid cycle, the tricarboxylic acid cycle, or the Krebs cycle. Named after the German biochemist Sir Hans Adolf Krebs (1900-1981). The citric acid cycle is a series of chemical reactions in which tissues use carbohydrates, fats, and proteins to produce energy; it is part of a larger series of enzymatic reactions known as oxidative phosphorylation. In the latter reaction, glucose is broken down to release energy, which is stored in the form of ATP—a catabolic sequence.

The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, by a sequence of enzymes. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy and will not occur by themselves, by coupling them to spontaneous reactions that release energy. As enzymes act as catalysts they allow these reactions to proceed quickly and efficiently. Enzymes also allow the regulation of metabolic pathways in response to changes in the cell's environment or signals from other cells.

Metabolism Explained by Professor (Video)

Cellular Respiration

Cellular respiration is the set of metabolic reactions and processes which occur in cells to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The type of reactions which take place in respiration are catabolic reactions which consist of the oxidation of one molecule and the reduction of another. In other words, cellular respiration is the process of oxidizing food molecules, like glucose, to carbon dioxide and water. Then the energy released is trapped in the form of ATP for use by all the energy-consuming activities of the cell.

Nutrients commonly used by animal and plant cells in cellular respiration include glucose, amino acids and fatty acids, and a common oxidizing agent (electron acceptor) is molecular oxygen (O2). Bacteria and archaea can also be lithotrophs and these organisms may respire using a broad range of inorganic molecules as electron donors and acceptors, such as sulfur, metal ions, methane or hydrogen. Organisms that use oxygen as a final electron acceptor in respiration are described as aerobic, while those that do not are referred to as anaerobic.

The process of cellular respiration occurs in two phases: 1) glycolysis, the breakdown of glucose to pyruvic acid; 2) the complete oxidation of pyruvic acid to carbon dioxide and water.

Cellular Respiration Animation Video

Chemiosmosis

Chemiosmosis is the process by which ions diffuse across a mitochondrial permeable membrane. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration.

In chemiosmosis, ions (electrons) of hydrogen diffuse from an area of high electron concentration to an area of lower electron concentration. An electrochemical concentration gradient of electrons across a membrane could be harnessed to make ATP. This process is similar to osmosis, which is the diffusion of water across a membrane, hence the name chemiosmosis.

ATP synthase is the enzyme that makes ATP by chemiosmosis. It allows electrons to pass through the membrane using the kinetic energy to phosphorylate ADP making ATP. The generation of ATP by chemiosmosis occurs in chloroplasts and mitochondria as well as in some bacteria.

Chemiosmotic phosphorylation is the third pathway that produces ATP from inorganic phosphate and an ADP molecule. This process is part of oxidative phosphorylation. The complete breakdown of glucose in the presence of oxygen is called cellular respiration. The last steps of this process occur in mitochondria. The reduced molecules NADH and FADH2 are generated by the Krebs cycle and glycolysis. These molecules pass electrons to an electron transport chain, which uses the energy released to create a proton gradient across the inner mitochondrial membrane. ATP synthase then uses the energy stored in this gradient to make ATP. This process is called oxidative phosphorylation because oxygen is the final electron acceptor and the energy released by reducing oxygen to water is used to phosphorylate ADP and generate ATP.

Electron Transport Chain (Video)

Catabolism

Catabolism is the set of metabolic pathways which break down molecules into smaller units and release energy. A simpler way to put it: catabolism is the metabolic breakdown of complex molecules into simpler ones, often resulting in a release of energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides and amino acids, respectively. As molecules such as polysaccharides, proteins and nucleic acids are made from long chains of these small monomer units.

People who are undernourished are sometimes said to be in a catabolic state, which means that they are catabolizing their body tissues, without replacing them. Hence, a proper relation between anabolism and catabolism is essential for the maintenance of bodily homeostasis and dynamic equilibrium.

The byproducts of catabolism are cellular wastes, which include lactic acid, acetic acid, carbon dioxide, ammonia, creatinine, and urea. The creation of these wastes is usually an oxidation process involving a release of chemical free energy, some of which is lost as heat, but the rest of which is used to drive the synthesis of adenosine triphosphate (ATP). This molecule acts as a way for the cell to transfer the energy released by catabolism to the energy-requiring reactions that make up anabolism.

Schematic diagram of catabolism

Anabolism

Anabolism, or anabolic process, is the set of metabolic pathways which build molecules from smaller units. It is the phase of metabolism in which simple substances are synthesized into the complex materials of living tissue. These reactions require energy. One way of categorizing metabolic processes, whether at the cellular, organ or organism level is as 'anabolic' or as 'catabolic', which is the opposite. Anabolism is powered by catabolism, where large molecules are broken down into smaller parts and then used up in respiration. Many anabolic processes are powered by adenosine triphosphate (ATP).

As anabolism is the metabolic synthesis of proteins, fats, and other constituents of living organisms from molecules or simple precursors, it has a tendency toward building up organs and tissues. These processes produce growth and differentiation of cells and increase in body size, a process that involves synthesis of complex molecules. Examples of anabolic processes include the growth and mineralization of bone and increases in muscle mass.

Endocrinologists have traditionally classified hormones as anabolic or catabolic, depending on which part of metabolism they stimulate. The classic anabolic hormones are the anabolic steroids, which stimulate protein synthesis and muscle growth. The balance between anabolism and catabolism is also regulated by circadian rhythms, with processes such as glucose metabolism fluctuating to match an animal's normal periods of activity throughout the day.

Citric Acid Cycle

The citric acid cycle is a series of enzyme-catalysed chemical reactions, which take place in the matrix of the mitochondrion. The citric acid cycle is very importance in all living cells that use oxygen as part of cellular respiration. The components and reactions of the citric acid cycle were established by seminal work from Albert Szent-Györgyi and Hans Krebs. The citric acid cycle is also called the tricarboxylic acid cycle (TCA cycle), or the Krebs cycle. The Citric Acid Cycle is one of 3 stages of cellular respiration. The other stages are glycolysis and electron transport/oxidative phosphorylation.

The citric acid cycle is part of a metabolic pathway which participates in the chemical conversion of carbohydrates, fats and proteins into carbon dioxide and water to generate a form of usable energy. Other relevant reactions in the pathway include those in glycolysis and pyruvate oxidation before the citric acid cycle, and oxidative phosphorylation after it. In addition, it provides precursors for many compounds including some amino acids and is therefore functional even in cells performing fermentation.

Crystal-Clear Explanation of Citric Acid Cycle / Krebs Cycle (Animation)



The Krebs Cycle Video

Pyruvate Decarboxylation

Pyruvate decarboxylation, also called oxidative decarboxylation, is the biochemical reaction that uses pyruvate to form acetyl-CoA, releasing NADH, a reducing equivalent, and carbon dioxide via decarboxylation. It is also known as the link reaction because it forms an important link between the metabolic pathways of glycolysis and the citric acid cycle. This reaction is usually catalyzed by the pyruvate dehydrogenase complex as part of aerobic respiration. In eukaryotes, pyruvate decarboxylation takes place exclusively inside the mitochondrial matrix; in prokaryotes similar reactions take place in the cytoplasm and at the plasma membrane.

Pyruvate decarboxylation occurs in the mitochondria, unlike the reactions of glycolysis which are cytosolic, and is very common in most organisms as a link to the citric acid cycle. The conversion of pyruvate to acetyl CoA by the pyruvate dehydrogenase complex is a key step in the liver in particular, as it removes any chance of conversion of pyruvate to glucose, or as a transmination substrate. It commits pyruvate to entering the citric acid cycle, where it is either used as a substrate for oxidative phosphorylation, or is converted to citrate for export to the cytosol to serve as a substrate for fatty acid and isoprenoid biosynthesis.

Pyruvate

Pyruvate is the carboxylate (COOH) ion (anion) of pyruvic acid, an organic acid which is a key intersection in several metabolic pathways. It is a chemical substance made in our bodies as a result of glucose metabolism. Pyruvate can be made from glucose through glycolysis and supplies energy to living cells in the citric acid cycle, and can also be converted to carbohydrates via gluconeogenesis, to fatty acids or energy through acetyl-CoA, to the amino acid alanine and to ethanol.

Pyruvate is the end product of glycolysis, which is used and synthesized by many metabolic pathways. In energy generation, it can be either converted to lactate, when the oxygen is not sufficient, or broken down to water and carbon dioxide in the presence of oxygen, generating large amounts of ATP.

Pyruvate, which is a natural metabolic fuel and antioxidant in myocardium and other tissues, exerts a variety of cardioprotective actions when provided at supraphysiological concentrations. Pyruvate increases cardiac contractile performance and myocardial energy state, bolsters endogenous antioxidant systems, and protects myocardium from ischemia-reperfusion injury and oxidant stress.

The Pyruvate Dehydrogenase Complex and Kreb's Cycle explanation ( Video )

Nicotinamide Adenine Dinucleotide

Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in all living cells. Along with the its relative nicotinamide adenine dinucleotide phosphate (NADP), the NAD is one of the most important coenzymes in the cell. The compound is a dinucleotide, since it consists of two nucleotides joined through their phosphate groups, with one nucleotide containing an adenine base and the other containing nicotinamide.

The nicotinamide adenine dinucleotide participates in redox reactions as it brings electrons from one reaction to the next. The coenzyme is therefore found in two forms in cells: NAD is an oxidizing agent, accepting electrons from other molecules and becoming reduced. This reaction forms NADH, which can then be used as a reducing agent to donate electrons. These electron transfer reactions are the main function of NAD. However, it is also used in other cellular processes, notably as a substrate of enzymes that add or remove chemical groups from proteins, in posttranslational modifications. Because of the importance of these functions, the enzymes involved in NAD metabolism are targets for drug discovery.

Because of the positive charge on the nitrogen atom in the nicotinamide ring (upper right), the oxidized forms of these important redox reagents are often depicted as NAD+ and NADP+ respectively. In cells, most oxidations are accomplished by the removal of hydrogen atoms. Both of these coenzymes play crucial roles in this. Each molecule of NAD+ (or NADP+) can acquire two electrons; that is, be reduced by two electrons. However, only one proton accompanies the reduction. The other proton produced as two hydrogen atoms are removed from the molecule being oxidized is liberated into the surrounding medium. For NAD, the reaction is thus: NAD+ + 2H -> NADH + H+

Adenosine Triphosphate

Adenosine triphosphate (ATP) is a molecule used in cells as a coenzyme. Adenosine triphosphate is the immediate source of energy for the mechanical work performed by muscle. ATP is a nucleotide which causes the contraction of the muscle protein actomyosin with the formation of adenosine diphosphate and inorganic phosphate. ATP is also involved in the activation of amino acids, a necessary step in the synthesis of protein. It is often called the "molecular unit of currency" of intracellular energy transfer.

Adenosine triphosphate is the only compound which the body can use directly as fuel for energy-consuming activities, including movement. Without it, we would die. ATP is a high-energy compound made using the energy derived from the breakdown of food during respiration. Physical activity uses enormous quantities of ATP. An active muscle cell requires about two million ATP molecules per second to drive its biochemical machinery.

Adenosine triphosphate transports chemical energy within cells for metabolism. It is produced by photophosphorylation and cellular respiration and used by enzymes and structural proteins in many cellular processes, including biosynthetic reactions, motility, and cell division. One molecule of ATP contains three phosphate groups, and it is produced by ATP synthase from inorganic phosphate and adenosine diphosphate (ADP) or adenosine monophosphate (AMP). Metabolic processes that use ATP as an energy source convert it back into its precursors. ATP is therefore continuously recycled in organisms, with the human body turning over its own weight in ATP each day.

ATP is used as a substrate in signal transduction pathways by kinases that phosphorylate proteins and lipids, as well as by adenylate cyclase, which uses ATP to produce the second messenger molecule cyclic AMP. The ratio between ATP and AMP is used as a way for a cell to sense how much energy is available and control the metabolic pathways that produce and consume ATP. Apart from its roles in energy metabolism and signaling, ATP is also incorporated into nucleic acids by polymerases in the processes of DNA replication and transcription.

Nucleotides

Nucleotides are organic compounds which consists of a nucleoside combined with a phosphate group. Nucleotides are molecules which join together to form the structural units of RNA and DNA. Nucleotides also play important roles in metabolism. In that capacity, they serve as sources of chemical energy (adenosine triphosphate and guanosine triphosphate), participate in cellular signaling (cyclic guanosine monophosphate and cyclic adenosine monophosphate), and are incorporated into important cofactors of enzymatic reactions (coenzyme A, flavin adenine dinucleotide, flavin mononucleotide, and nicotinamide adenine dinucleotide phosphate.

A nucleotide is composed of a nitrogenous base, a five-carbon sugar (either ribose or 2'-deoxyribose), and one to three phosphate groups. Together, the nitrogenous base and sugar comprise a nucleoside. The phosphate groups form bonds with either the 2, 3, or 5-carbon of the sugar, with the 5-carbon site most common. A nucleotide is one of the building blocks of ribonucleic acids (RNA) and deoxyribonucleic acid (DNA). Nucleotides are linked by enzymes in order to make long, chainlike polynucleotides of defined sequence. The order or sequence of the nucleotide units along a polynucleotide chain plays an important role in the storage and transfer of genetic information.

A nucleotide molecule contains three functional groups: a base, a sugar, and a phosphate. It may seem puzzling that a nucleic acid should contain a base. While the base portion does have weakly basic properties, the nucleotide as a whole acts as an acid, due to the phosphate group.

Glycolysis

Glycolysis is the metabolic pathway which converts glucose (C6H12O6) into pyruvate (CH3COCOO- + H+). Glycolysis is the anaerobic catabolism of glucose, occurring in virtually all cells. In eukaryotes, it occurs in the cytosol. The free energy, which is stored in 2 molecules of pyruvic acid, is somewhat less than that in the original glucose molecule. The free energy released in this process is used to form the high energy compounds, ATP (adenosine triphosphate) and NADH (reduced nicotinamide adenine dinucleotide).

Glycolysis is a definite sequence of ten reactions involving ten intermediate compounds, with one of the steps involving two intermediates. The most common type of glycolysis is the Embden-Meyerhof pathway, which was first discovered by Gustav Embden and Otto Meyerhof.

Glycolysis process explained by professor (video)

Metabolic Pathways

Metabolic pathways are series of chemical reactions which take place within a cell. In each metabolic pathway, a principal chemical is modified by chemical reactions. Enzymes catalyze these reactions, and often require dietary minerals, vitamins, and other cofactors in order to function properly. Because of the many chemicals that may be involved, metabolic pathways can be quite elaborate. In addition, many pathways can exist within a cell. This collection of pathways is called the metabolic network. Pathways are important to the maintenance of homeostasis within an organism.

There is a very large number of metabolic pathways. In humans, the most important metabolic pathways are:

1) glycolysis, which is glucose oxidation in order to obtain Adenosine triphosphate (ATP); 2) citric acid cycle (Krebs' cycle), which is acetyl-CoA oxidation in order to obtain GTP and valuable intermediates; 3) oxidative phosphorylation, which is the disposal of the electrons released by glycolysis and citric acid cycle; 4) pentose phosphate pathway, which is the synthesis of pentoses and release of the reducing power needed for anabolic reactions; 5) urea cycle, which is the disposal of NH4+ in less toxic forms; 6) fatty acid b-oxidation, which is the breakdown of fatty acids into acetyl-CoA, to be used by the Krebs' cycle; 7) gluconeogenesis, which is glucose synthesis from smaller percursors, to be used by the brain.

Oxidative Phosphorylation

Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate (ATP). Although the many forms of life on earth use a range of different nutrients, almost all carry out oxidative phosphorylation to produce ATP, which is the molecule that supplies energy to metabolism. This pathway is probably so pervasive because it is a highly efficient way of releasing energy, compared to alternative fermentation processes such as anaerobic glycolysis.

During oxidative phosphorylation, electrons are transferred from electron donors to electron acceptors such as oxygen, in redox reactions. These redox reactions release energy, which is used to form ATP. In eukaryotes, these redox reactions are carried out by a series of protein complexes within mitochondria, whereas, in prokaryotes, these proteins are located in the cells' inner membranes. These linked sets of enzymes are called electron transport chains. In eukaryotes, five main protein complexes are involved, whereas in prokaryotes many different enzymes are present, using a variety of electron donors and acceptors.

The energy released by electrons flowing through this electron transport chain is used to transport protons across the inner mitochondrial membrane, in a process called chemiosmosis. This generates potential energy in the form of a pH gradient and an electrical potential across this membrane. This store of energy is tapped by allowing protons to flow back across the membrane and down this gradient, through a large enzyme called ATP synthase. This enzyme uses this energy to generate ATP from adenosine diphosphate (ADP), in a phosphorylation reaction. This reaction is driven by the proton flow, which forces the rotation of a part of the enzyme; the ATP synthase is a rotary mechanical motor.

Although oxidative phosphorylation is a vital part of metabolism, it produces reactive oxygen species such as superoxide and hydrogen peroxide, which lead to propagation of free radicals, damaging cells and contributing to disease and, possibly, aging (senescence). The enzymes carrying out this metabolic pathway are also the target of many drugs and poisons that inhibit their activities.

Oxidative Phosphorylation Explained by a Professor (Video)