7.4 Oxidative phosphorylation - Biology 2e | OpenStax (2023)

science goals

At the end of this section, you can do the following:

  • Describe how electrons move in the electron transport chain and explain what happens to their energy levels during this process.
  • Explain how the proton (H+) the gradient is set and maintained by the electron transport chain

You just read about the two pathways of glucose catabolism (glycolysis and the citric acid cycle) that generate ATP. However, most of the ATP produced during the aerobic catabolism of glucose is not generated directly through these pathways. Instead, it is derived from a process that begins with the movement of electrons through a series of electron carriers that undergo redox reactions. This process causes the accumulation of hydrogen ions in the intermembrane space. This creates a concentration gradient in which hydrogen ions diffuse from the intermembrane space into the mitochondrial matrix via ATP synthase. The current of hydrogen ions drives the catalytic action of ATP synthase, which phosphorylates ADP to form ATP.

Electron transport chain.

Electron transport chain (Figure 7.12) is the last part of aerobic respiration and the only part of glucose metabolism that uses oxygen from the air. Oxygen constantly penetrates plant tissues (usually through stomata), but also fungi and bacteria; However, in animals, oxygen enters the body through various respiratory systems. Electron transport is a series of redox reactions resembling a relay or cube brigade in which electrons rapidly pass from one component to another until the end of the chain, where the electrons reduce molecular oxygen and, together with the accompanying protons, produce water. . . There are four complexes composed of proteins labeled I through IV.Figure 7.12, and the aggregation of these four complexes along with the associated mobile additional electron carriers is calledelectron transport chain. The electron transport chain exists in multiple copies in the inner mitochondrial membrane of eukaryotes and the cell membrane of prokaryotes.

7.4 Oxidative phosphorylation - Biology 2e | OpenStax (1)

Cipher7.12 The electron transport chain is a series of electron carriers embedded in the inner mitochondrial membrane that transports electrons from NADH and FADH back and forth.2to molecular oxygen. During this process, protons are pumped from the mitochondrial matrix into the intermembrane space and oxygen is reduced to form water.

complex I

First, two electrons are transported to the first complex via NADH. This complex, markedI, consists of flavin mononucleotide (FMN) and iron-sulphur-containing protein (Fe-S). FMN, which is a derivative of vitamin B.2(also called riboflavin) is one of many prosthetic groups or cofactors in the electron transport chain. Downprosthetic groupIt is a non-protein molecule necessary for the function of the protein. Prosthetic groups are non-peptide organic or inorganic molecules associated with a protein that facilitate its function. Prosthetic groups include coenzymes, prosthetic groups of enzymes. The complex I enzyme is NADH dehydrogenase and consists of 44 individual polypeptide chains. Complex I can pump four hydrogen ions from the matrix across the membrane into the intermembrane space, thus establishing and maintaining a hydrogen ion gradient between the two compartments separated by the inner mitochondrial membrane.

Q in Complex II

Complex II receives FADH directly2–which bypasses complex I. The relationship connecting the first and second complex with the third isubiquinonaB. The Q molecule is lipid soluble and moves freely through the hydrophobic core of the membrane. After shrinking (QH2), the ubiquinone donates its electrons to the next complex in the electron transport chain. Q receives electrons from complex I's NADH and electrons from FADH.2complex II. This enzyme and FADH2They form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex. Since these electrons bypass and therefore do not power the proton pump in the first complex, fewer ATP molecules are made from FADH.2electrons.The number of ATP molecules finally obtained is directly proportional to the number of protons pumped through the inner mitochondrial membrane.

Complejo III

The third complex consists of cytochrome b, another Fe-S protein, the Rieske center (2Fe-2S center) and cytochrome c proteins. This complex is also called cytochrome oxidoreductase. Cytochrome proteins have a heme prosthetic group. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. As a result, the iron ion in the nucleus is reduced and oxidized as it passes through the electrons, fluctuating between different oxidation states: Fe++(lowered) and Fe+++(rusty). Heme molecules in cytochromes have slightly different characteristics due to the action of the different proteins that bind to them, giving each complex slightly different characteristics. Complex III pumps protons across the membrane and donates electrons to cytochrome c for transport to the fourth protein-enzyme complex. (Cytochrome c receives electrons from Q; however, while Q transfers pairs of electrons, cytochrome c can only accept one at a time.)

complex IV

The fourth complex consists of cytochrome c a and proteins.3. This complex contains two heme groups (one on each of the two cytochromes, a and a3) and three copper ions (some CuAthanks, CuBfor cytochrome A3). Cytochromes keep the oxygen molecule tightly packed between iron and copper ions until the oxygen is completely reduced by gaining two electrons. The reduced oxygen then takes up two hydrogen ions from the surrounding medium to form water (H2OR). The removal of hydrogen ions from the system increases the ion gradient, which is the basis of the chemiosmosis process.


In chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen ions (protons) across the mitochondrial membrane. Uneven distribution of H+Ions passing through the membrane create electrical and concentration gradients (hence the electrochemical gradient) as a result of the positive charge of the hydrogen ions and their aggregation on one side of the membrane.

If the membrane were constantly open for easy diffusion of hydrogen ions, the ions would tend to diffuse back into the matrix, under concentrations that produce their electrochemical gradient. Note that many ions cannot diffuse across non-polar regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through an integral membrane protein called ATP synthase.Figure 7.13). This complex protein acts as a small generator powered by the hydrogen ions that diffuse through it along an electrochemical gradient. The rotating parts of this molecular machine facilitate the addition of phosphate to ADP, creating ATP.using the potential energy of the hydrogen ion gradient.

visual connection

7.4 Oxidative phosphorylation - Biology 2e | OpenStax (2)

Cipher7.13 ATP synthase is a complex molecular machine that uses a proton (H+) with the formation of ATP from ADP and inorganic phosphate (Pi). (Source: adaptation of Klaus Hoffmeier's work)

Dinitrophenol (DNP) is an "uncoupling" agent that causes protons to "leak" from the inner mitochondrial membrane. It was used as a slimming drug until 1938. What effect of DNP can be expected on changing the pH of the inner mitochondrial membrane? Why do you think it could be an effective slimming drug?

Chemiosmosis (Figure 7.14) is used to generate 90 percent of the ATP produced during the aerobic catabolism of glucose; It is also the method used in the light reactions of photosynthesis to harness the energy of sunlight during the photophosphorylation process. Recall that the production of ATP by the process of chemiosmosis in the mitochondria is called oxidative phosphorylation. The overall result of these reactions is the production of ATP from the energy of electrons extracted from hydrogen atoms. These atoms were originally part of the glucose molecule. At the end of the path, electrons are used to reduce the oxygen molecule to oxygen ions. The extra electrons in the oxygen attract hydrogen ions (protons) from the surrounding medium and water is formed. Thus, oxygen is the last electron acceptor in the electron transport chain.

visual connection

7.4 Oxidative phosphorylation - Biology 2e | OpenStax (3)

Cipher7.14 In oxidative phosphorylation, ATP synthase uses the pH gradient created by the electron transport chain to form ATP. Source: Rao, A., Ryan, K., Fletcher, S. and Tag, A. Department of Biology, Texas A&M University.

Cyanide inhibits cytochrome c oxidase, part of the electron transport chain. Can an increase or decrease in intermembrane space pH be expected in the event of cyanide poisoning? What effect would cyanide have on ATP synthesis?

ATP efficiency

The number of ATP molecules produced by glucose catabolism varies. For example, the amount of hydrogen ions that electron transport chain complexes can pump across the membrane varies from species to species. Another source of variation is the coiling of electrons on mitochondrial membranes. (NADH produced by glycolysis cannot easily enter the mitochondria.) Therefore, NAD captures electrons from inside the mitochondria.+Moda+. As you have already found out, these are FADs.+molecules can carry fewer ions; consequently, fewer ATP molecules are produced in the FAD+acts as a carrier. ABOVE+It is used as an electron carrier in the liver and FAD.+works on the brain.

Another factor affecting the production of ATP molecules made from glucose is that the intermediates in these pathways are also used for other purposes. Glucose catabolism is linked to the pathways that build or break down all other biochemicals in cells, and the result is a bit more complicated than the ideal situations described so far. For example, sugars other than glucose enter the glycolytic pathway to extract energy. In addition, the five-carbon sugars that make up nucleic acids are formed from the intermediates of glycolysis. Certain non-essential amino acids can be produced from intermediates of both glycolysis and the citric acid cycle. Lipids such as cholesterol and triglycerides are also produced from intermediates in these pathways, and both amino acids and triglycerides are broken down for energy via these pathways. Overall, in living organisms, these glucose catabolism pathways take up about 34 percent of the energy contained in glucose, with the remainder released as heat.


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