Chemiosmotic hypothesis

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The chemisosmotic hypothesis first postulated by Peter Mitchell in 1961 describes the process of ATP generation in the final stage of cellular respiration, namely oxidative phosphorylation in the mitochondria. This process was also discovered to take place in the thylakoids of chloroplasts as a means of generating ATP and reduced NADP in the light dependent reaction, key products needed for the light independent reaction to generate hexose sugars in plants. Chemiosmosis in both plants and animals are essentially the same however there are a few subtleties in the creation of the proton gradient related to the electron transport chain. [1]

The chemiosmotic hypothesis suggests that the action of ATP synthase is coupled with that of a proton gradient. It is the action of the proton gradient that causes a proton motive force that allows ATP synthase to phosphorylate ADP and inorganic phosphate to ATP. In mitochondria, the key site of ATP production in oxidative phosphorylation is the inner mitochondrial membrane. Pumping of hydrogen ions to generate a gradient is facilitated by transmembranal proteins called electron carriers. These electron carriers are sites of redox reactions for electrons and with each reaction across the electron carriers the electrons energy is transferred into the pumping of hydrogen ions across the membrane, this results in a high concentration in the intermembranal space than that of the matrix. The protons go through the ATPsynthase from an area of high concentration in the intermembranal space to an area of lower concentration, the mitochondrial matrix, through facilitated diffusion generating ATP. The proton-motive force is the mathematical sum of the chemical gradient, expressed as the difference in pH between the matrix and intermembranal space, and the charge gradient created by the disequilibrium (via proton pumping) of proton distribution either side of the inner membrane.[2]

Plants during the light dependent reaction gain energy from sunlight in photosystems as a means of getting electrons from a lower energy level to a higher energy level. The source of this electron is from the photolysis of water, which also generates protons needed for chemiosmosis. The source in mitochondria is derived from co-enzymes: FAD and NAD. During the previous steps of respiration, especially the Krebs cycle, the reduction of co-enzymes plays a pivotal role as a supply of electrons and protons for the electron transport chain and producing the proton-motive force. In both plants and animals, oxygen is used as the most common final electron acceptor so that the electron transport chain can continue so that chemiosmosis and production of ATP can continue.[3] Inhibitors such as cyanide can block the aforementioned process resulting in no ATP production and subsequently death.

References

  1. Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Chapter 14, Energy Conversion: Mitochondria and Chloroplasts.
  2. Berg JM, Tymoczko JL ,Stryer L. Biochemistry: International Edition Hardcover. WH Freeman Palgrave Macmillan; 2011. Chapter 18 p562
  3. Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 16.2, Electron Transport and Oxidative Phosphorylation.
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