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Explain chemiosmotic hypothesis for ATP synthesis.

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Chemiosmotic hypothesis (Oxidative Phosphorylation) : It is the synthesis of energy-rich ATP from ADP and inorganic phosphate, that is connected to oxidation of reduced coenzymes produced in cellular respiration. ATP synthesis is explained by chemiosmotic (coupling) theory given by British biochemist Peter Mitchell in 1961 (Nobel Prize in 1978). It is applicable to ATP synthesis both in respiration and photosynthesis. According to chemiosmotic theory, the energy liberated during electron transport performs the osmotic work of accumulating H+ ions, conserving energy in building a proton gradient or proton motive force (PMF) which is used to build ATP from ADP and inorganic phosphate.

(i) Development of Proton Gradient or Proton Motive Force : Oxidation of reduced coenzymes releases both electrons and protons. Electrons pass over an electron transport chain releasing energy at every step. The energy is employed in pushing not only the protons of their coenzymes but also other protons from mitochondrial matrix to the intermembrane space or outer mitochondrial chamber. Oxidation of one NADH (+H+ ) is linked to pushing of three pairs of protons into outer chamber while oxidation of FADH2 is linked to sending out two pairs of protons. As a result, proton concentration increases in the outer chamber (Fig.). The difference in the proton concentration on the two sides of a system is called proton gradient. pH of the outer chamber decreases as compared to mitochondrial matrix.

An electrochemical potential difference occurs across the inner mitochondrial membrane that tends to push protons back towards the matrix. It is called proton motive force (PMF).

(ii) Proton Driven ATP Synthesis : Proton gradient or proton motive force tends to push the protons from outer mitochondrial chamber towards the mitochondrial matrix through the inner membrane. However, inner membrane is permeable to protons only in the region of Fo or bases of elementary particles (FQ – F i particles). They function as proton tunnels or channels while F i head piece has enzyme ATP-ase that catalyses ATP synthesis. Two protons are required for synthesis of one molecule of ATP. They knock out one oxygen of inorganic phosphate and convert the latter into active phosphate (energy rich phosphate).

The latter immediately combines with ADP to form ATP or ADP-P. Since NADH (+H+ ) drives out 3 pairs of protons to outer chamber, its oxidation is linked to synthesis of 3 ATP molecules. Similarly, oxidation of FADH2 that causes pumping of two pairs of protons forms 2 ATP molecules. ATP molecules synthesised inside mitochondria come out of the latter through facilitated diffusion across inner membrane and diffusion across outer membrane.

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