Sunday, 22 September 2019

Chemiosmotic Hypothesis

Chemiosmotic Hypothesis 

Chemiosmotic Hypothesis

Chemiosmotic Coupling of Electron Transport and ATP Synthesis

The chemiosmotic hypothesis of ATP formation was proposed by Mitchell (1961). Electron transport, both in respiration and photosynthesis, produces a proton gradient. The gradient develops in the outer chamber or intermembrane space of mitochondria and inside the thylakoid lumen in chloroplasts. Synthesis of ATP, i.e., energy-rich molecules through chemiosmosis requires a membrane, a proton pump, a proton gradient, and enzyme ATP synthase.

Mechanism of chemiosmotic synthesis of ATP in chloroplast

Chloroplast has two pigment systems, PSI located in the non-appressed part of grana thylakoids and stroma thylakoids; PSII presents in appressed part of grana thylakoids. Both the systems have electron carriers arranged in series and carry out non-cyclic photophosphorylation (light-driven synthesis of ATP) in collaboration.

synthesis of ATP in chloroplast

ATP synthase enzyme

Adenosine triphosphate synthase (ATP synthase) enzymes are dynamos that interconvert rotational and chemical energy. The enzyme is a large (about 600,000 Da in the monomer state) multisubunit complex with a water-soluble complex (F 1 ) and a membrane complex (F 0 ). F 1 consist of five different polypeptides, with a stoichiometry of?. The catalytic sites are located largely on the b polypeptide. F 0 contains four different polypeptides, with a stoichiometry of a, b, b?, c 10-15 (in chloroplast and mitochondria the general structure is similar except that there are two isoforms and 7-9 additional subunits in mitochondria). F 1 has a central rotor that at one end is within the core of F 1 and at the other end is connected with c ring of F 0. During ATP synthesis, the c ring rotates driven by the movement of protons, in turn, the rotor rotates within F 1 which causes the conformational changes in the catalytic sites that provides the energy for phosphorylation of ADP. The peripheral stalk acts as a stator connecting F 1 with F 0 and prevents futile rotation of F 1 as the rotor spins within it.

ATP synthase enzyme

Mechanism of chemiosmotic synthesis of ATP in the mitochondrion

The inner mitochondrial membrane contains groups of electron and proton transporting enzymes arranged in series called electron transport chain. Five complexes of the inner membrane are, 
(i) NADH-UQ reductase or NADH dehydrogenase complex, 
(ii) Succinate UQ-reductase complex, 
(iii) UGH 2 -cytochrome c reductase complex
(iv) Cytochrome c oxidase and 
(v) ATP synthase.

atp synthesis in mitochondria

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