Chemiosmosis Theory - Edubirdie

University:
University of California, San Diego
Course:
BIMM 100 | Molecular Biology
Academic year:

2024
Views:

142

Pages:

1
Author:

customer-8930767

Explain the chemiosmotic theory of ATP synthesis in mitochondria and chloroplasts. How does proton motive force drive phosphorylation? Chemiosmotic theory, proposed by Peter Mitchell, explains ATP synthesis as a consequence of an electrochemical proton gradient across a membrane. In mitochondria, electron transport chain (ETC) complexes I, III, and IV pump protons (H?) from the mitochondrial matrix into the intermembrane space using energy from NADH and FADH? oxidation. This creates a high H? concentration in the intermembrane space relative to the matrix, generating both a chemical (pH) and electrical (membrane potential) gradient—collectively the proton motive force (PMF). In chloroplasts, lightdriven electron flow through photosystems II and I pumps H? from the stroma into the thylakoid lumen. ATP synthase (complex V) is a rotary molecular motor. The PMF drives H? flow back through the synthase’s F? subunit (the rotor), causing conformational changes in the F? subunit. Specifically, three catalytic ?-subunits undergo sequential changes: loose (ADP + Pi binding), tight (ATP formation), and open (ATP release). Each full rotation produces three ATP molecules. The PMF magnitude (about 200 mV in mitochondria) determines the rate of ATP synthesis. Uncouplers like 2,4-dinitrophenol dissipate the gradient without ATP production, generating heat—a mechanism used in brown adipose tissue thermogenesis. Thus, chemiosmosis couples electron transport to phosphorylation, contrasting with substrate-level phosphorylation.

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