Photosystems & Chemiosmosis- The …
To make a long story short, the primary function of ATP synthase in most organisms is ATP synthesis. Hence the name. However, in some cases the reverse reaction, i.e. transmembrane proton pumpingpowered by ATP hydrolysis is more important. A typical example: anaerobic bacteria produce ATP byfermentation, and ATP synthase uses ATP to generate protonmotive force necessary for ion transportand flagella motility.
Many bacteria can live both from fermentation and respiration or photosynthesis. In such case ATP synthasefunctions in both ways.
An important issue is to control ATP-driven proton pumping activity of ATP synthase in order to avoid wasteful ATP hydrolysis under conditions when no protonmotive force can be generated (e.g. leakydamaged membrane, uncoupler present, etc.). In such case ATP hydrolysis becomes a problem,because it can quickly exchaust the intecellular ATP pool. To avoid this situation,all ATP synthases are equipped with regulatory mechanisms that suppress the ATPaseactivity if no protonmotive force is present. The degree of ATP hydrolysis inhibitiondepend on the organism. In plants (in chloroplasts), where it is necessary to preserveATP pool through the whole night, the inhibition is very strong: the enzyme hardly has anyATPase activity. In contrast, in anaerobic bacteria where ATP synhase is the maingenerator of protonmotive force, such inhibition is very weak. Mitochondrial ATP synthase is somewhereinbetween.
How does chemiosmosis occur in photosynthesis? - …
Evidence in Support of the Chemiosmotic Coupling HypothesisAs Mitchell predicted (1-3), the mitochondrial, bacterial, and chloroplast membranes that couple ATP synthesis to electron transport are poorly permeable to protons, except when proton-linked processes, such as ATP formation, occur at high rates.
Mechanism of ATP synthesis is by substrate-level and oxidative phosphorylation/chemiosmosis; but in fermentation, substrate-level phosphorylation only during glycolysis.