Practice Test 12

A cell-free system contains actin filaments and myosin heads with bound ADP. When ATP is added, myosin detaches from actin; when ATP is then hydrolyzed to ADP and Pi, myosin binds actin and can perform a power stroke as Pi is released. If ATP is replaced with a nonhydrolyzable ATP analog, myosin detaches but repeated power strokes do not occur. The system contains no membranes, ion gradients, or electron carriers. This scenario examines how ATP hydrolysis energy is coupled to mechanical work at the molecular level.

A membrane vesicle contains a proton pump that uses energy from an electron donor to move H+ into the vesicle, making the inside acidic. When ADP and Pi are added, ATP is produced only if ATP synthase is present in the membrane with its catalytic side facing the exterior solution. If ATP synthase is inserted in the opposite orientation, little ATP accumulates outside even though the proton gradient still forms. The vesicle membrane is otherwise impermeable to nucleotides. This highlights how proton flow direction through ATP synthase determines where ATP is generated.