Practice Test 9

A scientist tracks movement of lactose into cells with a lactose-specific permease. When lactose is 15 mM outside and 1 mM inside, lactose enters rapidly in the dark and in the presence of ATP synthase inhibitors. When outside lactose is lowered to 1 mM, net transport becomes zero. Which explanation best accounts for these observations?

A cell has a membrane carrier that binds amino acid Y. Extracellular Y is 30 mM and intracellular Y is 3 mM. When a nonhydrolyzable ATP analog is added, Y uptake continues unchanged; when a mutation reduces Y binding affinity, uptake rate decreases at the same gradient. Which explanation best accounts for these results?