Membrane Transport Practice Test

A cell is placed in a solution where the extracellular glucose concentration is 10 mM and intracellular glucose is 2 mM. The plasma membrane is impermeable to glucose unless a specific carrier protein is present. When the carrier is expressed, glucose enters the cell rapidly; adding an ATP synthesis inhibitor does not change the rate. No vesicles form, and the carrier shows saturation at high extracellular glucose. Which mechanism best explains glucose entry under these conditions?

A bacterial cell imports lactose using a membrane symporter that requires a preexisting H+ gradient: extracellular pH is 6.0 and cytosolic pH is 7.5. When a chemical collapses the H+ gradient without affecting ATP levels, lactose uptake stops. Lactose is more concentrated inside than outside during uptake. Which mechanism best explains lactose entry under normal conditions?