Practice Test 11

A membrane carrier binds lactose on the outside and undergoes a conformational change to release lactose inside. Extracellular lactose is 8 mM and cytosolic lactose is 2 mM. Uptake occurs without ATP and is unaffected by collapsing ion gradients. Uptake saturates at high lactose concentration, indicating a limited number of carriers. Which mechanism best explains lactose uptake?

A cell is placed in a hypertonic solution with higher solute concentration than its cytosol. Over several minutes, the cell shrinks as water exits. The rate of water loss increases when aquaporins are inserted into the membrane and is unaffected by ATP depletion. Net water movement is from cytosol to outside, down the water concentration gradient. Which mechanism best explains the water movement?