In bacteria, the transport of many nutrients, including sugars and amino acids, is driven by the electrochemical H+ gradient across the plasma membrane. In E. coli, for example, an H+–lactose symporter mediates the active transport of the sugar lactose into the cell. Given what you know about coupled transport, which is likely true of the H+–lactose symporter?
A. If the H+ gradient were reversed, the transporter could serve as an H+–lactose antiport.
B. The transporter oscillates randomly between states in which it is open to either the extracellular space or the cytosol.
C. To undergo the conformational change that releases lactose into the cell, the transporter hydrolyzes ATP.
D. Lactose and H+ ions bind to two different conformations of the transporter.
E. The transporter goes through an intermediate state in which the lactose-binding site is open to both sides of the membrane.