In active transport, does the concentration gradient drive the transport rate?

In active transport, does the concentration gradient drive the transport rate?

• A. True
• B. Depends on the transporter
• C. Not sure
• D. False

Answer: (D)

Active transport uses energy to move substances against their concentration gradient, so the gradient of the solute itself does not drive the transport rate. Instead, the rate is set by how fast the transporter works (its turnover, reflected in Vmax) and how much energy is available (ATP supply for primary active transport, or an ion gradient for secondary active transport). If the transporter is saturated, increasing the substrate concentration won’t speed up the transport, because the pump can only cycle so fast. In primary active transport, energy from ATP directly powers the pump; in secondary active transport, the driving ion gradient provides the energy. So, the statement is not generally true—the gradient does not drive the rate in active transport as a whole.

Active transport uses energy to move substances against their concentration gradient, so the gradient of the solute itself does not drive the transport rate. Instead, the rate is set by how fast the transporter works (its turnover, reflected in Vmax) and how much energy is available (ATP supply for primary active transport, or an ion gradient for secondary active transport). If the transporter is saturated, increasing the substrate concentration won’t speed up the transport, because the pump can only cycle so fast. In primary active transport, energy from ATP directly powers the pump; in secondary active transport, the driving ion gradient provides the energy. So, the statement is not generally true—the gradient does not drive the rate in active transport as a whole.