The rate of transport of a substance into a cell is drastically reduced when the formation of ATP is blocked. The transport system must be a form of:

A.	active transport
B.	simple diffusion
C.	facilitated diffusion
D.	both active transport and simple diffusion
E.	both simple and facilitated diffusion

Tutorial

Active transport systems use an energy source (usually ATP) to drive the movement of molecules against a concentration gradient. The best example of active transport is the Na⁺/K⁺ATPase. This membrane protein transporter moves Na⁺ out of the cell and K⁺ into the cell, building up high Na⁺ outside and high K⁺ inside the cells.

Nearly a third of the energy we use each day drives this transport system. The transmission of signals through our brain and the control of water flow into and out of cells requires the activity of the Na⁺/K⁺ATPase. This system is a good example of the coupling of ATP hydrolysis with an unfavorable reaction (ion movement against a concentration gradient).