AAMC FL Practice Exam

Primary active transport is characterized by its requirement for energy input to move ions or molecules against their concentration gradient. This process directly utilizes energy, typically in the form of adenosine triphosphate (ATP), to transport substances across a cell membrane.

During primary active transport, ATP is hydrolyzed, which provides the necessary energy to change the conformation of transport proteins (often referred to as pumps), allowing them to transport ions or molecules from areas of lower concentration to areas of higher concentration.

This contrasts with passive transport mechanisms, which do not require energy and instead rely on the natural movement of substances down their concentration gradient. The other options reflect misunderstandings of the transport process: energy expenditure is indeed necessary, passive diffusion occurs without any energy input, and cellular mechanisms are central to the function of all membrane transport processes. Therefore, the defining characteristic of primary active transport is that it utilizes ATP as an energy source to drive the transport of substances against their concentration gradients.