AAMC FL Practice Exam 2025 - Free Practice Questions and Study Resources.

Primary active transport is a process that directly uses energy to move substances against their concentration gradient across a cell membrane. This energy is most commonly derived from the hydrolysis of ATP, which is the primary molecule that stores and transfers energy within cells. During this transport mechanism, ATP is broken down to release energy, which is then harnessed to transport ions or molecules from an area of lower concentration to an area of higher concentration. This is vital for maintaining cellular functions, such as nutrient uptake and ion homeostasis.

For example, the sodium-potassium pump is a classic illustration of primary active transport. It moves sodium ions out of the cell while bringing potassium ions in, using ATP to power the process. This transport mechanism differs fundamentally from others, as it requires direct energy input rather than relying on natural diffusion processes or existing gradients.

Thus, the correct answer appropriately identifies primary active transport as that which utilizes the hydrolysis of ATP for the transport of substances against their concentration gradients.