In primary active transport, the energy required to move molecules across the membrane comes directly from the hydrolysis of ATP (breaking a phosphate bond to release energy). The proteins that facilitate this are often called .

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This is where things get clever. Instead of using ATP directly, the cell uses the pre-existing gradient (usually Na⁺) created by primary transport to drag another molecule along for the ride. The review clearly distinguishes:

To understand active transport, we must first distinguish it from its counterpart, . Passive transport is the movement of molecules down their concentration gradient (from high concentration to low concentration), which requires no energy.

Active transport is the movement of molecules across a cell membrane from a region of lower concentration to a region of higher concentration, requiring the expenditure of energy. This process is essential for various cellular functions, such as maintaining proper ion balance, regulating pH, and generating nerve impulses.

Compare the across different active transport types.