Primary active transport moves a solute against its electrochemical or concentration gradient by coupling transport directly to an energy-releasing chemical reaction, commonly ATP hydrolysis.

The transport protein is often called a pump. A pump binds transported solutes and ATP in particular conformational states, uses ATP-dependent chemistry to change its conformation and then releases the solutes on the opposite side.

Because uphill transport has

$$\Delta G_{\rm transport}>0,$$

it cannot proceed sustainably on its own. Coupling to a sufficiently favorable ATP-dependent reaction can make the total cycle favorable:

$$\Delta G_{\rm cycle}=\Delta G_{\rm transport}+\Delta G_{\rm ATP}<0.$$

Example: sodium-potassium pump

The animal-cell Na$^+$/K$^+$ ATPase uses ATP to transport ions against their gradients. In one idealized cycle it moves

$$3,\mathrm{Na^+}\ \text{out}$$

and

$$2,\mathrm{K^+}\ \text{in}$$

per ATP hydrolyzed.

A simplified sequence is:

1. the inward-facing pump binds three intracellular $\mathrm{Na^+}$ ions;
2. ATP transfers a phosphoryl group to the pump; this phosphorylation changes the protein's favored conformation;
3. $\mathrm{Na^+}$ is released outside;
4. two extracellular $\mathrm{K^+}$ ions bind;
5. removal of the phosphoryl group, dephosphorylation, favors return toward the inward-facing state;
6. $\mathrm{K^+}$ is released inside.

Because three positive charges leave for every two entering, this pump is electrogenic: each cycle directly contributes a small net movement of positive charge outward.

Pumps maintain gradients rather than merely creating motion

Ions also leak passively through channels and other pathways. Cells therefore use pumps continuously to oppose this dissipation and maintain nonequilibrium concentration differences.

The stored gradients can then drive other processes. Primary active transport is thus a conversion process: favorable chemical free energy is used to build electrochemical free energy across a membrane.