Representation of active transport in cell membranes:

Enzyme E₁ near the surface of low concentration of A promotes the chemical reaction among A and B to yield a compound AB inside the cell membrane. Compound AB will diffuse by the membrane to the other side whereas there is another enzyme E₂.This enzyme does just the opposite of what E₁ does and expedites the decomposition of AB  within A and B.

Figure:  A schematic representation of active transport in cell membranes

The element A diffuses out of the membrane to the high concentration side since there is a local build up of A inside the membrane. A carrier molecule B is too large to pass by the cell membrane and is holed inside the membrane. The two enzymatic reactions constitute a metabolic pump that produced power to pump A against its concentration gradient. The accumulation of potassium ions in cells is a well known instance of active transport.