Concept preview

What you will be able to do

1. 01Explain active transport and why it requires ATP to move substances against the gradient.
2. 02Describe the sodium–potassium pump and its importance.
3. 03Explain secondary active transport (co-transport).
4. 04Describe bulk transport: endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis.
5. 05Compare passive and active transport.

Passive transport can only ever do one thing: let substances slide downhill, evening out their concentrations. But cells constantly need to do the opposite — to gather scarce nutrients from a dilute environment, to expel wastes into a crowded one, and to keep the inside of the cell chemically very different from the outside. Moving a substance against its concentration gradient, from low concentration to high, is like rolling a ball uphill: it does not happen by itself and it demands energy. This uphill movement is called active transport, and the fuel that powers it is the cell's universal energy currency, ATP (adenosine triphosphate).