How does active transport work in cells?
Active transport uses energy to move substances against their electrochemical gradients. Cells use it to maintain ion distributions, acquire nutrients, and regulate their internal environment. Not all protein-mediated transport is active; some carriers work passively.
Primary and secondary transport
Primary active transport draws directly on an energy source such as ATP hydrolysis. The sodium-potassium pump maintains ion distributions in animal cells using ATP. Secondary active transport couples movement to an existing ion gradient. Cotransport in the same direction is symport, while opposite directions constitute antiport. Carriers have limited capacity, so transport rate does not increase without limit.
Why gradients matter
Ion gradients contribute to nerve signaling, cell volume control, and nutrient uptake. Concentration alone does not determine whether ion movement is energetically uphill; electrical potential must also be included. Gradients maintained by pumps can dissipate if energy supply fails. Active transport through membrane proteins is distinct from endocytosis, which internalizes larger material in vesicles.