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Biology · Glossary

What is Channels, carriers, pumps?

Also known as: channel · aquaporin · carrier · facilitated diffusion · pump · active transport · sodium--potassium pump

Definition 7.9 University Biology — Year 1 · Chapter 7 — Membranes and Membrane Transport

Channels are integral proteins with a water-filled pore through which a specific ion or small molecule diffuses down its gradient at up to 10810^8 per second; many are gated, opening in response to a voltage, a ligand or a mechanical force. Aquaporins are water channels. Carriers bind their solute on one face, change conformation, and release it on the other, a thousand times a second at most; the glucose transporter of the red cell is one. Channels and carriers working down a gradient perform facilitated diffusion, passive but selective and saturable. Pumps are carriers that couple the transport of a solute against its gradient to the hydrolysis of ATP: primary active transport. The sodium–potassium pump of animal cells exports three Na+\mathrm{Na^+} and imports two K+\mathrm{K^+} per ATP; the proton pump of plant, fungal and bacterial membranes exports H+\mathrm{H^+}.

Five ways across. Simple diffusion through the lipid; a channel and a carrier (facilitated diffusion, down the gradient); the sodium–potassium pump (primary active transport, paid in ATP); a symporter that uses the sodium gradient the pump built to drag glucose uphill (secondary active transport).
Five ways across. Simple diffusion through the lipid; a channel and a carrier (facilitated diffusion, down the gradient); the sodium–potassium pump (primary active transport, paid in ATP); a symporter that uses the sodium gradient the pump built to drag glucose uphill (secondary active transport).

Examples

Example 7.11 (Kinetics tell the mechanism)

The flux of glucose into a red cell rises with the outside concentration, then levels off at a maximum, like an enzyme (Chapter 13): a carrier, saturable, with a KmK_m of about 1.5 mmol/L1.5\,\mathrm{mmol}/\mathrm{L}; a similar sugar, mannose, competes for it. The flux of urea rises in proportion to its concentration without limit: simple diffusion. The flux of glucose into an intestinal cell stops when sodium is removed from the lumen, or when the pump is poisoned with ouabain: secondary active transport.

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