AP Biology · Unit 2: Cell Structure & Function

2.9 Mechanisms of Transport

Putting the mechanisms together

Membrane structure enables several transport mechanisms. Passive transport (diffusion, osmosis, facilitated diffusion) needs no energy. Active transport uses ATP. Cotransport uses the gradient of one substance (built by a pump) to drag another substance along — indirect active transport.

Gradients and bulk transport

An electrochemical gradient combines a concentration gradient and an electrical (charge) gradient; pumps like proton pumps establish these gradients to do work. Bulk transport moves large amounts: endocytosis (including phagocytosis, 'cell eating') brings material in, and exocytosis sends it out.

Key terms

Cotransport: Using the gradient of one substance (from a pump) to move another against its gradient.
Endocytosis: Bringing large materials into the cell by engulfing them in a vesicle.
Exocytosis: Releasing materials from the cell by fusing vesicles with the membrane.
Electrochemical gradient: A combined concentration and electrical-charge gradient across a membrane.
Proton pump: An active transporter that moves H⁺ to build a gradient used for cellular work.

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Experiment

Intestinal cells absorb glucose even when glucose is more concentrated inside the cell than in the gut. The transporter moves Na⁺ and glucose together. Blocking the Na⁺/K⁺ pump stops glucose uptake.

Which process best describes glucose uptake here?

Secondary active transport powered by the Na⁺ gradient
Simple diffusion through the bilayer
Primary active transport directly using ATP
Facilitated diffusion down the glucose gradient

Show the answer

A. The cotransporter uses energy stored in the Na⁺ gradient (made by the ATP-driven pump) to move glucose against its gradient.

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