AP Biology · Unit 2: Cell Structure & Function

2.6 Membrane Transport

Passive vs. active

Passive transport needs no energy — substances move down their concentration gradient (from high to low). It includes simple diffusion, facilitated diffusion, and osmosis. Active transport uses energy (ATP) to move substances against their gradient (low to high).

Pumps and bulk transport

The sodium-potassium pump is a classic active transporter, pushing Na⁺ out and K⁺ in against their gradients. For very large materials, cells use bulk transport: endocytosis brings material in by engulfing it, and exocytosis releases material by fusing vesicles with the membrane.

Key terms

Passive transport: Movement down the concentration gradient with no energy required.
Active transport: Movement against the gradient, powered by ATP.
Concentration gradient: A difference in concentration that drives passive movement from high to low.
Sodium-potassium pump: An active transporter that pumps Na⁺ out and K⁺ in against their gradients.
Endocytosis vs. exocytosis: Endocytosis brings materials in; exocytosis releases them out (bulk transport).

Try an AP-style question

Experiment

A cell is treated with a poison that blocks ATP synthesis. Afterward, the cell can no longer maintain a high internal K⁺ concentration, but O₂ still enters normally.

Which conclusion is best supported?

K⁺ enters by simple diffusion, while O₂ requires an ATP-driven pump
The poison damaged the phospholipid bilayer, making it leak all ions
Both K⁺ and O₂ normally enter the cell by ATP-powered transport
K⁺ accumulation requires active transport, while O₂ enters by simple diffusion

Show the answer

D. Only the ATP-dependent process failed; O₂ movement, driven by its gradient, was unaffected.

Practice 8 AP-style questions with instant explanations, flashcards and spaced review.

Study 2.6 free in APrep