AP BiologyUnit 2: Cells2.5
AP Biology · Unit 2 · Topic 2.5

Membrane Transport

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson3 min read

Downhill is free. Uphill costs energy. A rate that plateaus means the carriers are full.

Most transport comes down to one question: is the solute moving down its gradient, or up it? Downhill is passive and costs the cell nothing; uphill is active and costs energy.

Key terms

  • Concentration gradient — a difference in concentration across a membrane. Left alone, net movement runs from high to low.
  • Passive transport — net movement down a gradient with no energy from the cell: simple diffusion, facilitated diffusion, and osmosis.
  • Active transport — moving a solute up its gradient, from low to high. It needs a carrier protein and energy, often ATP.
  • Channel protein — a selective tunnel through the membrane. An aquaporin passes water but turns ions away.
  • Carrier protein — binds a solute and changes shape to move it across. The number of carriers caps the top speed.
  • Endocytosis and exocytosis — bulk transport in vesicles: in by pinching off membrane, out by fusing with it. Both use energy.

How to name the mechanism

  1. Find the gradient. Solute moving from high to low through the bilayer, a channel, or a carrier? That's passive. No ATP.
  2. When concentrations match, net movement stops, but molecules don't. They keep crossing both ways at equal rates: dynamic equilibrium.
  3. A carrier in facilitated diffusion is a door, not a motor. More carriers raise the top speed; the gradient still sets the direction.
  4. Carriers can saturate. Once all are busy, extra solute can't raise the rate, so it levels off. Diffusion through the bilayer keeps rising with concentration.
  5. Solute moving from low to high? That's active. The sodium-potassium pump spends one ATP to move 3 Na+ out and 2 K+ in, both uphill.
  6. Big cargo, like a whole bacterium or a batch of insulin, moves in vesicles. That costs energy whichever way the gradient runs.

Why it matters

Transport questions are often data questions. You get uptake rates, inside and outside concentrations, or an ATP blocker, and you name the mechanism. Three clues do most of the work. A solute piling up inside above the outside level means active transport. A rate that levels off as concentration rises means carriers. An effect that disappears without ATP means energy was needed.

Exam traps

TrapActually
At equilibrium, molecules stop crossing the membrane.They keep crossing both ways at equal rates. Only the net change stops.
If a protein is involved, it must be active transport.Facilitated diffusion uses proteins and is passive. Active means uphill, and uphill costs energy.
Uptake levels off at high concentration because the cell is full.Every carrier is busy. The limit sits in the membrane, not in the supply.
Steroid hormones need a channel or a vesicle to get into cells.Steroids are mostly nonpolar. They dissolve straight through the bilayer, with no protein and no ATP.
Worked example

A nerve cell holds about 15 mM Na+ inside and 145 mM outside. A drug that blocks only the sodium-potassium pump is added. Predict what happens to the Na+ concentration inside the cell over the next few hours, and explain.

  1. Find the gradient. Na+ is about 10 times higher outside, so downhill for Na+ is into the cell.
  2. The pump moves Na+ out, which is uphill. That's active transport, paid for with ATP.
  3. Na+ also leaks in slowly through channels, downhill, all the time. Normally the pump keeps up with the leak.
  4. Block the pump and only the leak is left. Na+ keeps coming in, and nothing pumps it back out.

Na+ inside rises steadily, and the gradient runs down.

A gradient is not stored; it is maintained. Active transport has to keep working against a constant passive leak. The near-miss predicts no change because the drug opened no channels. But the leak was always there, and the pump was canceling it out.

Now try it on real exam-style questions

Ten questions on Membrane Transport, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 2.5 Membrane Transport

What is the term for a cell taking in small droplets of extracellular fluid in tiny vesicles?

  1. A Phagocytosis
  2. B Pinocytosis
  3. C Exocytosis
  4. D Osmosis
Show the answer

B — Pinocytosis, or cell drinking, is a form of endocytosis in which the membrane folds inward around a droplet of fluid and its dissolved solutes. Phagocytosis is the tempting choice, but it is cell eating, in which the cell engulfs large particles such as bacteria.

Question 2 · 2.5 Membrane Transport

In which directions does the sodium-potassium pump move Na+ and K+ across the plasma membrane?

  1. A Na+ out of the cell and K+ into it
  2. B Na+ into the cell and K+ out of it
  3. C Both Na+ and K+ out of the cell
  4. D Both Na+ and K+ into the cell
Show the answer

A — The pump uses ATP to export Na+ and import K+, keeping Na+ concentrated outside the cell and K+ concentrated inside. Students who reverse the directions are usually remembering that Na+ leaks in through channels, which is the passive movement the pump works against.

Question 3 · 2.5 Membrane Transport

Which process do pancreatic cells use to release insulin, a large protein hormone, into the blood?

  1. A Facilitated diffusion
  2. B Endocytosis
  3. C Simple diffusion
  4. D Exocytosis
Show the answer

D — Large molecules such as proteins are too big for channels or carriers, so cells package them in vesicles that fuse with the plasma membrane and release their contents outside, which is exocytosis. Endocytosis is the tempting mix-up, but it is the reverse process, bringing material into the cell.