AP Biology · Lesson 2

Membranes and transport

Know Know it

A membrane is a phospholipid bilayer. Each phospholipid has a polar, water-loving head and two nonpolar tails. The heads face the watery fluid inside and outside the cell, and the tails face each other in the middle. Proteins sit in or on the bilayer, and both lipids and many proteins drift sideways: the fluid mosaic model. Cholesterol in animal membranes helps keep fluidity steady as temperature changes, and unsaturated tails, with their kinks, keep a membrane fluid in the cold.

The nonpolar middle makes the membrane selectively permeable. Small nonpolar molecules such as oxygen and carbon dioxide cross it directly. Ions and large polar molecules such as glucose cross only through proteins. Water crosses slowly through the bilayer and much faster through aquaporin channels. A cell wall outside the membrane in plants, fungi and most bacteria gives support but does not do the selecting.

Passive transport moves a substance down its concentration gradient and uses no energy from the cell: simple diffusion through the bilayer, or facilitated diffusion through a channel or carrier. Active transport moves a substance against its gradient with a pump that uses energy, usually ATP. The sodium-potassium pump moves 3 sodium ions out and 2 potassium ions in for each ATP. In cotransport, an ion flowing down a gradient that a pump built carries a second substance against its own gradient. Large items move in vesicles: exocytosis out and endocytosis in, both using energy.

Trap A protein does not mean active

Facilitated diffusion uses a protein but no energy, and it moves a substance only down its gradient. Ask two questions: which way is the substance moving compared with its gradient, and does the process stop when ATP runs out?

Worked example Easy

Which substance can cross a phospholipid bilayer quickly on its own, with no protein to help it?

  1. Sodium ions
  2. Glucose
  3. Oxygen gasAnswer
  4. A protein hormone

How to solve it

  1. The middle of the bilayer is nonpolar.
  2. Charged, polar or large substances are held back by it.
  3. Only the small nonpolar molecule, oxygen, slips through on its own.

Why each choice is right or wrong

  • A. Incorrect. Sodium ions carry a charge, so the nonpolar middle of the bilayer blocks them.
  • B. Incorrect. Glucose is a large polar molecule, so it needs a carrier protein to cross at a useful rate.
  • C. Correct. Oxygen is small and nonpolar, so it dissolves through the nonpolar tails of the bilayer.
  • D. Incorrect. A protein hormone is far too large and polar to pass between the phospholipids.

Explain Explain it

Task verbs: explain, predict

To name a transport process, read two facts from the setup: the direction of movement compared with the concentration gradient, and whether the process needs ATP. Against the gradient with ATP is active transport. Down the gradient through a protein is facilitated diffusion. Then explain by tying those two facts to the process you named.

Worked example Medium

A fictional pond alga keeps potassium at 140 mM inside its cells while the pond water holds 5 mM. The cells keep taking in potassium. When a drug blocks ATP production, potassium uptake stops within minutes.

Which process brings potassium into these cells?

  1. Simple diffusion through the bilayer
  2. Facilitated diffusion through a potassium channel
  3. Active transport by a pump proteinAnswer
  4. Osmosis through aquaporins

How to solve it

  1. Direction: potassium moves from 5 mM outside to 140 mM inside, against its gradient.
  2. Energy: uptake stops when ATP is blocked, so it needs ATP.
  3. Against the gradient with ATP means active transport by a pump.

Why each choice is right or wrong

  • A. Incorrect. Simple diffusion through the bilayer only moves a substance down its gradient, and charged potassium ions cannot cross the bilayer anyway.
  • B. Incorrect. A channel lets potassium flow only from 140 mM inside toward 5 mM outside, which is the opposite direction.
  • C. Correct. Potassium moves from 5 mM to 140 mM, against its gradient, and uptake stops without ATP: active transport.
  • D. Incorrect. Osmosis is the movement of water, and aquaporins carry water rather than potassium ions.

Predict Your turn

An animal cell is treated with a fictional drug that stops it from making ATP. Predict what happens over time to the difference in sodium concentration between the inside and outside of the cell, and explain your prediction using the role of the sodium-potassium pump.

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Compare with a model answer

A full answer does this

  • Predicts that the sodium difference shrinks, with sodium inside the cell rising toward the outside level
  • Explains that the pump needs ATP to move sodium out against its gradient, so without ATP it stops while sodium keeps leaking in down its gradient

Model answer

The sodium difference across the membrane will shrink over time, with sodium inside the cell rising. The sodium-potassium pump uses ATP to push sodium out against its gradient. Without ATP the pump stops, but sodium keeps leaking into the cell down its gradient through channels, so nothing restores the difference.

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