AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/2/2-5)
Unit 2 · Topic 2.5
2.5 Membrane Transport
Because membranes are selectively permeable, cells can keep different concentrations of substances on each side, called concentration gradients. Passive transport lets substances move down their gradient without the cell spending energy, while active transport uses energy and can push substances up their gradient. For very large molecules or bulk material, cells use vesicles: endocytosis brings things in and exocytosis sends them out.
Key terms
- concentration gradient
- passive transport
- active transport
- endocytosis
- exocytosis
Concentration gradients
A concentration gradient is a difference in the concentration of a substance from one place to another, such as inside vs. outside the cell. Because the membrane controls what crosses, cells can build and keep these gradients. A gradient is a form of stored energy, a bit like water held behind a dam.
Molecules are always moving randomly. When there's a gradient, more molecules happen to move from the crowded side to the less crowded side than the other way. So the net movement is down the gradient, from high concentration to low. When concentrations become equal, molecules keep moving both ways, but there's no net change. That's called equilibrium.
Passive vs. active transport
Passive transport includes simple diffusion straight through the bilayer (for small nonpolar molecules, 2.4), facilitated diffusion through proteins (2.6) and osmosis, the diffusion of water (2.7). It runs on the energy stored in the gradient itself.
Active transport needs a direct input of energy and a membrane protein (2.8). It's how cells build and maintain gradients, for example keeping K⁺ much higher inside a cell than outside.
| Feature | Passive transport | Active transport |
|---|---|---|
| Direction | Down the gradient (high → low) | Can go up the gradient (low → high) |
| Energy from the cell? | No direct input of metabolic energy | Yes, direct input, usually from ATP |
| Proteins needed? | Not for simple diffusion; yes for facilitated diffusion | Yes, always (pumps and other transport proteins) |
| Examples | O₂ diffusing into a cell; osmosis; glucose through a carrier protein | Sodium-potassium pump; proton pumps |
Moving big things: endocytosis and exocytosis
Some things are too large for any channel or carrier: whole proteins, polysaccharides, bacteria, or a large amount of fluid at once. Cells move these in bulk using vesicles, and both directions require energy.
- Endocytosis (into the cell): the plasma membrane folds inward around material outside the cell and pinches off as a new vesicle inside the cell. When a white blood cell engulfs a bacterium, that's a type of endocytosis called phagocytosis ('cell eating'). The vesicle may then fuse with a lysosome so the contents can be digested (2.1).
- Exocytosis (out of the cell): a vesicle inside the cell moves to the plasma membrane and fuses with it, releasing its contents outside. Cells secrete hormones such as insulin, digestive enzymes and neurotransmitters this way.
- Both processes change the membrane: endocytosis removes a patch of it, and exocytosis adds the vesicle's membrane to the plasma membrane.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Is it active or passive? Use an energy poison
Cells take up substance X even when the concentration of X is already higher inside the cell than outside. When researchers add cyanide, which stops the cell from making most of its ATP, uptake of X stops. Uptake of substance Y, which only enters when its concentration is higher outside, continues normally. Classify how each substance enters, and justify.
Show the solutionHide the solution
- Step 1: X moves from low concentration (outside) to high concentration (inside), which is against its gradient. That can't happen passively.
- Step 2: Uptake of X stops when ATP production stops, so it depends on metabolic energy. X enters by active transport.
- Step 3: Y only moves from high to low, down its gradient.
- Step 4: Cyanide doesn't affect Y's uptake, so Y doesn't need the cell's energy. Y enters by passive transport (diffusion or facilitated diffusion).
Answer: X enters by active transport (it moves against its gradient and needs ATP); Y enters by passive transport (it moves down its gradient and doesn't need ATP).
- Example 2
Is it diffusion? Check the direction (classic trap)
A student says: 'Molecules diffuse from low concentration to high concentration until they're spread out evenly.' Find the error and fix it. Then describe what happens once concentrations are equal.
Show the solutionHide the solution
- Step 1: The direction is backward: net diffusion goes from high concentration to low concentration, down the gradient.
- Step 2: Rewrite: molecules diffuse from where they're more concentrated to where they're less concentrated until the concentrations are equal.
- Step 3: The second trap: at equilibrium, movement doesn't stop. Molecules still cross in both directions.
- Step 4: What stops is net movement, because equal numbers cross each way.
Answer: Net diffusion is from high to low concentration. At equilibrium, molecules keep moving both ways, but there is no net change in concentration.
Common mistakes
- Saying passive transport uses no proteins. Facilitated diffusion is passive and depends on channel or carrier proteins.
- Saying all active transport moves substances from low to high. It often does, but what defines it is the direct input of energy.
- Saying molecules stop moving at equilibrium. They keep moving; only the net movement stops.
- Forgetting that endocytosis and exocytosis need energy.
On the exam
- Expect experiments with an ATP poison, a temperature change or a missing protein, where you classify a transport type from the results. Look at direction relative to the gradient and whether energy is needed.
- Use precise language: 'down (or against) its concentration gradient' and 'requires (or doesn't require) a direct input of energy.'
Connected topics
Videos
Check yourself
4 questions on 2.5 Membrane Transport. Pick an answer to see if you got it, and why.
Nerve cells store neurotransmitter molecules in vesicles and release them in large amounts at once. Which of the following processes do they use to release the neurotransmitter?
A white blood cell surrounds a bacterium with its plasma membrane and pinches off a vesicle containing the bacterium. Which statement about this process is correct?
A gland cell releases a large number of secretory vesicles by exocytosis over a short time, while taking in very little material by endocytosis. Which change to the cell is most likely during this time?
The concentration of a sugar is 5 mM outside a cell and 50 mM inside. The cell continues to take in more of the sugar. Which of the following must be true about this transport?
0 of 4 answered