AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/2/2-8)
Unit 2 · Topic 2.8
2.8 Mechanisms of Transport
Active transport uses membrane proteins and a direct input of energy, usually from ATP, to move ions and molecules across a membrane, often against their gradient. That's how cells build and maintain electrochemical gradients. The sodium-potassium pump is the key example: each cycle moves 3 Na⁺ out and 2 K⁺ in, which helps set the voltage across the membrane, the membrane potential.
Key terms
- active transport
- ATP
- sodium-potassium pump
- electrochemical gradient
- membrane potential
What active transport needs
Active transport always needs two things: a membrane protein (often called a pump) and metabolic energy. The energy usually comes from ATP. When ATP is hydrolyzed to ADP and a phosphate, energy is released, and the pump uses it to change shape and move its cargo (3.3).
Pumps let a cell build gradients, not just use them. Without active transport, diffusion would slowly erase every difference between inside and outside until the cell reached equilibrium. A cell at equilibrium with its surroundings is a dead cell.
Electrochemical gradients
For an uncharged molecule, only its concentration gradient matters. For an ion, charge matters too. If the inside of the cell is negative, a positive ion like Na⁺ is pulled inward by charge as well as by concentration. The combined effect of the concentration difference and the charge difference is the electrochemical gradient. Pumps store energy in these gradients, and the cell spends it later, for example when Na⁺ rushes into a nerve cell to send a signal.
The sodium-potassium pump
The sodium-potassium pump (Na⁺/K⁺ ATPase) is found in the plasma membranes of animal cells. An 'ATPase' is an enzyme that hydrolyzes ATP. Each cycle goes like this:
- Three Na⁺ ions from inside the cell bind to the pump.
- ATP transfers a phosphate group to the pump (phosphorylation), and the pump changes shape so it opens to the outside.
- The three Na⁺ are released outside the cell, and two K⁺ ions from outside bind.
- The phosphate group comes off, and the pump returns to its original shape, opening to the inside.
- The two K⁺ are released inside, and the cycle can start again.
Building the membrane potential
After many cycles, Na⁺ is much more concentrated outside the cell and K⁺ is much more concentrated inside. Each cycle also moves 3 positive charges out but only 2 back in, a net of one positive charge out. The pump, along with K⁺ leaking out through open channels, leaves the inside of the cell negative compared with the outside.
That voltage across the membrane is the membrane potential. In a resting nerve cell it's roughly −70 millivolts (inside negative). The Na⁺ and K⁺ gradients the pump builds are what nerve and muscle cells use to send signals.
Cells also use stored gradients to power other transport. For example, some carrier proteins let Na⁺ flow back into a cell down its gradient and use that flow to drag glucose in with it, even against glucose's own gradient. This is called cotransport. It doesn't use ATP directly, but it depends on the gradient the ATP-powered pump built.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Counting ions and charges
A sodium-potassium pump completes 100 cycles. How many Na⁺ and K⁺ ions does it move, how many ATP molecules does it use, and what is the net change in positive charge inside the cell?
Show the solutionHide the solution
- Step 1: Each cycle uses 1 ATP, moves 3 Na⁺ out and moves 2 K⁺ in.
- Step 2: Na⁺ out: 3 × 100 = 300. K⁺ in: 2 × 100 = 200. ATP used: 100.
- Step 3: Charge: each ion carries +1. Out: 300 positive charges. In: 200 positive charges.
- Step 4: Net: 300 − 200 = 100 more positive charges leave than enter.
Answer: 300 Na⁺ out, 200 K⁺ in, 100 ATP used, and a net loss of 100 positive charges from the inside, which helps make the inside negative.
- Example 2
Blocking the pump
Ouabain is a chemical that blocks the sodium-potassium pump. Predict what happens to the Na⁺ concentration inside a cell, to the membrane potential, and to the cell's uptake of glucose by Na⁺-glucose cotransport, after the cell is treated with ouabain for a while.
Show the solutionHide the solution
- Step 1: With the pump blocked, Na⁺ that leaks in is no longer pumped out, so Na⁺ builds up inside the cell.
- Step 2: K⁺ is no longer pumped back in, so it slowly leaks out. The gradients shrink, and the net outward movement of positive charge stops.
- Step 3: The membrane potential becomes less negative over time, drifting toward zero.
- Step 4: Cotransport depends on Na⁺ flowing in down its gradient. A smaller Na⁺ gradient means less energy to drag glucose in, so glucose uptake drops.
Answer: Inside Na⁺ rises, the membrane potential becomes less negative as the gradients run down, and Na⁺-driven glucose uptake decreases.
Common mistakes
- Reversing the pump: it moves 3 Na⁺ out and 2 K⁺ in, not the other way around.
- Saying active transport doesn't need a protein. All active transport across a membrane uses membrane proteins.
- Forgetting that ions respond to charge as well as concentration. Use 'electrochemical gradient' for ions.
- Thinking that blocking ATP production stops all transport right away. Passive transport continues; what stops is pumping, so the gradients slowly run down.
On the exam
- Expect questions that give the pump's 3:2 ratio and ask about charge or gradients, or that ask you to predict the effect of a pump inhibitor or an ATP shortage.
- Use the full chain in answers: ATP → pump changes shape → ions moved against their gradients → electrochemical gradient and membrane potential.
Connected topics
Videos
Check yourself
4 questions on 2.8 Mechanisms of Transport. Pick an answer to see if you got it, and why.
| Condition | K⁺ concentration outside cells (mM) | K⁺ concentration inside cells after 1 hour (mM) |
|---|---|---|
| Normal oxygen | 1 | 60 |
| Normal oxygen plus a drug that blocks ATP production | 1 | 2 |
| No oxygen | 1 | 3 |
Experimental data: plant root cells were placed in a solution containing K⁺. Each value is the mean of four trials.
Which of the following is best supported by the data?
Why did removing oxygen have about the same effect as the drug?
Each cycle of the sodium-potassium pump moves 3 Na⁺ out of the cell and 2 K⁺ into the cell. Which of the following best describes how this helps create the membrane potential?
A drug blocks the sodium-potassium pump in animal cells. Which of the following would most likely happen over time?
0 of 4 answered