Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Passive transport (simple and facilitated diffusion) moves substances down their concentration gradient with no energy input.
- Active transport needs a membrane protein (a pump) and a direct input of metabolic energy, usually ATP. It can move substances against their gradient.
- Pumps are specific (binding sites fit particular ions) and saturable (a maximum rate when all pumps are busy).
- Sodium–potassium pump (an ATPase): 3 Na⁺ bind inside → ATP phosphorylates the pump → shape change releases Na⁺ outside → 2 K⁺ bind → phosphate released → shape returns, K⁺ released inside.
- An electrochemical gradient combines a concentration difference and a charge difference. The membrane potential of many resting animal cells is about −70 mV (inside negative).
- Other pumps: H⁺ pumps (plant cells, lysosomes), Ca²⁺ pumps (keep cytoplasmic Ca²⁺ low).
Key relationships
| Process | Energy | Protein | Direction |
|---|---|---|---|
| simple diffusion | none | none | high → low |
| facilitated diffusion | none | channel or carrier | high → low |
| active transport | ATP (direct) | pump | can be low → high |
| Per pump cycle | Amount |
|---|---|
| Na⁺ moved out | 3 |
| K⁺ moved in | 2 |
| ATP used | 1 |
| net positive charge moved out | 1 |
Assumptions behind the numbers
- Pump calculations assume every cycle is complete and uses exactly one ATP.
- “Starts to run down” predictions assume ions keep leaking through channels when the pumps stop.
- Uptake that stops when ATP production is blocked is good evidence for active transport, but a blocker can have side effects; a second method strengthens the case.
Mistakes to avoid
- Defining active transport only as “against the gradient” and forgetting the energy and the protein.
- Saying facilitated diffusion uses ATP.
- Writing a 1 : 1 exchange of Na⁺ and K⁺.
- Saying ATP is carried across the membrane.
- Forgetting that pumps must keep working because ions leak back.
- Saying ions cross the bilayer directly because they are small.
Quick self-check
- A cell’s Na⁺/K⁺ pumps use 9.0 × 10⁵ ATP per second. How many Na⁺ and K⁺ move? (2.7 × 10⁶ Na⁺ out and 1.8 × 10⁶ K⁺ in per second)
- Why is the pump said to help keep the inside negative? (Each cycle moves 3 positive charges out but only 2 in: one net positive charge leaves.)
- What happens to the Na⁺ gradient if ATP runs out? (Pumping stops, Na⁺ leaks in and the gradient gradually runs down.)
Next: practice questions.