Active transport
Biology· Unit 4: Cell Membranes and Transport· 15 min read
1. Definition and Key Features of Active Transport★★☆☆☆⏱ 4 min
Active Transport
The movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration (against the concentration gradient), powered by energy released from ATP hydrolysis.
Example:
Uptake of nitrate ions by plant root hair cells from soil.
Unlike passive transport (diffusion, osmosis, facilitated diffusion), active transport requires energy input to move substances against their concentration gradient. All active transport relies on specific carrier proteins embedded in the phospholipid bilayer that change shape to move substances across the membrane.
Explain why glucose uptake by intestinal epithelial cells can require active transport after a meal once most nutrients have been absorbed.
- 1
After most glucose has already been absorbed, glucose concentration in the gut lumen is lower than glucose concentration inside the epithelial cells.
- 2
Passive diffusion and facilitated diffusion only move substances down their concentration gradient (from high to low), so cannot absorb the remaining glucose.
- 3
Active transport uses energy from ATP hydrolysis to change the shape of glucose carrier proteins, moving glucose against its concentration gradient into the cell.
2. Mechanism: Carrier Proteins and the Sodium-Potassium Pump★★★☆☆⏱ 5 min
Carrier Protein
A transmembrane protein that binds to a specific solute and undergoes reversible conformational changes to transport the solute across the membrane.
The general sequence for primary active transport is: 1. Solute binds to the carrier on the low-concentration side of the membrane. 2. ATP is hydrolyzed, and a phosphate group attaches to the carrier, triggering a shape change. 3. Solute is released on the high-concentration side of the membrane. 4. Phosphate detaches, and the carrier returns to its original shape.
Outline the steps of active transport by the sodium-potassium (Na⁺/K⁺) pump.
- 1
Three intracellular sodium ions (Na⁺) bind to specific sites on the carrier protein.
- 2
ATP is hydrolyzed to ADP and inorganic phosphate (Pi), which binds covalently to the carrier, causing a conformational change.
- 3
The shape change exposes the Na⁺ ions to the extracellular space, where they are released (Na⁺ concentration is higher outside the cell).
- 4
Two extracellular potassium ions (K⁺) bind to the carrier, triggering release of the Pi group.
- 5
Loss of Pi causes the carrier to revert to its original shape, releasing the two K⁺ ions into the cytoplasm, where K⁺ concentration is higher.
Exam tip:
Always remember: 3 Na⁺ move out, 2 K⁺ move in per ATP molecule used. This is a very common exam question.
3. Comparing Active and Passive Transport★★★☆☆⏱ 4 min
Compare and contrast questions are extremely common for this topic, so it is important to clearly recall the key similarities and differences between active and passive transport processes.
Core features compared below:
Active Transport
Moves against concentration gradient, requires ATP hydrolysis, requires carrier proteins, maintains concentration gradients across membranes
+ Pros: Can accumulate nutrients against gradients, maintains cell resting potential
− Cons: Uses a large proportion of cellular energy
Passive Transport
Moves down concentration gradient, no ATP required, uses channel/carrier proteins for facilitated diffusion, no proteins for simple diffusion
+ Pros: No energy expenditure for movement down gradients
− Cons: Cannot accumulate substances against gradients, stops at equilibrium
Explain why oxygen enters red blood cells via diffusion but calcium ions are pumped out via active transport.
- 1
Oxygen is small and non-polar, so it diffuses freely across the phospholipid bilayer. It moves from higher concentration outside the cell to lower concentration inside, so diffusion is sufficient.
- 2
Calcium ions are maintained at a much lower concentration inside red blood cells than outside, so pumping calcium out requires movement against the concentration gradient.
- 3
Movement against the gradient requires energy from ATP hydrolysis, so active transport via a calcium carrier pump is required.
4. Key Biological Examples★★☆☆☆⏱ 3 min
Nitrate ion uptake by plant root hair cells from soil
Glucose absorption by epithelial cells in the mammalian ileum
Calcium ion pumping in muscle cells to maintain low cytoplasmic calcium
Na⁺/K⁺ pump maintaining resting membrane potential in animal neurons
Explain why plant roots cannot rely on diffusion to take up nitrate ions from soil.
- 1
Plants require nitrate ions to make amino acids, nucleic acids and other key molecules, so nitrates accumulate in root hair cell cytoplasm.
- 2
This means nitrate concentration is usually higher inside the root cell than in the surrounding soil water.
- 3
Diffusion only moves substances down their concentration gradient, so cannot move nitrates from low concentration in soil to high concentration in the root cell.
- 4
Active transport uses ATP from root cell respiration to move nitrates against their gradient. If roots are waterlogged and cannot respire aerobically, active transport stops, and the plant cannot take up enough nitrates to grow.
5. Common Pitfalls
Wrong move:
Claiming all ATP-dependent transport is carrier-mediated active transport
Why:
Bulk transport (endocytosis/exocytosis) also uses ATP but is a separate process not classified as active transport in CIE 9700
Correct move:
Only carrier-mediated transport against a concentration gradient is classified as active transport; bulk transport is a separate energy-dependent process
Wrong move:
Confusing carrier proteins for active transport with channel proteins for facilitated diffusion
Why:
Channel proteins form pores for passive diffusion, they do not change shape to move substances against gradients
Correct move:
All active transport uses carrier proteins; channel proteins are only used for passive facilitated diffusion
Wrong move:
Stating active transport moves substances down the concentration gradient
Why:
This is the defining feature of passive transport, not active transport
Correct move:
Active transport always moves against the concentration gradient, from low to high concentration
Wrong move:
Forgetting both Na⁺ and K⁺ are moved against their gradients by the Na⁺/K⁺ pump
Why:
Many students only remember Na⁺ is moved out against its gradient, but K⁺ is also moved against its gradient into the cell
Correct move:
3 Na⁺ out against (higher outside), 2 K⁺ in against (higher inside the cell)
Wrong move:
Claiming active transport does not require membrane proteins
Why:
Polar molecules and ions cannot cross the hydrophobic core of the membrane unaided, even with energy input
Correct move:
All active transport requires specific carrier proteins embedded in the cell membrane
6. Quick Reference Cheatsheet
Feature | Active Transport | Passive Transport |
|---|---|---|
Concentration gradient | Against (low → high) | Down (high → low) |
Energy required | Yes, from ATP hydrolysis | No, uses kinetic energy |
Requires membrane proteins | Always (carriers) | Yes for facilitated diffusion, no for simple diffusion |
Stops when | Gradient maintained (energy required) | Equilibrium reached across membrane |
Common examples | Na+/K+ pump, nitrate uptake, glucose absorption | Oxygen diffusion, osmosis, ion movement through channels |
7. Frequently Asked
Is all ATP-dependent transport active transport?
No. Bulk transport processes (endocytosis and exocytosis) also use ATP but are not classified as carrier-mediated active transport, which is covered in this topic.
How do I distinguish active transport from facilitated diffusion in exams?
Active transport moves against the concentration gradient and requires ATP, while facilitated diffusion is passive, moves down the gradient and uses no energy.
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 · 1
MCQ on active transport mechanism
- 2023 · 2
Explain sodium-potassium pump
- 2021 · 3
Compare active vs passive transport
Going deeper
What's Next
Active transport is a core cellular process that underpins many higher-level biological concepts, from membrane potential to nutrient absorption and osmoregulation. Mastering the mechanism and key comparisons with passive transport prepares you for topics involving bulk transport of substances across membranes, and the generation of electrical signals in excitable cells like neurons and muscle cells. It also provides foundational knowledge for understanding whole-organism processes like plant mineral nutrition and mammalian digestion. Follow the links below to continue building your knowledge of membrane transport.