# Active transport

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u4-active-transport/

This module covers energy-dependent carrier-mediated movement of molecules and ions across cell membranes, including key mechanisms, examples, and comparisons with passive transport to prepare you for exam questions.

**Prerequisites:** [Structure of cell membranes](https://www.owlsprep.com/study/cie-9700-u4-cell-membrane-structure/); [Passive transport (diffusion, osmosis)](https://www.owlsprep.com/study/cie-9700-u4-passive-transport/)

## Learning objectives

- Describe the mechanism of active transport across cell membranes
- Distinguish between active transport and passive transport processes
- Explain the roles of ATP and carrier proteins in active transport
- Describe key biological examples of active transport

## Definition and Key Features of Active Transport

**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.

> **info**
>
> Energy for most active transport comes from hydrolysis of ATP, which releases approximately 30.6 kJ of energy per molecule to power conformational changes in carrier proteins.

**Worked example:** 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.

## Mechanism: Carrier Proteins and the Sodium-Potassium Pump

**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.

**Worked example:** 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.

## Comparing Active and Passive Transport

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.

**Comparing methods**

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

**Worked example:** 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.

## Key Biological Examples

- 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

**Worked example:** 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.

## Common pitfalls

- **Wrong:** Claiming all ATP-dependent transport is carrier-mediated active transport
  - Why it fails: Bulk transport (endocytosis/exocytosis) also uses ATP but is a separate process not classified as active transport in CIE 9700
  - Correct: Only carrier-mediated transport against a concentration gradient is classified as active transport; bulk transport is a separate energy-dependent process
- **Wrong:** Confusing carrier proteins for active transport with channel proteins for facilitated diffusion
  - Why it fails: Channel proteins form pores for passive diffusion, they do not change shape to move substances against gradients
  - Correct: All active transport uses carrier proteins; channel proteins are only used for passive facilitated diffusion
- **Wrong:** Stating active transport moves substances down the concentration gradient
  - Why it fails: This is the defining feature of passive transport, not active transport
  - Correct: Active transport always moves against the concentration gradient, from low to high concentration
- **Wrong:** Forgetting both Na⁺ and K⁺ are moved against their gradients by the Na⁺/K⁺ pump
  - Why it fails: Many students only remember Na⁺ is moved out against its gradient, but K⁺ is also moved against its gradient into the cell
  - Correct: 3 Na⁺ out against (higher outside), 2 K⁺ in against (higher inside the cell)
- **Wrong:** Claiming active transport does not require membrane proteins
  - Why it fails: Polar molecules and ions cannot cross the hydrophobic core of the membrane unaided, even with energy input
  - Correct: All active transport requires specific carrier proteins embedded in the cell membrane

## 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 |

## 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.

- [Bulk Transport (Endocytosis & Exocytosis)](https://www.owlsprep.com/study/cie-9700-u4-bulk-transport/)
- [Passive Transport](https://www.owlsprep.com/study/cie-9700-u4-passive-transport/)
- [The Mitotic Cell Cycle](https://www.owlsprep.com/study/cie-9700-u5-overview/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-9700-u4-active-transport/