3.3 Active transport
- Syllabus
- 0610–2026–2027
- Topic
- 3.3
- Level
- —
Learning objectives
3.3.1Active transport as the movement• Describe active transport as the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (i.e. against a concentration gradient), using energy from respiration3.3.2Importance of active transport• Explain the importance of active transport as a process for movement of molecules or ions across membranes, including ion uptake by root hairs3.3.3Protein carriers move molecules or• State that protein carriers move molecules or ions across a membrane during active transport
Define active transport
Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration—against a concentration gradient—using energy from respiration.
| Feature | Active transport |
|---|---|
| barrier crossed | cell membrane |
| direction | lower → higher concentration |
| relation to gradient | against it |
| energy | required from respiration |
Moving particles against their gradient does not occur by their net random spreading. The cell must supply energy released by respiration to drive the movement.
| Process | Net direction | Energy from respiration |
|---|---|---|
| active transport | low → high concentration | required |
| diffusion | high → low concentration | not required |
A substance crossing a membrane is not automatically undergoing active transport. Check both direction and energy: movement down a gradient without respiratory energy is diffusion.
Explain why cells use active transport
Active transport allows cells to take up or remove useful molecules and ions even when the required movement is against their concentration gradient.
| Cell or tissue | Substance moved | Why active transport matters |
|---|---|---|
| root hair cell | mineral ions such as nitrate or magnesium | ions can enter from dilute soil solution even when their concentration is already higher inside the root |
| small-intestine epithelium | glucose | absorption can continue when glucose concentration in the gut is lower than in epithelial cells or blood |
| kidney tubule epithelium | glucose and salts | useful substances can be reabsorbed rather than lost in urine |
Because energy comes from respiration, cells performing much active transport often contain many mitochondria. Reduced oxygen can reduce aerobic respiration and therefore reduce active uptake.
A complete root-hair explanation states: mineral-ion concentration is lower in soil than in the root cell, ions cross the cell membrane against the gradient, and respiration supplies energy.
Water uptake by root hairs is osmosis, not active transport. Active transport in this context concerns mineral ions moving against their concentration gradient.
Describe the role of carrier proteins
Protein carriers in the cell membrane move molecules or ions across the membrane during active transport.
| Step | Carrier action |
|---|---|
| 1 | a particular molecule or ion binds to its carrier protein on one side of the membrane |
| 2 | energy from respiration enables the carrier to change shape |
| 3 | the particle is moved across the membrane and released on the other side |
| 4 | the carrier returns to its original shape and can repeat the cycle |
A carrier's binding region fits particular molecules or ions, so different substances may require different carrier proteins.
If uptake reaches a maximum even as external concentration rises, a possible explanation is that all available carriers are working; the number of carriers limits the rate.
Carrier proteins are part of the membrane; they do not supply the energy themselves. The energy that drives active transport comes from respiration.