3.1.7Movement into and out of Cells
Importance of Active Transport — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 8 of 8
Part 1 of 4What active transport is (Extended)
Against the gradient
- Against the gradient Active transport moves particles from a region of lower concentration to a region of higher concentration,
- which is the opposite direction to the one in which they would naturally diffuse.
Energy is required
- Energy is required Because the particles are being pushed uphill, the cell must supply energy, which comes from respiration.
- Diffusion and osmosis are passive and need no energy from the cell.
Carrier proteins
- Carrier proteins Active transport uses carrier proteins in the cell membrane that pick up specific molecules or ions and move them across.
Part 2 of 4Active transport compared with diffusion
| Feature | Diffusion | Active transport |
|---|---|---|
| Direction | Down the concentration gradient | Against the concentration gradient |
| Energy from respiration | Not needed | Needed |
| Carrier proteins | Not required for small molecules passing through the membrane | Always required |
| Effect on gradient | Reduces the gradient until it is equal | Builds up or maintains a large gradient |
Part 3 of 4Why it matters in living things
Active transport
Glucose in the small intestine
- Glucose in the small intestine Epithelial cells in the villi absorb glucose from the gut even when its concentration in the blood is higher than in the gut,
- so almost all the glucose from digested food is absorbed.
Glucose in the kidney
- Glucose in the kidney Cells in the kidney tubules (nephrons) reabsorb glucose back into the blood so that it is not lost in urine.
Ions in plant roots
- Ions in plant roots Root hair cells take up mineral ions such as nitrate from soil water,
- where the ion concentration is lower than inside the root cells.
Why this is vital
- Why this is vital Without active transport, cells could not obtain enough of these substances to grow, release energy or build proteins.
Part 4 of 4Linking energy and the cell
1
A root hair cell or epithelial cell needs particles that are scarce outside the cell.
2
A specific carrier protein in the membrane binds to the particle.
3
Energy released from respiration changes the shape of the carrier protein.
4
The particle is released on the other side of the membrane, inside the cell.
5
The carrier returns to its original shape and can pick up another particle. Cells doing lots of active transport therefore contain many mitochondria.
Key terms
| Term | Meaning |
|---|---|
| Active transport | The movement of particles through a membrane against a concentration gradient, using energy from respiration. |
| Concentration gradient | The difference in concentration between two regions. |
| Carrier protein | A membrane protein that binds to a specific particle and moves it across the membrane. |
| Root hair cell | A plant cell with a long extension that absorbs water and mineral ions from the soil. |
| Villus | A tiny finger-like projection of the small intestine wall that increases the surface area for absorption. |
Avoid these mistakes
1
Saying active transport 'uses ATP' without linking it to respiration — it is respiration that supplies the energy.
2
Claiming that active transport moves particles down a concentration gradient; that is diffusion.
3
Forgetting to give the directionactive transport is from low to high concentration.
4
Saying that plants absorb ions by osmosis; osmosis is the movement of water only.
5
Writing that active transport needs no membrane proteins; carrier proteins are essential.
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