Active transportSpec 4.1.3.3
- Active transport
- Moves substances from a more dilute solution to a more concentrated solution (against a concentration gradient). This requires energy from respiration.
- Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. Plants require ions for healthy growth.
- It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood, which has a higher sugar concentration. Sugar molecules are used for cell respiration.
Because active transport needs energy from respiration, cells that carry it out (such as root hair cells) have many mitochondria.
Comparing the three processes
You should be able to describe how substances are transported into and out of cells by diffusion, osmosis and active transport, and explain the differences between the three processes.
| Diffusion | Osmosis | Active transport | |
|---|---|---|---|
| What moves | Particles of a gas or of a substance in solution | Water only | Substances such as mineral ions and sugar |
| Direction | From higher to lower concentration (down the gradient) | From a dilute solution to a concentrated solution | From a more dilute to a more concentrated solution (against the gradient) |
| Membrane | Across a membrane or not | Through a partially permeable membrane | Across the cell membrane |
| Energy from respiration needed? | No | No | Yes |
Quick check
Name three factors that affect the rate of diffusion.
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The concentration gradient, the temperature and the surface area of the membrane.
Why do multicellular organisms need exchange surfaces and a transport system?
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They have a small surface area to volume ratio, so diffusion across the outside alone cannot supply all of their cells.
Define osmosis.
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The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
How is active transport different from diffusion?
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It moves substances against a concentration gradient and needs energy from respiration.
Give an example of active transport in plants.
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Absorption of mineral ions into root hairs from very dilute solutions in the soil.