ZIMSEC O Level · Biology · Form 3
Substances move into and out of cells by diffusion, osmosis and active transport.
Diffusion
The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It needs no energy from the cell. Example: oxygen diffuses from alveoli into the blood.
Rate increases with a steeper concentration gradient, higher temperature, a larger surface area and a shorter distance.
Osmosis
The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.
- Animal cells in pure water swell and may burst; in strong solutions they shrink.
- Plant cells in water become turgid (firm); in strong solutions they become plasmolysed (the membrane pulls away from the wall).
Active transport
Movement of substances against a concentration gradient, using energy from respiration. Root hair cells take in mineral ions this way.
Worked example: Potato strips are placed in pure water and in strong sugar solution. Explain the results.
In water, the strips gain water by osmosis and become longer and firmer.
In strong sugar solution, they lose water by osmosis and become shorter and soft.
Check yourself
- Define diffusion.
- Why does a plant cell not burst in pure water?
- Why do root hair cells need many mitochondria?
Show answers
- Net movement of particles from high to low concentration, down a concentration gradient.
- The cellulose cell wall resists further expansion.
- To release energy for active transport of mineral ions.
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