Transport Across the Cell Membrane
Senior 2 · Biology
Transport Across the Cell Membrane
Lesson 4 of 5
20 min
Let's begin
On a hot afternoon the bean leaves in a garden droop, and after an evening shower they stand up again. Nothing grew or broke. Water left the cells and then came back in.
The same process explains why a hospital drip is made of 0.9% salt solution and not pure water, and why salting or drying fish such as isambaza keeps it from rotting.
A plant cell has a strong cellulose cell wall outside its membrane, and a large vacuole full of cell sap.
An animal cell has only a membrane, so it cannot withstand much pressure.
| Where | What osmosis does |
|---|---|
| Root hairs | Soil water usually has a higher water potential than root-hair cell sap, so water enters the root by osmosis. |
| Support in plants | Turgid cells press against each other and hold up soft stems and leaves. When they lose water and become flaccid, the plant wilts. |
| Stomata | Guard cells take in water, become turgid and bend apart to open the pore; when they lose water, the pore closes. |
| Too much fertiliser | Heavy fertiliser can make soil water more concentrated than root cell sap, so water leaves the roots and the crop wilts ("fertiliser burn"). |
| Hospital drips | Drips use isotonic 0.9% sodium chloride (normal saline) so blood cells neither burst nor shrivel. |
| Food preservation | Salting fish, and making jam with lots of sugar, draw water out of bacteria and fungi by osmosis, so they cannot grow. |
| Freshwater protists | Amoeba and Paramecium in pond water gain water all the time; a contractile vacuole pumps it out to stop them bursting. |
Question: Explain why bean plants wilt on a hot, dry afternoon and recover after rain.
Solution: In hot, dry air the leaves lose water by evaporation faster than the roots can take it up from dry soil. The leaf cells lose water, their vacuoles shrink, and they become flaccid, no longer pressing on each other, so the leaves droop. After rain the soil water has a high water potential again, water enters the roots by osmosis and travels to the leaves, and the leaf cells take it in, become turgid and hold the leaves up again.
Tip
Memory hook: a plant cell in water becomes turgid but never bursts, because the wall is its "belt". An animal cell has no belt, so it bursts.
Watch the words: plasmolysis belongs to plant cells, haemolysis to bursting red blood cells, and crenation to shrivelled red blood cells.
Describe what happens to a plant cell placed in a concentrated sugar solution.
Solution: The solution has a lower water potential than the cell sap, so water leaves by osmosis. The vacuole shrinks, and the membrane and cytoplasm pull away from the wall. The cell becomes plasmolysed.
Why does a red blood cell burst in pure water while an onion cell does not?
Solution: Both take in water by osmosis. The onion cell has a strong cellulose wall that resists the swelling, so it becomes turgid but does not burst. The red blood cell has no wall, so it keeps swelling until the membrane breaks (haemolysis).
A nurse accidentally prepares a drip much more concentrated than 0.9% salt. Predict the effect on the patient's red blood cells.
Solution: The drip would be hypertonic to the blood cells, so water would leave them by osmosis. They would shrink and shrivel (crenation) and could not carry oxygen properly.
Explain how salting preserves fish.
Solution: The salt makes a solution with a very low water potential on the fish. Water leaves any bacteria and fungi on it by osmosis, so they become dehydrated and cannot grow and cause rotting.
Summary
Sources and credits
The Rwandan examples, worked examples and practice questions are written by Ganzaa.
