AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
If body cells lose or gain too much water by osmosis they do not function…
Full specification wording
Maintaining water and nitrogen balance in the body (biology only): If body cells lose or gain too much water by osmosis they do not function efficiently.
Body cells contain cytoplasm with dissolved substances, and water moves into or out of cells by osmosis across the partially permeable cell membrane.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 6 of 17
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Body cells contain cytoplasm with dissolved substances, and water moves into or out of cells by osmosis across the partially permeable cell membrane. If cells lose too much water, the cytoplasm becomes concentrated and the cell may shrink; if cells gain too much water, the cell may swell and burst. In both cases, cell reactions and processes do not work efficiently because enzymes and other molecules need the correct water balance. For example, if a red blood cell is placed in pure water, water enters by osmosis and the cell may burst; in a concentrated salt solution, water leaves and the cell shrinks. The kidneys help prevent this by controlling water and ion loss in urine, keeping the water balance of body fluids stable.
What earns the marks
- Water moves into or out of cells by osmosis through the partially permeable cell membrane.
- If cells lose too much water, they become dehydrated and their reactions do not work efficiently.
- If cells gain too much water, they may swell and burst, so they cannot function efficiently.
- The kidneys help maintain the water balance of body fluids by adjusting water loss in urine.
- Efficient cell function depends on keeping the water content of cells and body fluids within a narrow range.
Common mistakes and how to put them right
Thinking water movement by osmosis does not affect cell function.
Put it right: Too much water loss or gain disrupts cell processes.
Believing cells only burst when they gain water and never shrink when they lose water.
Put it right: Both too little and too much water cause inefficient function.
Confusing osmosis with diffusion of solutes.
Put it right: Osmosis is the movement of water across a partially permeable membrane.
Exam tips
- Use the terms swell and shrink to describe the effects of too much water gain and loss on cells.
- Link the effect on cells to the role of the kidneys in keeping water balance stable.
- State that osmosis involves water moving across a partially permeable membrane from a dilute to a concentrated solution.
Try the lesson
The start of the Maintaining water and nitrogen balance in the body (biology only): If body cells lose or gain too much water by osmosis they do not function efficiently. lesson, made from checked teaching notes for this specification point.
Have a go first
A person becomes dehydrated and their body cells lose water by osmosis. What happens to these cells?
Listen
Body cells contain cytoplasm with dissolved substances, and water moves into or out of cells by osmosis across the partially permeable cell membrane. If cells lose too much water, the cytoplasm becomes concentrated and the cell may shrink; if cells gain too much water, the cell may swell and burst. In both cases, cell reactions and processes do not work efficiently because enzymes and other molecules need the correct water balance. For example, if a red blood cell is placed in pure water, water enters by osmosis and the cell may burst; in a concentrated salt solution, water leaves and the cell shrinks. The kidneys help prevent this by controlling water and ion loss in urine, keeping the water balance of body fluids stable. A strong answer earns marks for these points. Water moves into or out of cells by osmosis through the partially permeable cell membrane. If cells lose too much water, they become dehydrated and their reactions do not work efficiently. If cells gain too much water, they may swell and burst, so they cannot function efficiently. The kidneys help maintain the water balance of body fluids by adjusting water loss in urine. A common slip: Thinking water movement by osmosis does not affect cell function. Instead, too much water loss or gain disrupts cell processes. Exam tip: Use the terms swell and shrink to describe the effects of too much water gain and loss on cells.
Key words
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Osmosis and cell function
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Water balance in body fluids
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Homeostasis and kidney function
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Consequences of imbalance
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See every definition
- Osmosis and cell function
- Water moves into or out of cells by osmosis across the partially permeable cell membrane, and if cells lose or gain too much water they do not function efficiently.
- Water balance in body fluids
- Keeping the water content of cells and body fluids within a narrow range, because efficient cell function depends on it.
- Homeostasis and kidney function
- The kidneys help maintain the water balance of body fluids by adjusting water loss in urine.
- Consequences of imbalance
- If cells lose too much water the cytoplasm becomes concentrated and the cell may shrink; if they gain too much water the cell may swell and burst.
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