Osmosis Edexcel International GCSE Biology revision
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In plain words
Soak a raisin in water and it swells up. Water has moved in. Osmosis is just the diffusion of water: it moves through a cell membrane from where there is more water (a dilute solution) to where there is less (a concentrated one).
Three things to know
- Osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution) through a partially permeable membrane.
- A plant cell in pure water takes in water and becomes turgid: firm, with its contents pressing on the cell wall. This is what holds plants upright.
- A plant cell in a concentrated solution loses water and becomes flaccid. If it loses enough, the membrane pulls away from the wall: plasmolysis. An animal cell in pure water bursts, because it has no cell wall.
Worked example
A potato cylinder has a mass of 5.0 g. After an hour in a sugar solution its mass is 4.4 g. Find the percentage change in mass.
- Change in mass = 4.4 − 5.0 = −0.6 g.
- Percentage change = change ÷ original × 100 = −0.6 ÷ 5.0 × 100.
- = −12%. It lost mass, so water left the potato by osmosis: the solution was more concentrated than the potato cells.
Tips and tricks
- Only water moves in osmosis. Never write that the sugar or salt moved.
- Water moves towards the more concentrated solution. "Dilute to concentrated" is the easy way to remember it.
It lands in your notebook with its questions as flashcards.
Osmosis: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What moves during osmosis?
Osmosis is the movement of water only.
What does water pass through in osmosis?
The membrane lets water through but not larger dissolved molecules.
A plant cell is placed in pure water. What happens?
The cell wall stops it bursting.
What is plasmolysis?
It happens when a plant cell loses a lot of water.
In which direction does water move by osmosis?
A dilute solution has the higher water potential.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
Osmosis
Edexcel International GCSE Biology 4BI1 · 7 marks · papermunch.org
Name ______________________________ Date ______________
State what is meant by osmosis.[3]
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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.
Explain why a plant wilts when it is short of water.[2]
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Its cells lose water and become flaccid, so they no longer press against each other and support the plant.
Explain why a red blood cell bursts in pure water but a plant cell does not.[2]
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Both take in water by osmosis. The plant cell has a strong cell wall that stops it bursting; the red blood cell has no cell wall.
Answers: Osmosis
- 1. 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.
- 2. Its cells lose water and become flaccid, so they no longer press against each other and support the plant.
- 3. Both take in water by osmosis. The plant cell has a strong cell wall that stops it bursting; the red blood cell has no cell wall.