Osmosis and Diffusion
Explain passive transport and predict water movement in cells.
Cells move substances across their membranes without always using energy.
- Define diffusion, osmosis and active transport.
- Predict water movement across a membrane.
- Explain turgid and plasmolysed plant cells.
Diffusion
Particles move randomly from an area of higher concentration to an area of lower concentration. Oxygen diffuses from the lungs into the blood; carbon dioxide diffuses from blood into the lungs.
Osmosis
Osmosis is the movement of water molecules across a partially permeable membrane from a dilute (high water) solution to a concentrated (low water) solution.
In Plant Cells
In pure water, plant cells absorb water, the vacuole swells and the cell becomes turgid. In a strong sugar solution, water leaves and the cell becomes plasmolysed.
A potato strip in pure water gains mass — water enters the cells.
A potato strip in strong salt solution loses mass — water leaves the cells.
Practice
- Define osmosis.
- Predict whether a red blood cell swells or shrinks in pure water. Explain.
- A potato strip loses mass in a salt solution. Explain why.
Recap
Diffusion moves particles down a concentration gradient. Osmosis moves water across a membrane and can make cells turgid or plasmolysed.
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Cell Biology and Transport
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