Movement Into And Out Of Cells: Diffusion And Osmosis
3.1 Diffusion And Osmosis
Diffusion
- Definition: Net movement of molecules/ions from high concentration to low concentration (down a concentration gradient), due to random movement.
- Energy source: Kinetic energy of molecules.
Factors Affecting Diffusion
- Surface area (↑ area → ↑ rate).
- Temperature (↑ temp → faster diffusion).
- Concentration gradient (steeper → faster).
- Distance (shorter → faster).
Examples
- Gas exchange in lungs (Oâ‚‚ into blood, COâ‚‚ out).
- Movement of glucose in small intestine.
Osmosis
- Definition: Net movement of water molecules from region of higher water potential to region of lower water potential across a partially permeable membrane.
Effects On Animal Cells
- In pure water → cell bursts (lysis).
- In concentrated solution → cell shrinks (crenation).
Effects On Plant Cells
- In pure water → cell becomes turgid (firm).
- In concentrated solution → cell becomes plasmolysed (cytoplasm pulls away from wall).
- Normal → firm but not over-swollen.
Written And Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions And 11 World Records For Educate A Change O Level And IGCSE Biology Full Scale Course
Turgor Pressure In Plants
- Water enters vacuole → pushes cytoplasm against cell wall.
- Provides support to plant tissues.
- Explains why plants wilt if water is lost.
Investigations
| Experiment | Description | Outcome |
|---|---|---|
| Diffusion in agar blocks | Coloured cube in acid | Smaller cubes diffuse faster |
| Potato strips in sucrose solutions | Strips gain or lose mass | Shows osmosis |
| Onion epidermis in salt solution | Cells plasmolysed | Water moves out |
Key Terms
- Turgid: Cell firm with water.
- Plasmolysis: Cytoplasm shrinks away from cell wall.
- Crenation: Animal cell shrinks in concentrated solution.
- Lysis: Animal cell bursts in pure water.
Written And Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions And 11 World Records For Educate A Change O Level And IGCSE Biology Full Scale Course