Cell Membrane, Diffusion & Osmosis
The selectively permeable cell membrane, how diffusion and osmosis move things in and out of a cell, and what happens to plant and animal cells in different solutions.
1. Cell membrane
The cell membrane, also called the plasma membrane, is a thin, flexible boundary surrounding the cell. It separates the contents of the cell from its surroundings.
- Controls movement of substances into and out of the cell
- Helps maintain the internal environment of the cell
- Separates the cell contents from the external environment
2. Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. It happens because particles tend to spread out.
- Movement is from higher concentration → lower concentration
- Does not require a selectively permeable membrane
- Can occur in gases, liquids and solids
3. Osmosis
Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
Exam definition: osmosis is the movement of water through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
4. Diffusion vs osmosis
| Feature | Diffusion | Osmosis |
|---|---|---|
| What moves? | Particles | Water molecules |
| Direction | Higher concentration → lower concentration | Higher water concentration → lower water concentration |
| Membrane required? | Not necessarily | Selectively permeable membrane required |
| Medium | Gases, liquids and solids | Usually aqueous solutions |
| Example | Perfume spreading | Water entering a raisin |
5. Effect of osmosis on cells
The effect of water movement depends on the concentration of the surrounding solution.
Hypotonic solution
Lower solute concentration than the cell → water enters the cell.
Hypertonic solution
Higher solute concentration than the cell → water leaves the cell.
Isotonic solution
Same effective solute concentration inside and outside → no net movement of water.
6. Plasmolysis
Plasmolysis is the shrinkage of the cell contents away from the cell wall due to loss of water from a plant cell. It occurs when a plant cell is placed in a hypertonic solution.
7. Turgidity
When water enters a plant cell, the cell becomes firm or turgid. Turgidity provides mechanical support to many plant tissues.
8. Real-life applications
Raisin in water
When a raisin is placed in water, water enters the raisin through osmosis, causing it to swell.
Food preservation
High concentrations of salt or sugar can draw water out of microorganisms by osmosis, helping prevent their growth. This is one reason salt and sugar are used in some food-preservation methods.
Exact exam keywords
- Cell membrane → selectively permeable
- Diffusion → higher concentration to lower concentration
- Osmosis → water + selectively permeable membrane
- Hypotonic → water enters
- Hypertonic → water leaves
- Isotonic → no net water movement
- Plasmolysis → cell contents shrink away from cell wall
- Turgidity → plant cell becomes firm
1-mark questions tap to reveal
1. What is the main property of the cell membrane?
2. What is diffusion?
3. What moves during osmosis?
4. What type of membrane is required for osmosis?
5. What happens to a plant cell in a hypertonic solution?
2-mark questions
6. Differentiate between diffusion and osmosis.
Diffusion is the movement of particles from higher to lower concentration and does not necessarily require a membrane. Osmosis is the movement of water through a selectively permeable membrane from higher water concentration to lower water concentration.
7. What happens when a plant cell is placed in a hypotonic solution?
Water enters the plant cell by osmosis. The cell becomes swollen and turgid because the cell wall prevents it from bursting easily.
3-mark question
8. Explain plasmolysis.
When a plant cell is placed in a hypertonic solution, water moves out of the cell by osmosis. As water is lost, the cell contents shrink and the plasma membrane pulls away from the cell wall. This condition is called plasmolysis.
Assertion–Reason
A. Both A and R are true, and R is the correct explanation of A.
B. Both A and R are true, but R is not the correct explanation of A.
C. A is true, but R is false.
D. A is false, but R is true.
1. Assertion: Osmosis requires a selectively permeable membrane.
Reason: Osmosis involves the movement of water across such a membrane.
Answer: A
2. Assertion: Diffusion always requires a selectively permeable membrane.
Reason: Particles can move from higher to lower concentration without such a membrane.
Answer: D
3. Assertion: A plant cell placed in a hypertonic solution may undergo plasmolysis.
Reason: Water moves out of the cell by osmosis.
Answer: A
4. Assertion: A plant cell becomes turgid when water enters it.
Reason: Entry of water increases the internal pressure of the cell against the cell wall.
Answer: A
HOTS questions
HOTS 1. A raisin is placed in plain water and another in a concentrated sugar solution. Predict what happens.
The raisin in water gains water by osmosis and swells. The raisin in concentrated sugar solution tends to lose water, since the surrounding solution has a lower water concentration, and it may shrink.
HOTS 2. Why doesn’t a plant cell normally burst when placed in a hypotonic solution?
Water enters the plant cell by osmosis, but the rigid cell wall resists excessive expansion. This helps the cell become turgid rather than bursting.
HOTS 3. A student says, “Osmosis is simply diffusion of any substance through a membrane.” Is this correct?
No. Osmosis specifically involves the movement of water molecules through a selectively permeable membrane. Diffusion can involve different types of particles and does not necessarily require such a membrane.
10 fill in the blanks
| No. | Question | Answer |
|---|---|---|
| 1 | The cell membrane is ______ permeable. | selectively |
| 2 | Diffusion occurs from higher concentration to ______ concentration. | lower |
| 3 | Osmosis involves the movement of ______ molecules. | water |
| 4 | Osmosis requires a ______ permeable membrane. | selectively |
| 5 | Water enters a cell when it is placed in a ______ solution. | hypotonic |
| 6 | Water leaves a cell in a ______ solution. | hypertonic |
| 7 | A plant cell becomes ______ when it gains water. | turgid |
| 8 | Loss of water causing the cell contents to pull away from the cell wall is called ______. | plasmolysis |
| 9 | Perfume spreading through a room is an example of ______. | diffusion |
| 10 | Water entering a raisin placed in water occurs by ______. | osmosis |