Osmosis and Diffusion (Copy)

Osmosis and Diffusion – Expanded Section (O Level & IGCSE Biology Alternate to Practical)

1. Key Definitions

• Diffusion: The net movement of particles from a region of higher concentration to lower concentration, down a concentration gradient.
  • Happens in gases and liquids.
  • Passive process (no energy required).
• Osmosis: The diffusion of water molecules across a partially permeable membrane from a region of high water potential (dilute solution) to low water potential (concentrated solution).

2. Diffusion in Practice

Examples of practical observations:

• Gas exchange in lungs or leaves (O₂ in, CO₂ out)
• Dye spreading in water
• Smell of perfume spreading through a room

Common experiment setup:

• Agar jelly cube with indicator placed in acid:
  • Acid diffuses into cube → color change
  • Smaller cubes lose color faster due to larger surface area:volume ratio

3. Osmosis in Practice

Typical Exam Practical:

• Potato osmosis experiment:
  • Potato cylinders placed in various sugar solutions (0%, 5%, 10%, etc.).
  • Leave for 30 mins to 1 hour.
  • Measure mass or length before and after.

Expected Results:

4. Designing Osmosis Experiments

Variables:

• Independent: Concentration of solution
• Dependent: Change in mass or length
• Control variables: Volume of solution, time, size of potato cylinder, temperature

Control Setup:

• Potato in distilled water (0% sugar solution) to represent no solute condition

Sample Method:

1. Cut equal-sized potato cylinders (e.g., 5 cm).
2. Blot dry and measure initial mass with balance.
3. Place in different concentrations of sucrose solution.
4. Leave for a fixed time (e.g., 30 mins).
5. Blot dry and reweigh.
6. Calculate change in mass and percentage change.

5. Calculating % Change in Mass

Formula:
% change = (final mass – initial mass) / initial mass × 100

Example:
Initial mass = 2.00 g
Final mass = 1.80 g
→ % change = (1.80 – 2.00)/2.00 × 100 = -10%

6. Interpreting Osmosis Results

• Gain in mass → water entered → dilute solution → cell was in hypotonic environment
• Loss in mass → water left → concentrated solution → hypertonic environment
• No change → isotonic solution → no net osmosis

7. Turgidity and Plasmolysis

• Turgid: Plant cell in dilute solution gains water → cell swells but wall prevents bursting
• Flaccid: Cell loses water → volume shrinks slightly
• Plasmolysis: In strong sugar solution, cytoplasm pulls away from wall due to water loss

8. Diffusion vs Osmosis – Table Comparison

Common Errors to Avoid

• Mixing up osmosis and diffusion
• Forgetting to dry potato before measuring final mass
• Not using equal size potato pieces
• Leaving samples for unequal time
• Describing osmosis as water moving “into the solution” instead of “into or out of the cell”

Marking Points for Osmosis/Diffusion Practicals