Starter

1. Starter Explain why active transport allows substances to be accumulated in an area where as facilitated diffusion doesn`t.

2. Answer • Active transport uses energy to ensure that materials can pass only one way through the membrane- against a concentration gradient. Facilitated diffusion allows the movement of molecules both ways and so the concentration will reach an equilibrium over time.

3. Active transport • Make a flow diagram to show the steps involved in active transport • e.g. Start with The cell needs to contain a higher level of potassium ions in the cell than the concentration outside the cell

4. Objectives • Recall a definition for osmosis • Explain osmosis in terms of water potential • Recognise and evaluate the effects of solutions of different water potentials on plant and animal cells

5. Osmosis is the diffusion of water from a region of high water concentration to a region of low water concentration through a partially permeable membrane Osmosis

6. Osmosis is the diffusion of water from a region of high water potential to a region of low water potential through a partially permeable membrane Osmosis

7. Water molecules can diffuse easily across membranes Many solute molecules, e.g. Na+, cannot diffuse across membranes (without help) Osmosis is the diffusion of water, down water potential (Ψ) gradient, across a selectively permeable membrane Osmosis

8. Ψ is the tendency of a solution to lose water, it is decreased by the addition of a solute, it is increased by external pressure Ψ is measured in kPa Ψ is always negative, i.e. Ψ of pure water is 0, Ψ of sugar solution may be -2000 kPa Water always moves to the lowest (most negative) Ψ Hypotonic = higher Ψ than cell Hypertonic = lower Ψ than cell Isotonic = same Ψ as cell Water potential (Ψ)

9. Task 1 • Complete the spaces on the osmosis sheet

10. Task 2 • In pairs Summarise what happens to plant an animal cells in the following solutions, 1. Isotonic 2. Hypotonic 3. Hypertonic Use pages 31and 32. 10 minutes time limit. Then teach each other the other ½ (10 minutes)

11. Plenary • Fresh water amoebae must continually move large volumes of water out of the well. Explain why this is the case and suggest how they might carry this out.

12. Answer • The water potential of the cell is lower than that of the surrounding water- so water moves into the cell. • Amoebae move excess water into a membrane bound vacuole called a `contractile vacuole`. This vacuole empties its contents out of the cell via process called exocytosis. His is an active process.