1. components of cell membrane
Quiz on functions of
components of cell membrane

2. Crossing membranes (passive processes) pages 22-27
Starting activity:
What do you remember about diffusion?
Define it...
What affects its rate?
What state can it occur in?

3. Diffusion
The net movement of molecules or ions from a region of their high concentration to a region of their lower concentration, down a concentration gradient.
What substances have to diffuse into and out of living cells?
Metabolism
Cross membranes

4. The rate of diffusion Temperature Concentration gradient
Stirring/moving
Surface area
Distance/thickness
Size of molecule

5. Diffusion
Lipid soluble, small, uncharged molecules can diffuse across the phospholipid bilayer down a concentration gradient – random process relying on kinetic energy of particles
Substances e.g. CO2, O2, H2O, steroid hormones (lipid soluble)

6. Larger or charged molecules
Facilitated diffusion

7. Facilitated diffusion
Large or polar molecules can’t pass easily through the bilayer
These molecules move through gaps in channel proteins
Transport depends on a concentration gradient
(facilitated = made easy)
Substances e.g. glucose, sodium ions, amino acids

8. Summary table Substance moved by Examples Simple diffusion
Facilitated diffusion using channel proteins
Facilitated diffusion using carrier proteins
Compare and contrast facilitated diffusion and simple diffusion

9. Osmosis – a special case
The net movement of water molecules from a region of higher water potential to a region of lower water potential (down a water potential gradient) across a partially permeable membrane. Water potential = a measure of the concentration of water molecules which are able to diffuse.

10. Key definitions Solvent – a liquid that dissolves solids
Solute – a solid that dissolves in a liquid
Solution – a liquid containing dissolved solids

11. Water Potential
Water potential (Ψ = greek letter psi) of a solution is the tendency of water to diffuse out of it (unit is one of pressure) i.e. how frequently water molecules are hitting the membrane – kPa, kilopascals
Pure water has a water potential of 0
Adding a solute to pure water decreases the concentration of water molecules in a given space - the chance of water molecules leaving the solution is less likely as the water molecules are attracted to the solute molecules and move less freely
Solutions have a negative water potential
17g sucrose in 1dm3 water Ψ = -130kPa

12. Osmosis – a special case of diffusion
Although water molecules are polar they are small enough to pass through the membrane
Osmosis is…..the diffusion of water from a high water potential to a low water potential (down its water potential gradient) across a partially permeable membrane

13. Understanding water potential and osmosis
Units = Greek letter psi ψ
Measured in kPa.
Pressure created by water molecules
Pure water has a ψ 0 kPa.
As the water potential decreases value for ψ becomes more negative.
The more solute dissolved, the lower the water potential of the solution.
Diagram 4 from books page 27

14. Osmosis in animal and plant cells
crenated
haemolysed
Incipient plasmolysis
plasmolysed

17. Potato Cylinders Investigation
Results
Examine the potato cylinders and measure their mass (g)
Copy and complete this table.
How could we display this on a graph?
Sucrose Concentration (M)
0.0
0.1
0.2
0.4
0.6
0.8
1.0
Initial mass (g)
Final mass (g)
Mass change (+/- g)
Percentage mass change

18. Discussion 
Why do you calculate the percentage change in mass rather than the actual change in mass
Estimate the concentration of the potato from the graph.
3. From your knowledge of osmosis in plant cells, what do you suppose has happened to the cells of the potato tissue (a) in 1M sucrose, (b) in water? 
How would these changes in the cells account for any changes in size of the potato cylinder?
 What do you think limits the change in mass of the potato cylinder which was immersed in water?
How could the results of this experiment be used to explain the extension growth of plant shoots and roots ?
7. How could the experiment be modified to find out the osmotic concentration of cell sap ?

19. Red onion practical

20. Crossing membranes (active processes)
Diffusion alone will not meet the needs of a cell.
Speed
Concentration gradient (glucose, magnesium ions)
Ensuring a one way flow
Moving large amounts

21. Active transport
The movement of molecules or ions across membranes, which uses ATP to drive protein ‘pumps’ within the membrane. Eg Na⁺ in nerve axons
Some carrier proteins act as pumps
Complementary shape to molecule they carry
The molecule can fit on
just one side ensuring
unidirectionality of movement

22. Active transport requires energy:
It acquires energy from respiration so cells that carry out a lot of A.T. will need lots of mitochondria.
What materials are required for aerobic respiration to occur?
Root cells actively transport ions e.g. NO₃⁻, K⁺ into roots.
This process stops when soil is waterlogged such as in peat bogs. Can you suggest why?

24. Moving large amounts – bulk transport
Pinocytosis (small molecules / liquids)
Phagocytosis (large solids)
Animation

25. Movement across membranes

26. Summary of movement across membranes:
Passive processes
Diffusion
Facilitated diffusion
Osmosis
Active processes
Active transport
Endocytosis and exocytosis

27. Tasks to try:
Explain why active transport allows substances to be accumulated in an area, whereas facilitated diffusion doesn’t.
Answer Q’s 10, 11 &12
Homework: Due next lesson
Finish Q’s above and Q9 from booklet