Facilitated diffusion is the passive transport of molecules across a cell membrane using carrier proteins. There are two main types of carrier proteins: carrier proteins that bind to specific molecules and undergo a conformational change to transport them across the membrane, and ion channels that have pores lined with charged groups to transport ions. Facilitated diffusion is saturated at high concentration differences because there are a limited number of carrier proteins.

1. Facilitated diffusion

2. Spot the difference!

3. Key features It is passive – does not require energy but It uses carrier proteins Here the solute molecules “combine” with carrier proteins in the membrane. These carrier molecules speed (or facilitate) the passage of the solute molecules across the membrane.

4. The molecules that use it: Large molecules such as glucose or charged ions.

5. Two types of protein are involved CARRIER PROTEINS – bind to a specific type of diffusing molecule. They have a highly specific hydrophilic region to which the solute molecule binds. This binding cause the protein to undergo a change in shape that moves the solute across the bilayer and release it on the other side

6. Carrier proteins

7. Carrier proteins

8. ION CHANNELS These are formed by proteins with a central pore that is lined with charged groups. They help the diffusion of charged particles such as Ca 2+, Na + , K + , HCO 3- and Cl ions. Some channels are gated and allow cells to regulate the flow of ions from one cell to another.

9. Ion channel

11. Comparison of diffusion and facilitated diffusion. What factors affect the diffusion rate? Concentration difference

12. In facilitated diffusion a point will be reached where increase in concentration differences are not accompanied by an increased rate. Why? The system is saturated – there are only a limited number of carrier molecules per unit area of membrane. The rate of movement reaches a maximum when all the carrier molecules are fully loaded with solute molecules

13. Osmosis The net movement of water molecules by diffusion from a solution of less negative water potential to a solution of more negative water potential through a partially permeable membrane.

14. Osmosis Osmosis is a “special kind of diffusion”. It’s when water diffuses from a concentrated area to a less concentrated area through a partially permeable membrane (i.e. one that allows water to move through but not anything else): Water Sugar solution In this example the water molecules will move from left to right (along the concentration gradient) and gradually dilute the sugar solution. This is how water moves into root hair cells

15. Water potentials Water moves from a place with a LESS NEGATIVE (higher) water potential to a place with a MORE NEGATIVE (lower) water potential. The water potential of pure water is 0 (zero). Solutions have negative water potentials – the more concentrated the solution, the more negative the water potential.

16. Water diffuses from a solution with a less negative water potential (hypotonic) into one with a more negative water potential (hypertonic) until the solutions are the same concentration (isotonic). EXAM TIP: You MUST use these sort of terms, e.g. more negative / less negative / hypotonic / hypertonic. If you talk – larger / smaller or bigger / smaller you won’t get any marks!!!!!!!

17. Draw the diagrams Label which solution has the more negative water potential. Which solution is: Hypotonic Hypertonic Isotonic

18. Strong sugar solution Medium sugar solution Weak sugar solution Potato cells

19. More facts…. The cell surface membrane is partially (and selectively) permeable. The membrane around the vacuole in plant cells is also partially permeable. Ions (and sugars) in the vacuole produce a negative water potential and water enters the vacuole by osmosis. The vacuole expands, pushes against the cell wall and makes the cell TURGID. Cell turgor provides support in plants.

20. Water potential formula: Water potential= solute potential + pressure potential

21. Osmosis The water potential of pure water is ZERO Water diffuses from a Less negative water potential hypotonic To a More negative water potential hypertonic hypertonic Until the solutions are ISOTONIC

22. Active Transport

23. Spot the difference

24. Definition Active transport is the transport of molecules or ions across a membrane by carrier proteins against a concentration gradient.

25. Key facts It requires energy from respiration Factors that reduce respiration will also reduce active transport: e.g. lower temperature; lack of oxygen; metabolic and respiratory inhibitors. Active transport involves carrier proteins in the membrane. The hydrolysis of ATP (process of decomposition where compound reacts with water to produce other compounds) releases the energy required for active transport.

26. Cells involved in active transport have a LARGE NUMBER of MITOCHONDRIA to provide the ATP required via AEROBIC RESPIRATION.

29. Example The sodium – potassium pump. Exists in most cell membranes. Actively removes sodium ions from the cell while actively accumulating potassium ions into them from their surroundings

31. Endocytosis and Exocytosis Endocytosis is the transport of large particles into the cell in vesicles formed by invagination of the cell surface membrane. Exocytosis is the reverse process and is used to secrete proteins, e.g digestive enzymes, out of the cells.

32. Endocytosis and Exocytosis

33. Endocytosis

34. Exocytosis