1. 4-1 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Chapter 4: Movement across membranes

2. 4-2 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Functions of membrane Plasma membranes control the passage of substances into and out of a cell –maintain stable conditions inside cell  homeostasis –membranes also control movement in and out of organelles Permeability of molecules depends on –size –electrical charge –lipid solubility

3. 4-3 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Diffusion of solutes in water Diffusion is the passive movement of molecules along a concentration gradient –high concentration → low concentration Diffusion of certain substances occurs across membranes –O 2, CO 2, alcohol Rate of diffusion depends on –permeability –magnitude of concentration gradient

4. 4-4 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Diffusion of ions Diffusion of ions occurs along an electrochemical gradient Electrochemical gradient is combination of –chemical gradient  high to low concentration –electrical gradient  difference in charge across membrane (opposites attract, like repels) Direction of net passive movement depends on relative strength of these two gradients

5. 4-5 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 4.2: Concentration and electrical gradients

6. 4-6 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Membrane transporters Membrane transporters accelerate the movement of less permeable molecules across membranes Transport proteins are specific to one or a small number of solutes Rate of transport across membrane depends on number of transport proteins –rate levels off if all transport proteins are occupied (cont.)

7. 4-7 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Membrane transporters (cont.) Transport proteins assist movement of molecules down concentration gradient through facilitated diffusion –requires no energy input Channels –conduits allow direct passage from one side of the membrane to the other Carriers –binding of solute on one side of membrane produces conformational change in protein moving solute through

8. 4-8 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Channels Channels allow molecules to move in or out of cells rapidly –faster than carriers, which bind and release solutes Most channels transport ions –high specificity  calcium channels  potassium channels  chloride channels

9. 4-9 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 4.5: Channels and carriers

10. 4-10 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Gating mechanisms Channels have open and closed states Channels are opened and closed by signals –voltage-gated channels  respond to changes in voltage across membranes –ligand-gated channels  activated by binding of specific molecules (ligands) –mechanically-gated channels  respond to physical disturbance

11. 4-11 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Facilitated transport Properties of facilitated transport that distinguish it from simple diffusion –transport is faster –transport proteins become saturated as substrate concentration increases –transport proteins are specific for particular substrates or types of substrate –transport is inhibited by similar substrates that compete for the binding site

12. 4-12 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Aquaporins Aquaporins are membrane-spanning proteins that allow water and urea to diffuse across membranes Tissues with high water permeability have high concentrations of aquaporins in their cell plasma membranes Concentration of some aquaporins in plasma membranes can be controlled by hormones –ACTH increases deposition of aquaporins in cell membranes of kidney collecting tubules

13. 4-13 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Active transport Active transport uses energy to move solute against electrochemical gradient Transport ATPases –direct pumping coupled to ATP hydrolysis Co-transport –diffusion of one molecule down its electrochemical gradient is used to pump a second molecule against its electrochemical gradient (cont.)

14. 4-14 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Active transport (cont.) Plant cells –H + pumped from cell by H + ATPase creates electrochemical gradient for inward movement of H + –inward movement of H + drives the pumping of other solutes against their electrochemical gradients Animal cells –gradients of Na + established by Na + –K + ATPase are used for active transport of solutes

15. 4-15 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Voltage differences Difference in voltage across membrane is membrane potential Membrane potential due to –negative charge of proteins and other polymers inside cell –transport of ions across plasma membrane Removal of positive ions from cell increases negative charge inside cell –animal cells c. –50 to –80 mV –plant cells –200 mV (cont.)

16. 4-16 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 4.8: Na + –K + ATPase

17. 4-17 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Voltage differences (cont.) Functions of membrane potentials –provide favourable electrochemical gradient for passive uptake of cations –inward electrochemical gradient can be used in active transport of other ions –changes can be used as signals, either locally or for transmission between cells

18. 4-18 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Osmosis Water travels passively across membranes by osmosis –higher free energy → lower free energy Free energy levels determined by –dissolved solutes –physical pressure (tension) Free energy of water decreased by –solutes –reduced pressure (cont.)

19. 4-19 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 4.9: Diffusion

20. 4-20 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Osmosis (cont.) Water flows across a membrane towards a region of highest solute concentration (= lowest water free energy) –in plants, pressure is also important as cell walls are rigid Overall free energy of water is water potential ( Ψ ) –sum of  osmotic potential ( Ψ π )  pressure potential ( Ψ P ) Ψ = Ψ π + Ψ P (cont.)

21. 4-21 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Osmosis (cont.) Osmosis is the net passive movement of water from a region of higher water potential to one of lower water potential through a selectively permeable membrane Iso-osmotic solutions –have same solute concentration (same osmotic potential) Hyperosmotic solutions –more concentrated Hypo-osmotic solutions –less concentrated (cont.)

22. 4-22 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 4.10a: Animal cells

23. 4-23 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 4.10b: Plant cells

24. 4-24 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Osmosis (cont.) Differences in water potential across the plasma membrane of cells without rigid walls are due only to differences in osmotic potential (concentration of solutes) In cells with a rigid wall, osmotic potential and pressure potential contribute to water potential –water enters when cells are placed in solution with less negative water potential –can only expand by c. 10 per cent before pressure (turgor) causes intracellular water potential to equal that of external solution

25. 4-25 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Turgor Shape of plants maintained by cell turgor Water continually moves from cells to atmosphere, which has very low water potential If lost water is not replaced, pressure potential (and turgor) is reduced

26. 4-26 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Vesicle-mediated transport Large molecules transported in membrane-bound vesicles Endocytosis –plasma membrane encloses substances outside cell –pinches off to form vesicle  phagocytosis (solids)  pinocytosis (fluids) –invagination may be receptor-mediated Exocytosis –intracellular vesicles fuse to plasma membrane and release contents to outside