1. Chapter 7 – Membrane Structure and Function

2. Membrane Structure Selectively permeable
Phospholipids are amphipathic – has both a hydrophobic and a hydrophilic region
Fluid mosaic model by Singer and Nicolson; proteins embedded in and moving throughout a fluid-like phospholipid bilayer

3. Membrane Structure

4. Membrane Fluidity Phospholipids Fluidity Cholesterol Lateral movement
Flip-flop movement
Fluidity
Unsaturated fatty acids = fluid
Saturated fatty acids = viscous
Cholesterol
Reduces fluidity at moderate temps
Hinders solidification at low temps

5. Do proteins move? Most membrane proteins are amphipathic
may move laterally, but not flip-flop
Some are virtually immobile, held in place by cytoskeletal attachments
Experiment to show movement:

6. Plasma Membrane Detailed

7. Membrane Proteins
Transport – hydrophilic channel, substrate specific, actively pump substances
Enzymes – can act in tandem
Signal transduction – relays an external signal (hormone) to inside of cell
Cell-cell recognition – gylocoprotein ID tags
Intercellular joining – gap or tight junctions
Attachment – maintain cell shape, stabilizes location of proteins, coordinate extracellular and intracellular changes

8. Membrane Permeability
Diffusion – tendency for molecules to spread out evenly into the available space
Concentration gradient
Passive transport – diffusion of a substance across a biological membrane

9. Diffusion

10. Osmosis The diffusion of water across a selectively permeable membrane
Tonicity – cells ability to gain/lose water

11. Osmosis

12. Water Balance Cells without walls Cells with walls
Isotonic – equal conc. of solutes
Hypertonic – higher conc. of solutes
Hypotonic – lower conc. of solutes
Osmoregulation – the control of water balance, example contractile vacule
Cells with walls
Turgid – very firm
Flaccid – limp
Plasmolysis – plasma membrane pulls away from wall

13. Osmoregulation

14. Plasmolysis

15. Turgid

16. Facilitated Diffusion
Movement of substances across a membrane with the aid of protein channels; no energy required
Channel proteins
Carrier proteins – alternates between 2 conformations

17. Active Transport
Movement of a substance against its concentration gradient with the help of cellular energy
Example: sodium/potassium pump

18. Active Transport

19. Bulk transport
Exocytosis – secretion of macromolecules by the fusion of vesicles with the plasma membrane
Endocytosis – import of macromolecules by forming new vesicles with the plasma membrane
Phagocytosis “cellular eating”
Pinocytosis “cellular drinking”
Receptor-mediated endocytosis (ligands), example cholesterol

20. Exocytosis/Endocytosis

21. Phagocytosis

22. Pinocytosis

23. Receptor Mediated Endocytosis