Biological Membranes

1. Biological Membranes Chapter 5

2. Fluid Mosaic Model • Amphipathic molecules: nonpolar, hydrophobic portion (2 fatty acid chains in a phospholipid) linked to a polar, hydrophilic portion (glycerol head in a phospholipid) • Cylindrical shape – allows the phospholipid to orient itself with tails toward the center of the bilayer and heads out • The embedded proteins in the bilayer are free to move about like icebergs floating on the sea • This can be thought of as a liquid crystal

3. Figure 5-4Page 98 Lateral movement only Time

4. Other membrane lipids: • Cholesterol – in animal cell membranes. At low temperatures it helps prevent solidifying. At higher temps it helps maintain stability • Glycolipids – carbohydrates combined with lipids; found on cell surfaces of animal cells; allow cells to recognize and interact with each other

5. In addition to phospholipids • Integral proteins – tightly bound to the membrane. Some do not extend all the way through the membrane. Transmembrane proteins do • Peripheral proteins – not embedded in the lipid bilayer and are located on the inner or outer surface • Glycoproteins – carbohydrates and proteins; on the outer surface of cells; may allow cells to adhere or provide protection

6. helix a Carbohydrate chains Glycoprotein Carbohydrate chain Extracellular fluid Hydrophobic Hydrophilic Glycolipid Cholesterol Hydrophilic Peripheral protein Integral proteins Cytosol

7. Functions of membrane proteins • Anchoring: anchor the cell to the extracellular matrix and connect to microfilaments within the cell • Passive transport: for passage of certain ions or molecules • Active transport: used to pump solutes across the membrane opposite to diffusion

8. Functions of membrane proteins… • Enzymatic activity: catalyze reactions that occur along the surface • Signal transduction: bind to signal molecules such as hormones • Cell recognition: identification tags • Intercellular junctions: attach membranes of adjacent cells

9. Passage through the cell membrane • Diffusion concepts: • Kinetic energy of all matter • Concentration gradient • Equilibrium • Cellular diffusion: • Passivetransport • Osmosis (water + plasma membrane) • Facilitated diffusion

10. Membranes are selectively permeable • Physical processes • Diffusion • Osmosis • Carrier-mediated processes • Channel proteins • Carrier proteins

11. Diffusion 1 2 3

12. Osmosis: water passes throughselectively permeable membranefrom region of higherconcentrationto lower

13. Osmosis • Isotonic • Equal solute concentration • Cell remains in equilibrium • Hypertonic • High in solute, low in water • Water moves out of cell • Cell shrinks • Hypotonic • Low in solute, high in water • Water moves into cell • Cell swells

14. Effects of osmosis • In plant cells: • Turgor pressure – cells push against rigid cell walls • Plasmolysis – cell contents pull away from cell wall  wilting • In animal cells: • Cells may swell to bursting - cytolysis • Some have contractile vacuoles to remove excess water to prevent bursting

15. (b) (c) (a) Plasma membrane Nucleus Vacuole Vacuole Vacuolar membrane (tonoplast) Plasma membrane Cytoplasm

16. Outside cell Outside cell Outside cell Inside cell Inside cell Inside cell (a) No net water movement Net water movement out of the cell Net water movement into the cell (b) (c) 10µm Isotonic solution Hypertonic solution Hypotonic solution

17. Carrier-mediated transport • Uses carrier proteins to allow substances to cross the nonpolar interior of the phospholipid bilayer • Ions, large polar molecules (glucose, amino acids) • Two types: • Facilitated diffusion – passive transport; does not require an additional source of energy; works with the concentration gradient • Carrier-mediated active transport – requires an additional energy source, usually ATP; works against the concentration gradient

18. Facilitated Diffusion • Makes use of the potential energy of the concentration gradient • As a molecule moves from a high concentration to a low concentration – energy is released • Specific carrier proteins are involved – the shape of the protein determines the solute particle it can transport and changes as it moves the particle across the membrane • This is a form of passive transport

19. Carrier-mediated active transport • For materials that the cell requires in high concentrations • The cell must ‘pump’ the material from low to high concentration – up the concentration gradient • An example is the sodium-potassium pump found in animal cells

20. Sodium-potassium pump • Uses energy in the form of ATP • Pumps two K+ ions into the cell for every three Na+ ions it pumps out • This causes an electrical as well as chemical gradient across the cell membrane – an electrochemical gradient • This gradient stores energy for the cell and can be used to help drive other transport systems

21. Na+/K+ pump

22. Transportation in vesicles • Exocytosis – cells eject wastes or specific products such as hormones • The vesicle fuses with the cell membrane • Endocytosis – extracellular materials are incorporated into the cell: • Phagocytosis • Pinocytosis • Receptor-mediated endocytosis

23. Exocytosis

24. Phagocytosis • ‘cell eating’ • large, solid materials such as other cells • White blood cells ingest whole bacteria

25. Phagocytosis

26. Pinocytosis • ‘cell drinking’ • Droplets of fluid collect in a fold of the plasma membrane which pinches off into the cytosol in a vesicle • Contents of the vesicle diffuse into the cytosol and the vesicle shrinks

27. Pinocytosis

28. Receptor-mediated endocytosis • Specific molecules combine with receptor molecules on the plasma membrane • Cholesterol is taken up this way

29. Receptor-mediated endocytosis