Chapter 7: CELL MEMBRANE

1. Chapter 7: CELL MEMBRANE Membranes = “the edge of life”

2. What must a membrane do? • let some things leave cell / enter cell • keep stuff inside or outside of cell • display cell antigens / markers • hold receptors for big molecules • be able to seal, fuse, be flexible as cell changes shape

3. When we understand the cell membrane, we will understand: • diseases like CF, diabetes • how to better design drug delivery systems • transplants without dangerous immunosuppressive drugs • how to better design anesthetics

4. CELL MEMBRANE STRUCTURE: • The lipid layer that forms the foundation of cell membranes is composed of molecules called PHOSPHOLIPIDS. • Recall: • one end is strongly nonpolar (hydrophobic); • one end is extremely polar (hydrophilic)

5. Result of phospholipid structure: cell membrane forms spontaneously in water  LIPID BILAYER • nonpolar lipid “tails” are repelled by polar water molecules; the polar “heads” of the molecules form hydrogen bonds with water molecules. • Protein channels penetrate the lipid bilayer & prevent complete impermeability

6. Lipid bilayer membranes are: • PERMEABLE to: • lipids • nonpolar molecules: O2, CO2 • small polar molecules: H2O • IMPERMEABLE to: • ions (Na+, K+, Cl-) • large polar molecules: sugars, proteins

7. 3 Main Types… • To allow things to pass through the membrane, and to allow the cell to communicate with its environment, there are 3 main types of proteins, which are embedded within the cell membrane: • Channels • Receptor proteins • Cell surface markers

8. 1. CHANNELS: • a given channel will transport only certain kinds of molecules...which gives the cell membrane its selectively permeable nature

9. 2. RECEPTOR PROTEINS: • collect & transmit information from the cell’s environment; binds a molecule on the outer surface of the cell (i.e. a hormone) which causes a change on the other end of the protein on the inner surface of the cell; this triggers a response within the cell **LIGAND: molecule that binds to specific receptor site on another molecule

12. 3. CELL SURFACE MARKERS: • many proteins (or lipids in some cases) have carbohydrate “flags” (usually <15 monomers) attached to the exterior end of the protein; identify your body‘s cells as belonging to you • useful in cell-cell recognition (sorting of animal embryo’s cells into tissues and organs; rejection of foreign cells by the immune system)

13. “glyco” = carbohydrate, so: *glycoprotein = protein w/ carbo chain *glycolipid = lipid w/ carbo chain

14. CHOLESTEROL: • found in cell membranes of eukaryotes; makes the membrane: 1)less fluid at warmer temps. 2) more fluid at lower temps.

15. Ch. 7: Transport Across the Cell Membrane

16. Selectively permeable: • property of biological membranes which allows some substances to pass more easily than others Selectively Permeable Membrane Water molecule: can pass through pore Protein: too big to pass through Enlargement of Membrane

17. Transport proteins: • membrane proteins that transport specific molecules or ions across biological membranes: • may provide hydrophilic tunnel through membrane • may bind to a substance and physically move it across the membrane • are specific for the substance they move

18. GLUCOSE Binding Recovery Transport Dissociation

19. Movement across the cell membrane can be: • Passive • cell does not have to expend energy • Active • energy-requiring process during which a transport protein pumps a molecule across a membrane, against its conc. gradient; is energetically “uphill”

20. Passive Transport: DIFFUSION • net movement of a substance down a concentration gradient • results from KE of molecules • results from random molecular movement • continues until equilibrium is reached (molecules continue to move but there is no net directional movement) Animation

21. Passive Transport: OSMOSIS • diffusion of water across a selectively permeable membrane; water moves down its concentration gradient • continues until equil. is reached • at equil. water molecules move in both directions at same rate

22. OUTSIDE THE CELL INSIDE THE CELL

24. relationship between solute concentration and movement of water can be described as: • HYPERTONIC: a solution w/a greater solute concentration than that inside a cell • HYPOTONIC: a solution w/a lower solute concentration compared to that inside a cell • ISOTONIC: a solution w/an equal solute concentration compared to that inside a cell Animation

26. Osmotic potential: potential for water to move from an area based on the amount of solute present in solution • osmotic potential of pure water = 0 • osmotic potential of a solution is proportional to solute concentration • more solute = lower osmotic potential • water moves from HI  LO water concentration

28. In cells with cell walls: • in a hypertonic environment, PLASMOLYSIS occurs; cells shrivel and usually die • in a hypotonic environment, water moves into cell, causing it to swell; cell becomes more TURGID.