Ch. 8 Membrane Structure and Function (Ch. 36 pgs. 750-752)

1. Ch. 8 Membrane Structure and Function(Ch. 36 pgs. 750-752)

2. Structure • Selectively Permeable • Fluid Mosaic model • The components of the membrane move laterally through membrane • Phospholipids move rapidly(length of bacterial cell every second) • Proteins move slower due to size • Some proteins may be controlled by microtubules in cytoskeleton • Some are held still by cytoskeleton • Composed mainly of phospholipids • Amphipathic molecule-hydrophobic and hydrophilic • Contain glycerol, phosphate group, 2 fatty acid tails

3. Proteins-typically amphipathic • Determine function of cell • Each cell has unique membrane proteins • Ex. RBC’s have up to 50 different type of membrane proteins • Integral Proteins • Penetrate into or through (transmembrane) the hydrophobic region • Peripheral Proteins • Bound to the surface of the membrane or to the integral proteins

5. Carbohydrates • Important for cell to cell recognition and distinction • Tissue recognition (A, B, AB, O blood), foreign cells, organs. • Many are oligosaccharides-short 15 sugar polysaccharides • Some glycolipids • Most often are glycoproteins • Typically only found on the external surface of the plasma membrane

6. Function • Any hydrophobic molecules can dissolve in the lipid bilayer and cross it (O2 and CO2) • Not ions and polar molecules (hydrophilic) • Includes sugars or water • Transport Proteins • Hydrophilic channels, or move them using ATP • Each are specific for a certain substance (or class) • Diffusion • The ability of a substance to spread into available space • Molecules move down the concentration gradient • Areas of higher concentrations to areas of lower concentrations • Cause a net movement of molecules • Molecules continue to diffuse in dynamic equilibrium (no net) • Each molecule will diffuse down its own gradient not affected by other molecule concentrations • Molecules can diffuse across a membrane (O2 and CO2) • Passive transport-no energy required

7. Facilitated Diffusion • Diffusion across a membrane with the help of a transport proteins • Can only perform at a X rate, and therefore can reach maximum rate easily • Can be inhibited • Channel Proteins-open corridor for passage of specific molecules • Aquaporins-allow for diffusion of water across a membrane • Gated Channels Proteins-a stimulus is needed to open or close the gate (electrical or chemical) • Na+ or K+ channels in nerve cells • Others are not gates at all, but undergo change in shape to allow passage of molecules • Still always down the concentration gradient (passive)

9. Active Transport • Against the concentration gradient (uses energy) • Uses transport proteins • Helps to keep high concentrations or low concentrations • Sodium/potassium pump (Na/K pump) • Membrane potential is -50 to -200 millivolts (inside of cell is negative) • This causes a chemical and electrical force driving molecules across the membrane (electrochemical gradient) • Lets look at Na+ rushing in • The pump transfers 3 Na+ for 2 K+ • Electrogenic (proton) pump-generates voltage across a membrane and therefore stores energy • Co-Transport • Links the diffusion of one molecule with the transport of another against its concentration • Plants use a H+ pump to help co-transport sucrose in

11. Transportation of large molecules across the membrane • Exocytosis • Vesicles from the golgi apparatus fuse to cell membrane • Used to secrete insulin by pancreas, neurotransmitter by nerves • Endocytosis • Phagocytosis • Cell eating • Wraps pseudopods around substance • Forms a vacuole that connets to a lysosome • Pinocytosis • Cell drinking • Forms smaller vesicles than phagocytosis • Engulfs some extracellular fluid • Unspecific • Receptor-mediated endocytosis • Specific • Extracellular substance binds to ligand before being engulfed • Can engulf large quantities of unconcentrated substances

13. Osmosis • Diffusion of water across a membrane due to a concentration gradient. • You must determine the total solute concentration • It does not matter what the solute is. • Typically you will need to know Molar concentrations • Occurs in cells because the membrane is permeable to water because of transport proteins (facilitated diffusion) • Passive Transport • Terms • Hypertonic-solution with higher concentration • If a cell is placed in a hypertonic solution, it will shrivel and die • Hypotonic-solution with lower concentration • If I move a cell to a hypotonic solution, it will swell and burst • Isotonic-solution with equal concentration • In an isotonic solution osmosis still occurs, but there is no net movement of water

14. Osmoregulation-animals ability to control water balance • Paramecium have contractile vacuoles • Kidneys • Plant cells • Hypotonic solutions will not cause the plant cell to burst • Due to the cell wall • Turger pressure-the back pressure exerted on the cell by the water (the cell is Turgid) • This is the preferred for a plant • Isotonic solutions will cause the plant to wilt (become flaccid) • Hypertonic solution cause the cell membrane to pull away from the wall • Called Plasmolysis • Cause the plant to die

15. Water potential (pgs 750-752)-psi or ψ • Potential refers to potential energy • By putting a cell in a solution that has a higher water potential, the cell will grow. • Takes pressure and solute concentration in account for osmosis • Ψ= ψs + ψp • s=solute concentration, p=pressure exerted on the solution • Measured in megapascals (MPa)(10 atmospheres of pressure) or bars (1 atmosphere of pressure) • Pure water has a ψ of zero • The more solute the more negative the potential is • The higher the concentration, the lower the water potential • Water will move from area of higher potential to areas of lower potential • cell filled with solute-low water potential (negative) • Pure water is zero, so it has the highest water potential • Pressure is directly proportional • The higher the pressure, the higher the potential

16. Solute concentration is either zero or negative • Water potential can be positive or negative based on the pressure • If there is no pressure on the system the water potential equals the solute concentration

17. If I had a solution with ψs of -2.4 and a solution with a ψs of -3.6 separated by a selectively permeable membrane, on which side and how much pressure would I have to exert to keep the chamber at equilibrium? • You must consider the water potential of the cell and the solution to determine the net flow of water in a living system • What would happen to a cell that had a ψ of -8 and was placed in a solution with a ψ of -6 • Cell will grow

18. Calculating Solute Potential • Ψs = -iCRT • i=ionization constant • C=molar concentration (moles/liter) • R=pressure constant (.o831 liter bars/mole degrees K) • T= temp degrees K (273+ degrees Celsius) • Ψs of a .3 molar solution of sucrose at 22 degrees Celsius is………… • -1 X .3 X .0831 X 295= -7.35 bars • Ψs of a .3 molar solution of NaCl at 22 degrees Celsius is ……… • -2 X .3 X .0831 X 295= -14.71 bars • What would the ψs potential be of a .1 molar solution of glucose at standard temperature and pressure? • What would happen to a cell if it had a .4 molar concentration of sucrose and it was placed in a .3 molar concentration of glucose? • What if that cell was placed in a .3 molar concentration of NaCl? • How much pressure would I have to exert on a .3 molar solution of glucose to keep it at equilibrium with pure water at 22 degrees Celsius? • If I put a cell with a ψ -12 in a .6 molar solution of sucrose, what will happen?

19. If I placed a cell with a .6 molar solution of sucrose under 2 bars of pressure and placed it in a beaker with a .2 molar solution of NaCl under 5 bars of pressure both at 28 degrees Celsius. What will happen to the cell? • Any system that has sat, will be at equilibrium and the water potential will be the same for both the cell and solution. • If one solution is pure water, the solution of pure water will be completely gone (or the cell will burst). • In a plant cell with pure water, the turger pressure keeps the water potential at zero.