1. MEMBRANE STRUCTURE AND FUNCTION CH 7

2. I. The membrane is a fluid mosaic A. The phospholipid bilayer Composed of two layers of phospholipids hydrophobic tails face in, polar heads face out prevents soluble material from passing across the membrane and allows for diffusion of small nonpolar molecules across the membrane

4. B. proteins partially or completely span the membrane

5. 6 main functions of membrane proteins – Transport – Enzymes – Cell recognition – Cell communication – Cell anchorage – Attach to cytoskeleton and extracellular matrix

7. C. Membranes are fluid To allow efficient transport across the membrane

8. 1.Factors that affect fluidity The amount of unsaturated fatty acids – Unsaturated fatty acids have carbon-to-carbon double bonds – Double bonds form kinks – Kinks reduce hydrophobic interactions between tails – Less hydrophobic interactions the more fluid Temperature – The higher the temp the more fluid the membrane

9. Amount of cholesterol – Animal membranes – At high temps it inhibits mobility – At low temps it maintains fluidity by preventing tight packing of phospholipids

10. 2. Variations in lipid composition of cell membranes Membranes of different species can have different lipid compositions due to different environments This is an adaptation Many species that live where temps vary can alter the lipid composition of their membranes

11. II. Membranes are Selectively Permeable They regulate what enters and leaves Hydrophobic molecules can easily slip thru the bilayer (gases, steroids) Polar molecules and large molecules can’t cross as easily (sugar, ions, proteins) Hydrophobic molecules slip thru bilayer Smaller hydrophilic molecules must pass thru transport proteins

12. III. Mechanisms of transport across the membrane A. Passive transport general features – high to low concentration (down the concentration gradient) – no energy required – Types: Diffusion Osmosis Facilitated diffusion

13. 1. Diffusion Movement of small nonpolar molecules across the phospholipid bilayer Down the gradient No energy Ex: O2 into cells CO2 out of cells. Only photosynthetic cells go both ways

14. 2. facilitated diffusion movement of slightly larger molecules through transport proteins down the gradient no energy

15. as solute concentration increases, rate of transport increases, till all transport proteins are saturated and rate levels off

17. 3. osmosis movement of WATER across the phospholipid bilayer thru aquaporins no energy down the gradient (hypotonic to hypertonic; high water potential to low water potential) a hypotonic solution has more water and less solute than a hypertonic solution

19. Water potential = solute potential + pressure potential (Ψ=Ψs +Ψp) – Water potential: tendency for water to move across a membrane from higher water potential to lower water potential – In a living cell it is zero or negative – solute potential results from the presence of solutes and is always negative (Ψs = -iCRT) – the greater the solute concentration (C) the smaller the solute potential – pressure potential is zero unless a force (like turgor pressure from cell wall) is applied. In plant cells as water flows in Ψp becomes positive – therefore Ψ=Ψs – the higher the concentration of solute, the lower the water potential and the greater the tendency of water to move

20. If a cell is placed in a hypotonic solution it will _________________ If a cell is placed in a hypertonic solution it will _________________ http://www.northland.c c.mn.us/biology/biology 1111/animations/active1.swf

21. B. Active transport Movement of ions UP the concentration gradient (low to high) thru transport protein specific to ion requires the expenditure of energy The energy changes is the shape of the transport protein so ions can be transported

22. http://www.northland.cc. mn.us/biology/biology11 11/animations/active1.s wf

23. Many active transport pumps are electrogenic pumps – They help maintain membrane potential by generating voltage across the membrane – Voltage is due to differences in positive and negative charges across the membrane – Electrogenic pumps store energy for cellular work Proton pump in ATP synthesis Na+/K+ pump in nerve impulse transission

24. C. Bulk movement Transport of large molecules across the membrane in vesicles using energy 1. Endocytosis – movement of material into the cell – stuff ingested is engulfed in a vesicle – The vesicles from endocytosis fuse with lysosomes for digestion

26. 2. Exocytosis Movement of large molecules out of cell thru vesicles especially proteins The vesicles with proteins to be secreted by exocytosis came from RER and Golgi