1. MEMBRANE STRUCTURE AND FUNCTION CHAPTER 7

2. Life on the Edge  Plasma membranes serve as the boundary between the living cell and the non-living environment surrounding it  The plasma membrane controls the traffic of substances into and out of the cell  Plasma membranes are selectively permeable, that is it allows some substances to cross while restricting others  Amphipathic molecules which mean they have hydrophilic and hydrophobic regions

3. Membrane Protein Example of a transport protein 4 identical subunits bind to form a channel What level of structure is demonstrated? Which end faces out? In?

4. Developing the Membrane Model  Gorter and Grendel. 1925  Prior knowledge composed of lipids and proteins  Phospholipid bilayer  Stable boundary between 2 aqueous compartments  Hydropphobic and –philic is respective locations  Protein location unknown  Adhesion connection Pure phospholipid not as strong

5. Sandwich Model  Danielli and Davson, 1935  Protein formed two continuous layers outside of the phospholipid layers  Same for internal membranes  Problems  Different functions have different membranes  Proteins amphipathic too  Hydrophobic toward outside aqueous Protein layer

6. Fluid Mosaic Model  Singer and Nicolson, 1972  Proteins embedded irregularly throughout the membrane  Proteins with hydrophobic and –philic regions

7. Supporting the Fluid Mosaic Model  Freeze-fracture splits membrane along bilayer  Prepares for electron microscopy  Hydrophobic interactions hold together  Lipids and proteins drift laterally  Phospholipids move fast laterally, but rarely change orientation  Some proteins more slowly and directionally others not at all

8. Fluidity Influences  Temperature  Decrease temps move from liquid to solid state  Pack tighter, e.g cold bacon grease to lard  Components  High in unsaturated fatty acid tails, liquid at lower temp.  Hydrocarbon tails with kinks prevent tight packing  Cholesterol  Prevents changes from temperature fluctuations, temp. buffer Warm = maintain phospholipids; reduce fluidity by restraining phospholipid movement Cool = prevent tight packing; maintains fluidity

9. Membrane Examples

10. Membrane Proteins Differ  Determinants of membrane function  Specific to each type of plasma membrane  2 major protein types  Peripheral attach outside the cell  Integral span entire or partially through bilayer  Functions vary according to area  6 major areas

11. Protein Functions  A) Channel and carrier proteins Selective or changes shape  B) Encourages reaction  May facilitate sequential reaction steps  C) Shape for a chemical messenger  D) Glycoproteins to serve as recognition tags  E) Form junctions to hold together  F) Maintains cell shape and stabilizes

12. Cellular Movement  Plasma membrane is selectively permeable  Nutrients in and waste products out  Characteristics of cells determine movement Size Charge Polarity Types of proteins available  Movement based on concentration gradients  [solutes] vary on membrane sides  Continue till equilibrium reached No net movement

13. Transport Types  Passive transport is movement that doesn’t require energy  Moves down [gradient] Substance independent  Simple is movement from high to low concentration  Osmosis is solvent movement (water) from high concentration to low concentration  Facilitated utilizes the same process, but has channels or carriers to transport  Active transport is movement that uses ATP

14. Diffusion of Two Solutes

15. Tonicity  Ability of a solution to make a cell gain or lose water  Depends on [solutes] impermeable to membrane relative to inside of cell Hypertonic solution Hypotonic solution Isotonic solution  Effects vary in animal and plant cells  Be able to diagram effects of all 3 solutions in both cell types  Know associated terms

16. Cellular Effects  Isotonic solutions have SAME [solute]  No net movement across membrane  Animal = stable  Plant = flaccid (limp)  Hypertonic solutions have MORE [solute] than the cell  Water moves out to environment Animal = crenation or shriveling Plant = plasmolysis, so lose water and PM shrinks  Hypotonic solutions have LESS [solute] than the cell  Water moves in Animal = lyse or burst Plant = turgor pressure (central vacuole)

17. Recognizing Tonicity

18. Osmoregulation  Control of water balance  Paramecium use contractile vacuole to regulate water balance  Excess water out so cell doesn’t lyse  Plants that reside in drier climates  Central vacuole

19. Cell plasma membrane is selectively permeable to water and simple sugars (which ones?) What direction does each solute move? What type of solution is this ‘cell’ in? Practice Problem

20. Active Transport  Pumps molecules against their [gradients]  Carrier proteins responsible for this movement  Costs ATP to supply energy  Transfers a phosphate group to protein and forms ADP  Na + /K + pump in animals cells 3 Na + out and 2 K + in maintains resting state of the cell  Proton pump in plant, fungi, and bacteria cells Hydrogen ions pumped out of cell Separates charges, (+) out and (-) in Charge separation stores energy Cotransport moves solutes indirectly  Uses left over ATP from above

21. Bulk Movement  Exocytosis  Move substances out of the cell from golgi apparatus  Endocytosis  Move substances into the cell by forming new vesicles  Phagocytosis Cellular eating (engulfing solid foodstuffs)  Pinocytosis Cellular drinking (engulfing liquid stuff)  Receptor mediated Specific binding must occur first before engulfing Vessicles fuse or form from plasma membrane