Chapter 5: Biological Membranes

1. Chapter 5: Biological Membranes AP Biology Chapter 5

2. Plasma Membrane Functions: • separates the cell's insides from outside • regulates passage of materials into/out of cell • transmitting signals and info. between the cell and environment • participates in chemical reactions • essential part of energy transfer and storage systems

3. Composition: - lipid bilayer - proteins - in constant motion

4. Membrane Proteins • *Important area of research: how membrane proteins function in health/disease • many are enzymes • function in transport of materials/information • connect cells together to form tissues

5. PhospholipidBilayer • Phospholipid: 2 fatty acid chains linked to a glycerol molecule - nonpolar, hydrophobic ends (fatty acids) - hydrophilic ends (phosphate group) - hydrophobic "tails" turn to inside of membrane - hydrophilic "heads" turn to outer ends of membrane • - amphipathic - have distinct hydrophilic and hydrophobic regions

7. PhospholipidBilayer • Hydrogen bonds form between the phospholipid "heads" and the watery environment inside and outside of the cell • Hydrophobic interactions force the "tails" to face inward • Phospholipids are not bonded to each other, which makes the double layer fluid

9. Fluid Mosaic Model • Cell membrane consists of a fluid bilayer of phospholipid molecules in which proteins are embedded - like the tiles in a mosaic picture • NOT static - proteins can move • p.107 diagram of fluid mosaic model

10. Membrane Proteins • Functions: • Transport of small molecules • Enzymes • Information transfer • Identification tags - allow for cell-cell recognition

11. Membrane Proteins • Integral Proteins - firmly bound to the membrane, usually do not extend all the way through • amphipathic • hydrophilic regions extend out of the cell or into cytoplasm - hydrophobic regions interact with fatty acid tails of the phospholipids

13. Membrane Proteins • Transmembrane Proteins - extend through the membrane, also amphipathic

14. Membrane Proteins • Peripheral Proteins - not embedded in the lipid bilayer • located on the inner or outer surfaces of the plasma membrane • can be removed from the membrane without disrupting the structure

15. Cell Membrane is Selectively Permeable • Most membranes are permeable to small molecules and lipid-soluble or polar molecules • Water molecules may pass through the lipid bilayer • gases: such as O2 and CO2

17. Transport Across Membranes • Passive Transport - does not require energy, moves with the concentration gradient • Active Transport - requires ATP, moves against the concentration gradient

18. Simple Diffusion • Process based on random motion • Particles move down concentration gradient -from an area of high concentration to low concentration • can occur rapidly • occurs until equilibrium is reached

20. Types of Diffusion: • Osmosis - diffusion of water across a selectively permeable membrane • p.114 Figure 5-11 • Dialysis - diffusion of a solute across a selectively permeable membrane • p.113 Figure 5-10

21. High H2O potential Low solute concentration Low H2O potential High solute concentration

22. Osmotic Pressure • the tendency of water to move into a solution by osmosis • solution with high solute concentration, low water, has a high osmotic pressure • solution with a low solute concentration, high water, low osmotic pressure

23. Isotonic - equal solute concentration • ex. blood plasma isotonic to blood cells • Hypertonic - higher solute concentration - if a cell is placed in a hypertonic environment, water will leave the cell, the cell shrinks - plasmolysis occurs: plasma membrane separates from cell wall • Hypotonic - lower solute concentration - a cell placed in a hypotonic environment will gain water, swell, and possibly burst

26. Turgor Pressure • Turgor Pressure - internal pressure of cells with cell walls • Plants, Algae, and Bacteria • Enables them to withstand a low solute concentration outside the cell • Cell is hypertonic to environment • Water moves into cell, cell swells, building pressure -> turgor pressure against cell wall • Cell does not burst b/c of cell wall, resist stretching and water molecules must stop moving into the cell

27. Carrier-Mediated Transport • Membrane proteins move ions or molecules across a membrane • 2 types: 1. facilitated diffusion (passive) 2. carrier-mediated active transport

28. Carrier-Mediated Transport • Facilitated diffusion - with concentration gradient, requires transport protein • energy comes from concentration gradient • ex. glucose permease - transports glucose into red blood cells

30. Carrier-Mediated Active Transport • Carrier-mediated Active Transport - movement of solutes across membrane against concentration gradient • particles must be "pumped" from region or low conc. to region of high conc. • requires energy source - ATP, and transport protein • ex. sodium-potassium pump: in all animal cells, pump sodium ions out of cell and potassium ions into cell

31. Other Types of Active Transport • large particles such as food, cell parts • requires ATP • Endocytosis and Exocytosis • Active Transportvideo clip

32. Exocytosis • Cell ejects waste products or hormones by the fusion of a vesicle with the plasma membrane • Vesicle releases contents from the cell

33. Endocytosis • Materials are taken into the cell • Types: phagocytosis, pinocytosis • Endocytosis Video clip

34. Phagocytosis • "cell eating" • cell ingests large solid particles such as bacteria and food • ex. protists, white blood cells • plasma membrane folds enclose particle, forms a vacuole, fuses, then enters the cell and fuses with lysosomes

35. Pinocytosis • "cell drinking" • cell takes in dissolved materials • droplets of fluid are trapped by folds in the membrane • pinch off into the cytosol as tiny vesicles • liquid is slowly transferred into the cytosol • vesicles become smaller, then disappear