1. Membrane Structure and Function Chapter 7 A. P. Biology

2. Plasma Membrane Also called the plasmalemma. Plasma Membrane = Phospholipid Bilayer + Transmembrane Proteins + Supporting Fibers + Glycoproteins and Glycolipids

3. Scientists studying the plasma membrane –Reasoned that it must be a phospholipid bilayer Figure 7.2 Hydrophilic head Hydrophobic tail WATER

4. Phospholipid Bilayer Glycerol + 2 Fatty Acids + Phosphorylated Alcohol = Phospholipid Hydrophilic or Polar Region = Phosphate Hydrophobic or Nonpolar region = Fatty Acids

5. The Davson-Danielli sandwich model of membrane structure –Stated that the membrane was made up of a phospholipid bilayer sandwiched between two protein layers. –Was supported by electron microscope pictures of membranes

6. In 1972, Singer and Nicolson –Proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer Figure 7.3 Phospholipid bilayer Hydrophobic region of protein Hydrophobic region of protein

7. Freeze-fracture studies of the plasma membrane –Supported the fluid mosaic model of membrane structure Figure 7.4 A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers. Extracellular layerCytoplasmic layer APPLICATION A cell membrane can be split into its two layers, revealing the ultrastructure of the membrane’s interior. TECHNIQUE Extracellular layer Proteins Cytoplasmic layer Knife Plasma membrane These SEMs show membrane proteins (the “bumps”) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer. RESULTS

10. Lipid Bilayer Nonpolar interior prevents passage of water-soluble, polar compounds. Only very small, uncharged molecules like O 2 and H 2 O can enter through the lipid bilayer. Also, allows nonpolar compounds to freely enter.

11. The Fluidity of Membranes Phospholipids in the plasma membrane –Can move within the bilayer Figure 7.5 A Lateral movement (~10 7 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids

12. The type of hydrocarbon tails in phospholipids –Affects the fluidity of the plasma membrane Figure 7.5 B FluidViscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails (b) Membrane fluidity

13. The steroid cholesterol –Has different effects on membrane fluidity at different temperatures Figure 7.5 (c) Cholesterol within the animal cell membrane Cholesterol

14. Figure 7.7 Glycoprotein Carbohydrate Microfilaments of cytoskeleton Cholesterol Peripheral protein Integral protein CYTOPLASMIC SIDE OF MEMBRANE EXTRACELLULAR SIDE OF MEMBRANE Glycolipid Membrane Proteins and Their Functions A membrane –Is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Fibers of extracellular matrix (ECM)

15. Lipid Bilayer is Fluid Fluid = Moving, Dynamic. Each lipid can rotate, move laterally Fluidity depends on temperature and type of fatty acid used. Unsaturated fatty acids are more fluid. Fluid Mosaic Model

16. Transmembrane Proteins (Integral Proteins) Part of the protein that extends through the bilayer is nonpolar (several nonpolar amino acids in this region). Usually is an alpha helix or beta barrel. Used to anchor protein in the membrane. Beta-barrels = form a pore and are called a porin protein.

18. Integral proteins –Penetrate the hydrophobic core of the lipid bilayer –Are often transmembrane proteins, completely spanning the membrane EXTRACELLULAR SIDE Figure 7.8 N-terminus C-terminus  Helix CYTOPLASMIC SIDE Extracellular Side

21. An overview of six major functions of membrane proteins Figure 7.9 Transport Enzymatic activity Signal transduction (a) (b) (c) ATP Enzymes Signal Receptor

22. Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM). (d) (e) (f) Glyco- protein Figure 7.9

23. Transmembrane Proteins Channels = passive transport of molecules across membrane. Carriers = transport of molecules against the gradient. Receptors = transmit information into the cell. Cell Adhesion Proteins = connect cells to each other. Cytoskeleton Attachment Proteins = to attach actin.

24. Membrane Receptors Conduct Signals

26. Integral Proteins Laterally Diffuse in the Membrane

28. Movement Across the Membrane Diffusion = random motion of molecules that causes a net movement from areas of high concentration to areas of low concentration. Osmosis = diffusion of water across a selectively permeable membrane.

29. Passive Diffusion

30. Factors that Affect the Direction of Diffusion The concentration gradient; High  Low. Temperature; High heat  Low Heat. Pressure; High Pressure  Low Pressure

31. Factors that Affect the Rate of Diffusion The steepness of the gradient. The molecular weight of the solute.

32. Concentrations Osmotic concentrations = concentrations of all solutes in a solution. If unequal concentrations… Hyperosmotic = solution with the higher solute concentration. Hypoosmotic = solution with the lower solute concentration. Isosmotic = solutions with the same osmotic or solute concentration.

33. Plasmolyzes Crenate

34. Osmotic Pressure If a cell’s cytoplasm is hyperosmotic to the extracellular fluid, then water diffuses into the cell and it swells. Pressure of the cytoplasm pushing out against the membrane- hydrostatic pressure. Osmotic pressure is the pressure needed to stop the osmotic movement of water across a membrane.

35. How do living things maintain osmotic balance? Some oceanic eukaryotes adjust internal [solutes]- they are isosmotic. Animals – circulate an isosmotic fluid around their cells. Must constantly monitor the fluid’s [solute] Ex. Humans secrete albumin into the plasma to match the body cells.

36. Protozoa- are hyperosmotic, so use extrusion to remove excess water; may have special organelles-contractile vacuoles. Plants- are hyperosmotic, but do not circulate an isosmotic solution; are usually under osmotic pressure- turgor pressure-presses the plasma membrane against the cell wall.

39. Water balance in cells with walls Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environ- ment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. (b) H2OH2OH2OH2O H2OH2O H2OH2O Turgid (normal)Flaccid Plasmolyzed Figure 7.13

41. Bulk Movement through Membranes Endocytosis- the cytoskeleton extends the membrane outward toward food particles. Bulk transport into cell. Extended membrane encircles the particle, fuses with itself, and contracts. Forms a vesicle around particle.

43. Three Kinds of Endocytosis Phagocytosis - “cell eating”- large, amounts of organic material;white blood cells and protists. Pinocytosis- “cell drinking”- liquid material brought into cell; mammalian ova and follicle cells. Receptor-mediated Endocytosis- use receptors in the membrane for specific transport into cell.

44. Receptor-Mediated Endocytosis Have indentations on the plasma membrane. Indentations = are clathrin-coated pits. Pits have receptor proteins on the extracellular side = trigger When receptor binds to target molecule, clathrin proteins on the cytoplasmic side begins endocytosis. Forms a clathrin -coated vesicle. Very specific.

45. Receptor-mediated Endocytosis

46. Exocytosis The release of material from vesicles at the cell surface. Examples: protists using a contractile vacuole to release water, gland cells secreting hormones, neurons releasing neurotransmitters.

48. Secretion Vesicles

50. Exocytosis and Neurons

51. Problems with Bulk Transport Endocytosis and Exocytosis are energy-intensive. Not highly selective.

52. Channel \Proteins –Provide corridors that allow a specific molecule or ion to cross the membrane Figure 7.15 EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM A channel protein (purple) has a channel through which water molecules or a specific solute can pass. (a)

53. Selectively Permeable Transport Channels- proteins in the cell membrane that transport specific ions into and out of the cell. Water-filled pores span the membrane. Ions do not interact with the channel protein. Diffusion is passive, [high] --> [low].

54. Channels are somewhat selective

56. Selectively Permeable Transport Carriers- proteins that transport specific ions, sugars, and amino acids into and out of the cell. The proteins facilitate movement by binding to the solute-facilitated diffusion. The proteins bind to the solute on one side of the membrane and release them on the other.

57. Uniport Facilitated Diffusion

58. Facilitated Diffusion It is specific, only certain molecules transported by a given carrier. It is passive, net movement is [high]-->[low]. It may become saturated if all protein carriers are occupied.

59. Active Transport A method of transporting specific ions, sugars, amino acids, nucleotides against conc. gradient. Involves protein carriers in the membrane and energy (ATP). How cells accumulate molecules internally.

60. Active Transport

61. The Sodium-Potassium Pump Is one type of active transport system

62. An Example of Active Transport Sodium-Potassium Pump- Active transport of Na + and K + ions. Normally, inside the cell: the [Na+] is low the [K+] is high The cell maintains this by actively pumping Na+ out and K+ in.

63. Sodium-Potassium Pump The protein uses ATP as an energy source for this movement against the gradient [low]--> [high]. See Fig. 6.21, p. 135 for how. Uses ~1/3 of all ATP in resting cell. This pump can transport 300 Na+ ions/second. All animals use it.

64. Cotransport: active transport driven by a concentration gradient Figure 7.19 Proton pump Sucrose-H + cotransporter Diffusion of H + Sucrose ATP H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ + + + + + + – – – – – –

65. Cotransport and Countertransport Many amino acids and sugars are transported into the cell through coupled channels. Their active transport is coupled with the movement of Na + inside the cell. [Na + ] high --> [Na + ] low into cell. [Amino acid] low--> [Amino acid] high.

67. An Electrogenic Pump –Is a transport protein that generates the voltage across a membrane Figure 7.18 EXTRACELLULAR FLUID + H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Proton pump ATP CYTOPLASM + + + + – – – – – +

68. The Proton Pump A transmembrane protein that moves H + against their concentration gradient, from [low] --> [high] outside of cells or organelles. Example: mitochondria move H+ across the inner membrane during electron transport. Energy to power this pump comes from NADH and FADH molecules.

70. Proton Pump The proton pump moves H + out of the matrix, through the inner membrane. ATP synthase channels H + back into the matrix, DOWN the gradient. PROVIDES ENERGY! ATP synthesis is coupled to H + movement. Almost all of the energy for cells is made this way.

71. Electron Transport Chain