Chapter 7

1. Membrane Structure and Function Chapter 7

2. Cell Membrane

3. Plasma Membrane • Boundary that separates living cell from surroundings • May have been 1 of first evolutionary steps • Exhibits selective permeability • Fluid mosaic model

4. What is selective permeability? • Allows some substances to cross it more easily than others • Encloses a solution different from the surrounding solution • Permits the uptake of nutrients and elimination of wastes

5. What makes a membrane? • Lipids and Proteins and carbohydrates • Proteins • Phospholipids are most abundant

6. What makes phospholipids unique? • They are amphipathic • Have both hydrophobic and hydrophilic regions

7. AmphipathicPhospholipids WATER Hydrophilic head Hydrophobic tail WATER

8. Hydrophilic region of protein AmphipathicProteins Phospholipid bilayer Hydrophobic region of protein

9. What is the fluid mosaic model? • Membrane is “fluid” structure • “Mosaic” because of various proteins embedded in or attached to the membrane • Bilayer – double layer due to phospholipids

10. How is the membrane fluid? • Not static sheets • Held together by hydrophobic interactions • Can shift laterally • rapid

11. How is the membrane fluid? • Can shift transversely across membrane, but it’s rare

12. Fluid Mosaic Model and Phospholipids Lateral movement (~107 times per second) Flip-flop (~ once per month) Movement of phospholipids

13. How does temp affect fluidity? • As temp decreases the phospholipids settle into a closely pack arrangement and the membrane solidifies • What role do unsaturated hydrocarbon tails play?

14. How does temp affect fluidity? • As temp decreases the phospholipids settle into a closely pack arrangement and the membrane solidifies • What role do unsaturated hydrocarbon tails play? • Membrane remains fluid to a lower temp if rich in phospholipids with unsaturated hydrocarbon tails • Why?

15. How does temp affect fluidity? • As temp decreases the phospholipids settle into a closely pack arrangement and the membrane solidifies • What role do unsaturated hydrocarbon tails play? • Membrane remains fluid to a lower temp if rich in phospholipids with unsaturated hydrocarbon tails • Why? • Because of the kinks n tails

16. How does temp affect fluidity? • As temp decreases the phospholipids settle into a closely pack arrangement and the membrane solidifies • Steroid cholesterols effect the fluidity at different temperatures

17. What are the membrane proteins? • Adds to the “mosaic” part of the model • TONS OF THEM!! • More than 50 in plasma of RBC • Proteins determine most of the membrane’s function

18. What are the membrane proteins? • Two types • Integral • Peripheral

19. What are the membrane proteins? • Two types • Integral • Penetrate the hydrophobic core of lipid bilayer • Some are transmembrane • Span the entire membrane

20. What are the membrane proteins? • Two types • Integral • Penetrate the hydrophobic core of lipid bilayer • Some are transmembrane • Span the entire membrane • Peripheral • Not embedded in lipid bilayer

21. 6 Major Functions of the Proteins of the Plasma Membrane • Transport • Enzymatic Activity • Signal Transduction • Cell-cell recognition • Intercellular Joining • Attachment to the cytoskeleton and Extracellular Matrix (ECM)

22. Signal Enzymes Membrane Protein Functions Receptor ATP Enzymatic activity Transport Signal transduction

23. Glyco- protein Membrane Protein Functions Attachment to the cytoskeleton and extra- cellular matrix (ECM) Cell-cell recognition Intercellular joining

24. 6 Major Functions of the Proteins of the Plasma Membrane • Transport • Provides a hydrophilic channel across the membrane that is selective for a particular solute • Shuttle substance from one side to the other by changing shape

25. 6 Major Functions of the Proteins of the Plasma Membrane • Transport • Enzymatic Activity • Protein may be an enzyme with its active site exposed to substances in adjacent solution • Can work as a team to carry out sequential steps

26. 6 Major Functions of the Proteins of the Plasma Membrane • Transport • Enzymatic Activity • Signal Transduction • Receptor protein may have binding site with specific shape that fits the shape of a chemical messenger • Example: hormone • External messenger may cause shape change

27. 6 Major Functions of the Proteins of the Plasma Membrane • Transport • Enzymatic Activity • Signal Transduction • Cell-cell recognition • Glycoproteins serve as identification tags that specifically recognized by membrane proteins of other cells

28. 6 Major Functions of the Proteins of the Plasma Membrane • Transport • Enzymatic Activity • Signal Transduction • Cell-cell recognition • Intercellular Joining • Membrane proteins of adjacent cells may hook together in various kinds of junctions • Example: • Gap junctions • Tight junctions

29. 6 Major Functions of the Proteins of the Plasma Membrane • Transport • Enzymatic Activity • Signal Transduction • Cell-cell recognition • Intercellular Joining • Attachment to the cytoskeleton and Extracellular Matrix (ECM) • Microfilaments may be noncovalently bound to membrane proteins • This helps maintain cell shape and stabilize location of certain membrane proteins

30. When is this important? • Sorting tissues into organ as embryo • For rejection of foreign objects by immune system

31. How is this recognition accomplished? • Cells recognize other cells by binding to surface molecules • Often carbohydrates • These carbohydrates are usually short, branched chains • 15 or few sugars

32. How is this recognition accomplished? • Carbohydrates on Extracellular side of plasma membrane vary: • from species to species • among individuals of same specie • Even from cell to cell

33. What is a glycolipid? • Carbohydrate covalently bonded to lipid

34. What is a glycoprotein? • Carbohydrate covalently bonded to protein

35. Synthesis of Membrane Proteins and Lipids

36. Synthesis of Membrane Proteins and Lipids • Synthesis of proteins and lipids in ER • Carbohydrates are added to make them glycoproteins • Carbohydrates are then modified • Inside Golgi Complex, glycoproteins undergo further carbohydrate modification. Lipids also acquire carbohydrates (glycolipids) • Transmembrane proteins, membrane glycolipids, and secretory proteins are transported in vesicle to plasma membrane • Vesicles fuse with membrane, releasing secretory proteins from the cell

37. Synthesis of Membrane Proteins and Lipids

38. Plasma Membrane is Supramolecular Structure • What is a Supramolecular Structure? • Many molecules ordered into a higher level of organization • Has emergent properties

39. Movement across the membrane • Steady movement of small molecules and ions in both directions • Sugars, amino acids, and nutrients enter cell • Waste leave cell • Regulation of inorganic ions • Movement occurs at different rates

40. Movement across the membrane • Nonpolar molecules are hydrophobic and can dissolve in the lipid bilayer of the membranes and cross it easily without membrane proteins • Hydrophobic core of membrane impedes direct passage of ions and polar molecules (hydrophilic molecules)

41. Membrane Permeability • Transport Proteins • Channel Proteins- provide a channel for hydrophilic molecules to move through. • Aquaporins- allow water to pass through the cell membrane quickly. • Carrier Proteins- bind to molecules and shuttle them across the membrane.

42. Diffusion • Diffusion- movement of molecules of any substance until they spread out evenly in the available space. (equilibrium). • Diffusion is a spontaneous process, needing no energy input. • Rule of Diffusion: in the absence of a force, a substance will diffuse from high concentration to low concentration.

43. Diffusion • A substance diffuses down its own concentration gradient, unaffected by the concentration of other substances. • Diffusion is a form of passive transport- movement that does not require the cell to use energy.

44. Osmosis • Osmosis- the diffusion of water. Water diffuses from the region of lower solute concentration (higher free water concentration) to the area of higher solute concentration (lower free water concentration)- until equilibrium is reached. • Osmosis is a method of passive transport

45. Osmosis

46. Osmosis • For dilute solution (like that found in most biological fluids), solutes don’t affect water concentration • Instead, tight clustering of water molecules around the hydrophilic solute molecules makes some of water unavailable to cross membrane • This is NOT FREE WATER

47. Osmosis • It’s FREE WATER that moves • Water moves from areas of low Solute concentration to high solute concentration

48. Osmosis • Tonicity- the ability of a surrounding solution to cause a cell to gain or lose water. • Hypertonic- concentration of solution is more than the cell. Cell will lose water, shrivel, and probably die. Hyper = “more” (when talking about nonpenetrating solutes) • Hypotonic- concentration of solution is less than the cell. Water will enter the cell and the cell will swell and lyse (burst). • Isotonic- concentration of solutions is the same on both sides of the membrane. No net movement of water = stable volume.

49. Osmosis • Osmoregulation- the control of solute concentrations and water balance. • Less permeable membrane, contractile vacuole, etc.

50. Facilitated Diffusion • Facilitated Diffusion- passive transport aided by proteins. • Frequently involves polar molecules.