Chapter 7: Membranes

1. Chapter 7: Membranes

2. Life at the Edge • Plasma membrane • Boundary, separates the living cell from nonliving

3. Figure 7.1 • Plasma membrane: selective permeability • Some substances cross more easily than others

4. Membranes are fluid mosaics of lipids and proteins • Phospholipids • Most abundant lipid in plasma membrane • Amphipathic, containing both hydrophobic and hydrophilic regions

5. WATER Hydrophilic head Hydrophobic tail WATER Figure 7.2 • Phospholipid bilayer

6. Davson-Danielli sandwich model of membrane structure

7. Hydrophobic region of protein Phospholipid bilayer Figure 7.3 Hydrophobic region of protein • Singer and Nicolson (1972) • Proteins dispersed in phospholipid bilayer

8. A cell membrane can be split into its two layers, revealing the ultrastructure of the membrane’s interior. APPLICATION TECHNIQUE Extracellular layer Proteins Knife Plasma membrane Cytoplasmic layer These SEMs show membrane proteins (the “bumps”) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer. RESULTS • Freeze-fracture studies of the plasma membrane • Supported the fluid mosaic model of membrane structure 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. Figure 7.4 Extracellular layer Cytoplasmic layer

9. Lateral movement (~107 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids Figure 7.5 A Fluidity of Membranes • Phospholipids can move within the bilayer

10. Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails (b) Membrane fluidity Figure 7.5 B • Hydrocarbon tails in phospholipids affects fluidity

11. Cholesterol Figure 7.5 (c) Cholesterol within the animal cell membrane • Steroid, cholesterol affects fluidity at different temperatures

12. Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Microfilaments of cytoskeleton Peripheral protein Cholesterol Integral protein CYTOPLASMIC SIDE OF MEMBRANE Figure 7.7 • Membrane • Collage of different proteins embedded in the fluid matrix of the lipid bilayer Fibers of extracellular matrix (ECM)

13. N-terminus C-terminus CYTOPLASMIC SIDE a Helix Figure 7.8 • Proteins • Some penetrate hydrophobic core of lipid bilayer • Transmembrane proteins: completely spanning membrane EXTRACELLULAR SIDE

14. Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy ssource to actively pump substances across the membrane. (a) ATP (b) Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. Enzymes Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell. (c) Signal Receptor Figure 7.9 • Major functions of membrane proteins

15. (d) Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Glyco- protein (e) Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (see Figure 6.31). (f) Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (see Figure 6.29). Figure 7.9

16. Cell-cell recognition • Ability to distinguish one type of cell from another

17. Membrane carbohydrates facilitates cell-cell recognition

18. Membranes have inside and outside faces • Affects movement of proteins

19. 1 Transmembrane glycoproteins Secretory protein Glycolipid 2 Golgi apparatus Vesicle 3 Plasma membrane: Cytoplasmic face 4 Extracellular face Transmembrane glycoprotein Secreted protein Membrane glycolipid Figure 7.10 • Membrane proteins and lipids synthesized in ER and Golgi ER

20. Cell must exchange materials with surroundings  controlled by plasma membrane (selectively permeable)

21. Hydrophobic molecules • Lipid soluble, can pass through membrane rapidly • Polar molecules • Do not cross the membrane rapidly

22. Transport proteins allow passage of hydrophilic substances

23. Passive transport: diffusion of a substance across a membrane w/o energy investment

24. (a) Molecules of dye Membrane (cross section) Diffusion of one solute. The membrane has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions. Equilibrium Net diffusion Net diffusion Figure 7.11 A • Diffusion

25. (b) Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concen- tration gradient. There will be a net diffusion of the purple dye toward the left, even though the total solute concentration was initially greater on the left side. Equilibrium Net diffusion Net diffusion Net diffusion Equilibrium Net diffusion Figure 7.11 B • Substances diffuse down their concentration gradient

26. Osmosis • Movement of water across a semipermeable membrane

27. Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar Selectively permeable mem- brane: sugar mole- cules cannot pass through pores, but water molecules can Water molecules cluster around sugar molecules More free water molecules (higher concentration) Fewer free water molecules (lower concentration) Osmosis  Water moves from an area of higher free water concentration to an area of lower free water concentration Figure 7.12 • Affected by conc. gradient of dissolved substances

28. Tonicity • Ability of a solution to cause a cell to gain or lose water • Great impact on cells w/o walls

29. Isotonic conc. • Conc of solutes is same as it is inside the cell • No net movement of water

30. Hypertonic soln. • Conc. of solutes is greater than it is inside the cell • Cell loses water

31. Hypotonic soln. • Conc of solutes is less than it is inside the cell • Cell gains water

32. Hypotonic solution Hypertonic solution Isotonic solution (a) Animal cell. An animal cell fares best in an isotonic environ- ment unless it has special adaptations to offset the osmotic uptake or loss of water. H2O H2O H2O H2O Normal Shriveled Lysed Figure 7.13 • Water balance in cells w/o walls

33. Cell walls help maintain water balance

34. Turgid plant cell • Hypotonic environment • Firm, a healthy state

35. Flaccid cell • Isotonic or hypertonic environment

36. (b) 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. H2O H2O H2O H2O Turgid (normal) Flaccid Plasmolyzed Figure 7.13 • Water balance in cells with walls

37. Facilitated diffusion • Transport proteins speed the movement of molecules across memb.

38. EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein (purple) has a channel through which water molecules or a specific solute can pass. Figure 7.15 • Channel proteins • allow specific molecule to pass

39. Solute Carrier protein (b) A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute. Figure 7.15 • Carrier proteins • Change in shape  translocates molecule

40. Active transport • Moves substances against conc gradient • Requires energy, (ATP)

41. 2 1 6 5 3 4 [Na+] high [K+] low Na+ Na+ Na+ Na+ Na+ ATP [Na+] low [K+] high P Na+ ADP CYTOPLASM Na+ binding stimulates phosphorylation by ATP. Cytoplasmic Na+ binds to the sodium-potassium pump. Na+ Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. Na+ Na+ K+ P K+ K+ is released and Na+ sites are receptive again; the cycle repeats. K+ K+ K+ K+ Loss of the phosphate restores the protein’s original conformation. Extracellular K+ binds to the protein, triggering release of the Phosphate group. • e.g. Sodium-potassium pump EXTRACELLULAR FLUID P P i Figure 7.16

42. Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP. ATP Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer. Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins. • Review: Passive and active transport compared Figure 7.17

43. Membrane potential • Voltage difference across a membrane

44. Electrochemical gradient • Caused by the conc/ electrical gradient of ions across a membrane, e.g. H+

45. – EXTRACELLULAR FLUID + – ATP + H+ H+ Proton pump H+ + – H+ H+ + – CYTOPLASM + H+ + – • Electrogenic pump (a protein), generates voltage across a membrane Figure 7.18

46. – + H+ ATP H+ + – H+ Proton pump H+ – + H+ – + H+ Diffusion of H+ Sucrose-H+ cotransporter H+ – + – Sucrose + Figure 7.19 • Cotransport

47. Bulk transport: exocytosis and endocytosis • Large molecules

48. Exocytosis • Vesicles migrate to the plasma memb, fuse, and release contents

49. Endocytosis • takes in macromolecules

50. EXTRACELLULAR FLUID 1 µm CYTOPLASM Pseudopodium Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). In pinocytosis, the cell “gulps” droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosisis nonspecific in the substances it transports. PINOCYTOSIS 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). Vesicle • Types of endocytosis In phagocytosis, a cell engulfs a particle by Wrapping pseudopodia around it and packaging it within a membrane- enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. PHAGOCYTOSIS Figure 7.20