Membrane Structure and Function: Fluid Mosaic Model and Transport Mechanisms

1. Chapter 7 notes Membrane Structure and Function

2. Concept 7.1 Most abundant lipids in membranes are phospholipids. - phospholipids are amphipathic (head is hydrophilic, tail is hydrophobic ) Phospholipids and proteins are arranged in the “fluid mosaic model”: membrane is fluid w/ proteins embedded in or attached to the bilayer

3. WATER Hydrophilic head Hydrophobic tail WATER Concept 7.1

4. Concept 7.1 Phospholipids and proteins are arranged in the “fluid mosaic model”: membrane is fluid w/ proteins embedded in or attached to the bilayer - disproved the Davson-Danielli “sandwich” model

5. Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein Concept 7.1

6. Concept 7.1 The membrane is fluid -membranes are not static sheets of molecules locked in place -the membrane is held together primarily by hydrophobic interactions

7. Concept 7.1 Lateral movement (107 times per second) Flip-flop ( once per month) (a) Movement of phospholipids

8. Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- carbon tails (b) Membrane fluidity Concept 7.1

9. Concept 7.1 Membranes are mosaics of structure and function - proteins are embedded in the fluid matrix; the lipid bilayer is the main fabric of the membrane, but proteins determine its specific fcn.

10. Fibers of extracellular matrix (ECM) Carbohydrate Glyco- protein Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE Concept 7.1

11. Concept 7.1 Two major types of membrane proteins: - Integral proteins: penetrate the hydrophobic core of the bilayer; many are transmembrane proteins - Peripheral proteins: appendages loosely bound to the surface of the membrane

12. EXTRACELLULAR SIDE N-terminus C-terminus CYTOPLASMIC SIDE  Helix Concept 7.1

13. Concept 7.1 Membrane carbohydrates are important for cell-cell recognition - cell-cell recognition is the ability of a cell to distinguish one type of neighboring cell from another - membrane carbohydrates are usually oligosaccharides (can vary greatly)

14. Signaling molecule Enzymes Receptor ATP Signal transduction (b) Enzymatic activity (c) Signal transduction (a) Transport Concept 7.1

15. Glyco- protein (e) Intercellular joining (d) Cell-cell recognition (f) Attachment to the cytoskeleton and extracellular matrix (ECM) Concept 7.1

16. Concept 7.2 Hydrophobic molecules can cross the bilayer with ease. However, ions and polar molecules cannot pass through because they are hydrophilic. - proteins play keys roles in regulating transportation.

17. Concept 7.2 Transport proteins: allow hydrophilic molecules to enter and exit the cell. The selective permeability of a membrane depends on the specific transport proteins built into the membrane.

18. Concept 7.3 Passive transport involves diffusion across a membrane. - Diffusion: the tendency for molecules of any substance to spread out into available space - any substance will move down a [gradient]. [high]  [low]

19. Molecules of dye Membrane (cross section) WATER Equilibrium Net diffusion Net diffusion (a) Diffusion of one solute Concept 7.3

20. Equilibrium Net diffusion Net diffusion Net diffusion Net diffusion Equilibrium (b) Diffusion of two solutes Concept 7.3

21. Concept 7.3 Passive transport: diffusion of a substance across a biological membrane. (no energy is used) Osmosis is the passive transport of water - sln. w/ a higher [solute] = hypertonic - sln. w/ a lower [solute] = hypotonic - slns. w/ equal [solute] = isotonic

22. Concept 7.3

23. Concept 7.3 Organisms without cell walls that live in hypertonic or hypotonic environments must have adaptations for osmoregulation, the control of water balance

24. Concept 7.3 Organisms with cell walls - turgid (very firm) when placed in a hypotonic sln. - flacid (limp) if the sln. is isotonic - plasmolysis (shriveled) occurs when put in a hypertonic sln.

25. Concept 7.3 Hypotonic solution Isotonic solution Hypertonic solution H2O H2O H2O H2O (a) Animal cell Lysed Normal Shriveled H2O H2O H2O H2O (b) Plant cell Turgid (normal) Flaccid Plasmolyzed

26. Concept 7.3 Facilitated diffusion: passive transport of molecules through transport proteins - each protein is specific for the solute it transports

27. EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein Solute Carrier protein (b) A carrier protein Concept 7.3

28. Concept 7.4 Active transport: movement of molecules across a membrane against the gradient (uses ATP) - sodium-potassium pump: movement of 3 Na+ for every 2 K+ ions

29. EXTRACELLULAR FLUID Na+ [Na+] high Na+ [K+] low Na+ Na+ Na+ Na+ Na+ Na+ ATP [Na+] low P Na+ P [K+] high CYTOPLASM ADP 2 3 1 K+ K+ K+ K+ K+ P K+ P 6 5 4 Concept 7.4

30. Concept 7.4 Some ion pumps generate voltage across membranes - membrane potential: the voltage across a membrane - electrogenic pump: a transport protein that generates voltage across a membrane (ex. Sodium-potassium pump)

31. Concept 7.4 The main electrogenic pump for plants and fungi is a proton pump which transports H+ ion out of the cell.

32. – EXTRACELLULAR FLUID + – ATP + H+ H+ Proton pump H+ – + H+ H+ H+ – + CYTOPLASM H+ – + Concept 7.4

33. Concept 7.4 In cotransport, a ATP powered pump can drive the transport of other solutes. 1) active transport of a substance against a gradient 2) cotransport through a protein w/ 2nd substance

34. – + H+ ATP H+ – + H+ Proton pump H+ – + H+ H+ – + Diffusion of H+ H+ Sucrose-H+ cotransporter H+ – + Sucrose – + Sucrose Concept 7.4

35. Concept 7.5 Exocytosis: the secretion of macromolecules by the fusion of vesicles with the plasma membrane Endocytosis: the cell takes in macromolecules by forming new vessicles - 3 types: phagocytosis, pinocytosis, and receptor-mediated endocytosis

36. PHAGOCYTOSIS EXTRACELLULAR FLUID CYTOPLASM 1 µm Pseudopodium Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM) Concept 7.5