1. 1 Membrane Structure and Function

2. 2 Plasma Membrane boundary Is the boundary that separates the living cell from its nonliving surroundings Selectively Permeable Selectively Permeable (chooses what may cross the membrane) Fluid mosaic Fluid mosaic of lipids and proteins bilayer Lipid bilayer Contains embedded proteins

3. 3

4. 4Phospholipids most abundant Are the most abundant lipid in the plasma membrane amphipathic Are amphipathic, containing both hydrophilic (head) and hydrophobic regions (tails) Head hydrophilic Head composed of phosphate group attached to one carbon of glycerol is hydrophilic tailshydrophobic Two fatty acid tails are hydrophobic

5. 5 Hydrophilic head Hydrophobic tail WATER Phospholipid Bilayer

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

7. 7 Fluid Mosaic Model fluid structure A membrane is a fluid structure with a “mosaic” of various proteins embedded in it when viewed from the top Phospholipidslaterally Phospholipids can move laterally a small amount and can “flex” their tails Membrane proteins laterall Membrane proteins also move side to side or laterally making the membrane fluid

8. 8 Freeze-fracture Freeze-fracture studies of the plasma membrane support the fluid mosaic model of membrane structure fracture plane often follows the hydrophobic interior of a membrane two separated layers membrane proteins go wholly with one of the layers 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.

9. 9 The Fluidity of Membranes Phospholipids in the plasma membrane Can move within the bilayer two ways Lateral movement (~10 7 times per second) Flip-flop (~ once per month)

10. 10 type of hydrocarbon tails The type of hydrocarbon tails in phospholipids Affects the fluidity of the plasma membrane FluidViscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails The Fluidity of Membranes

11. 11 The Fluidity of Membranes steroid cholesterol The steroid cholesterol Has different effects on membrane fluidity at different temperatures Figure 7.5 Cholesterol

12. 12 Membrane Proteins and Their Functions collage of different proteins embedded A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Fibers of extracellular matrix (ECM)

13. 13 Types of Membrane Proteins Integral proteins Penetrate the hydrophobic core of the lipid bilayer transmembrane proteins Are often transmembrane proteins, completely spanning the membrane EXTRACELLULAR SIDE

14. 14 Types of Membrane Proteins Peripheral proteins Are appendages loosely bound to the surface of the membrane

15. 15 Six Major Functions of Membrane Proteins Figure 7.9 Transport.. Enzymatic activity. Signal transduction. (a) (b) (c) ATP Enzymes Signal Receptor

16. 16 Cell-cell recognition. Intercellular joining. Attachment to the cytoskeleton and extracellular matrix (ECM). (d) (e) (f) Glyco- protein Six Major Functions of Membrane Proteins

17. 17 Membrane Permeability cell must exchange materials with its surroundings A cell must exchange materials with its surroundings, a process controlled by the plasma membrane Hydrophobic molecules lipid soluble rapidly Are lipid soluble and can pass through the membrane rapidly Polar molecules rapidly Do NOT cross the membrane rapidly

18. 18 Transport Proteins Transport proteins hydrophilic substances Allow passage of hydrophilic substances across the membrane

19. 19 Passive Transport Passive transport no energy Passive transport is diffusion of a substance across a membrane with no energy investment CO 2, H 2 O, and O 2 CO 2, H 2 O, and O 2 easily diffuse across plasma membranes diffusion animation Osmosis Diffusion of water is known as Osmosis

20. 20 Simple DiffusionDiffusion spread out evenly Is the tendency for molecules of any substance to spread out evenly into the available space high to low concentration Move from high to low concentration Down Down the concentration gradient

21. 21 Effects of Osmosis on Water BalanceOsmosis Is the movement of water across a semipermeable membrane affected by the concentration gradient of dissolved substances called the solution’s tonicity Is affected by the concentration gradient of dissolved substances called the solution’s tonicity osmosis animation

22. 22 Water Balance of Cells Without Walls Tonicity Is the ability of a solution to cause a cell to gain or lose water impact on cells without walls Has a great impact on cells without walls

23. 23 Three States of Tonicity

24. 24 Isotonic Solutions isotonic If a solution is isotonic concentration of solutes same The concentration of solutes is the same as it is inside the cell NO NET There will be NO NET movement of WATER

25. 25 Hypertonic Solution hypertonic If a solution is hypertonic concentration of solutesgreater The concentration of solutes is greater than it is inside the cell lose water (PLASMOLYSIS) The cell will lose water (PLASMOLYSIS)

26. 26 Hypotonic Solutions hypotonic If a solution is hypotonic concentration of solutesles The concentration of solutes is less than it is inside the cell gain water The cell will gain water

27. 27 Water Balance in Cells Without Walls isotonic Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.

28. 28 Water Balance of Cells with Walls Cell Walls Help maintain water balance Turgor pressure Is the pressure of water inside a plant cell pushing outward against the cell membrane turgid If a plant cell is turgid hypotonic It is in a hypotonic environment firm, a healthy state in most plants It is very firm, a healthy state in most plants flaccid If a plant cell is flaccid isotonic or hypertonic It is in an isotonic or hypertonic environment

29. 29 Water Balance in Cells with Walls turgid (firm hypotonic environment Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.

30. 30 Facilitated Diffusion Facilitated diffusion Passive Proteins Is a type of Passive Transport Aided by Proteins In facilitated diffusion Transport proteins Transport proteins speed the movement of molecules across the plasma membrane

31. 31 Facilitated Diffusion & Proteins Channel proteins Provide corridors that allow a specific molecule or ion to cross the membrane EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM A channel protein (purple) has a channel through which water molecules or a specific solute can pass.

32. 32 Facilitated Diffusion & Proteins Carrier proteins Undergo a subtle change in shape that translocates the solute-binding site across the membrane carrier proteinalternates between two conformations can transport the solute in either direction down the concentration gradient 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.

33. 33 Active Transport Active transport Uses energy against Uses energy to move solutes against their concentration gradients ATP Requires energy, usually in the form of ATP

34. 34 sodium-potassium pump The sodium-potassium pump Is one type of active transport system Active Transport P P i EXTRACELLULAR FLUID Na+ binding stimulates phosphorylation by ATP. 2 Na + Cytoplasmic Na + binds to the sodium-potassium pump. 1 K + is released and Na + sites are receptive again; the cycle repeats. 3 Phosphorylation causes the protein to change its conformation, expelling Na + to the outside. 4 Extracellular K + binds to the protein, triggering release of the Phosphate group. 6 Loss of the phosphate restores the protein’s original conformation. 5 CYTOPLASM [Na + ] low [K + ] high Na + P ATP Na + P ADP K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ [Na + ] high [K + ] low

35. 35 Maintenance of Membrane Potential by Ion Pumps Membrane potential Is the voltage difference across a membrane electrochemical gradient An electrochemical gradient Is caused by the concentration electrical gradient of ions across a membrane electrogenic pump An electrogenic pump Is a transport protein that generates the voltage across a membrane

36. 36 Proton Pump EXTRACELLULAR FLUID + H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Proton pump ATP CYTOPLASM + + + + – – – – – +

37. 37 CotransportCotransport active transport Occurs when active transport of a specific solute indirectly drives the active transport of another solute membrane protein Involves transport by a membrane protein concentration gradient Driven by a concentration gradient

38. 38 Example of Cotransport driven by a concentration gradient Cotransport: active transport driven by a concentration gradient

39. 39 Bulk Transport exocytosis and endocytosis Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Large proteins Cross the membrane by different mechanisms

40. 40 Exocytosis & Endocytosis exocytosis In exocytosis Transport vesicles Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents endocytosis In endocytosis forming new vesicles from the plasma membrane The cell takes in macromolecules by forming new vesicles from the plasma membrane

41. 41 Endocytosis

42. 42 Exocytosis

43. 43 phagocytosis Three Types of Endocytosis PHAGOCYTOSIS pinocytosis

44. 44 0.25 µm RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Ligand Coat protein Coated pit Coated vesicle A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs). Plasma membrane Coat protein Receptor-mediated endocytosis