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

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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. Phospholipid structure Consists of a hydrophilic “head” and hydrophobic “tails” CH 2 O P O O O CH CH 2 OO C O C O Phosphate Glycerol (a) Structural formula (b) Space-filling model Fatty acids (c) Phospholipid symbol Hydrophobic tails Hydrophilic head Hydrophobic tails – Hydrophilic head CH 2 Choline + Figure 5.13 N(CH 3 ) 3

6. Actually four major phospholipids phospholipids a. phosphatidylcholine b. sphingomyelinsphingomyelin c. phosphatidylserine d. phosphatidylethanolamine 6 Outside of cell Inside of cell

7. 7 Hydrophilic head Hydrophobic tail WATER Phospholipid Bilayer

8. Danielli-Davson model – sandwich model Protein covering every part of the membrane’s outside 8

9. 9 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 Hydrophilic region of protein Hydrophobic region of protein

10. 10 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

11. 11 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.

12. 12 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)

13. 13 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

14. 14 The Fluidity of Membranes steroid cholesterol The steroid cholesterol Has different effects on membrane fluidity at different temperatures Figure 7.5 Cholesterol

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16. Conceptual model of cholesterol effects on the traffic of microbial pathogens into or out of the eukaryotic cell. Goluszko P, Nowicki B Infect. Immun. 2005;73:7791-7796

17. Conceptual model of cholesterol effects on the traffic of microbial pathogens into or out of the eukaryotic cell. (A) The bacterial pathogen interacts directly with membrane cholesterol, which serves as a “docking site” and stabilizes microbial interaction with membranes. (B) The bacterial pathogen interacts directly with a GPI- anchored protein receptor embedded in lipid rafts. Cholesterol is required to maintain lipid raft integrity. (C) The internalized microorganism resides within a phagosome with a cholesterol-enriched membrane. Cholesterol is required to prevent phagolysosomal fusion. (D) Viral particles (HIV-1) are targeted to lipid rafts. Cholesterol is required to maintain virion integrity and the release of infectious virions.

18. 18 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) Fibers of ECM

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20. 20 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

21. 21 Types of Membrane Proteins Peripheral proteins Are appendages loosely bound to the surface of the membrane

22. 22 Six Major Functions of Membrane Proteins Figure 7.9 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 source to actively pump substances across the membrane. 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. 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. (a) (b) (c) ATP Enzymes Signal Receptor

23. 23 Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions 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 (d) (e) (f) Glyco- protein Six Major Functions of Membrane Proteins

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25. 25 The Role of Membrane Carbohydrates in Cell-Cell Recognition Cell-cell recognition I Is a cell’s ability to distinguish one type of neighboring cell from another Membrane carbohydrates Interact with the surface molecules of other cells, facilitating cell-cell recognition

26. 26 Synthesis and Sidedness of Membranes distinct inside and outside faces Membranes have distinct inside and outside faces affects the movement proteins synthesized endomembrane system (Golgi and ER) This affects the movement of proteins synthesized in the endomembrane system (Golgi and ER)

27. 27 Synthesis and Sidedness of Membranes Membrane proteins and lipids ER and Golgi apparatus Membrane proteins and lipids are made in the ER and Golgi apparatus ER

28. 28 Membrane Permeability structure selective permeability Membrane structure results in selective permeability cell must exchange materials with its surroundings A cell must exchange materials with its surroundings, a process controlled by the plasma membrane

29. 29 Permeability of the Lipid Bilayer Hydrophobic molecules lipid soluble rapidly Are lipid soluble and can pass through the membrane rapidly Polar molecules rapidly Do NOT cross the membrane rapidly

30. 30 Transport Proteins Transport proteins hydrophilic substances Allow passage of hydrophilic substances across the membrane

31. Channels vs Carriers Channels are distinguished from carriers based on how much and how fast molecules move through them. Channels transport materials faster and in greater quantities. Carriers – Uniport one substance or Symport two substances same direction or Antiport two substances opposite directions 31

32. Aquaporins – transport water only. A single human aquaporin-1 channel facilitates water transport at a rate of roughly 3 billion water molecules per second. 10 human aquaporins have been discovered 32

33. 33 What determines the rate of diffusion? There 4 factors: 1. The steepness of the concentration gradient. The bigger the difference between the two sides of the membrane the quicker the rate of diffusion. 2. Temperature. Higher temperatures give molecules or ions more kinetic energy. Molecules move around faster, so diffusion is faster. 3. The surface area. The greater the surface area the faster the diffusion can take place. This is because the more molecules or ions can cross the membrane at any one moment. 4. The type of molecule or ion diffusing. Large molecules need more energy to get them to move so they tend to diffuse more slowly. Non-polar molecules diffuse more easily than polar molecules because they are soluble in the non polar phospholipid tails.

34. 34 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 Osmosis Diffusion of water is known as Osmosis

35. 35 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

36. 36 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

37. 37 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

38. 38 Three States of Tonicity

39. 39 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

40. 40 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)

41. 41 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

42. 42 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.

43. 43 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

44. 44 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.

45. 45 How Will Water Move Across Semi-Permeable Membrane? Solution A has 100 molecules of glucose per ml Solution B has 100 molecules of fructose per ml How will the water molecules move? no net movement of water There will be no net movement of water since the concentration of solute in each solution is equal

46. 46 How Will Water Move Across Semi-Permeable Membrane? Solution A has 100 molecules of glucose per ml Solution B has 75 molecules of fructose per ml How will the water molecules move? There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of solute

47. 47 How Will Water Move Across Semi-Permeable Membrane? Solution A has 100 molecules of glucose per ml Solution B has 100 molecules of NaCl per ml How will the water molecules move? Each molecule of NaCl will dissociate to form a Na+ ion and a Cl- ion, making the final concentration of solutes 200 molecules per mil. Therefore, there will be a net movement of water from Solution A to Solution B until both solutions have equal concentrations of solute

48. 48 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

49. 49 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.

50. 50 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.

51. http://highered.mcgraw- hill.com/sites/0072495855/student_view0 /chapter2/animation__how_osmosis_work s.html

52. 52 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

53. 53 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

54. 54 Comparison of Passive & Active Transport 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. 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. ATP

55. 55 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

56. 56 Proton Pump EXTRACELLULAR FLUID + H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Proton pump ATP CYTOPLASM + + + + – – – – – +

57. 57 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

58. 58 Example of Cotransport driven by a concentration gradient Cotransport: active transport driven by a concentration gradient

59. 59 Bulk Transport exocytosis and endocytosis Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Large proteins Cross the membrane by different mechanisms

60. 60 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

61. 61 Endocytosis

62. 62 phagocytosis 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 vacuole as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. Three Types of Endocytosis PHAGOCYTOSIS pinocytosis In pinocytosis, the cell “gulps” droplets of extracellular fluid 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, pinocytosis is nonspecific in the substances it transports.

63. 63 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 enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle.

64. 64 Exocytosis

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