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07 membranes text

  1. 1. Membrane Structure and Function
  2. 2. Plasma MembranePlasma Membrane Is the boundaryboundary that separates the living cell from its nonliving surroundings Selectively PermeableSelectively Permeable (chooses what may cross the membrane) Fluid mosaicFluid mosaic of lipids and proteins Lipid bilayerbilayer Contains embeddedContains embedded proteinsproteins
  3. 3. PhospholipidsPhospholipids Are the most abundantmost abundant lipid in the plasma membrane Are amphipathicamphipathic, containing both hydrophilic (head) and hydrophobic regions (tails) HeadHead composed of phosphate group attached to one carbon of glycerol is hydrophilichydrophilic Two fatty acid tailstails are hydrophobichydrophobic
  4. 4. Hydrophilic head HydrophobicHydrophobic tailtail WATER WATER Phospholipid Bilayer
  5. 5. Singer and NicolsonSinger and Nicolson In 19721972, Singer and Nicolson, Proposed that membrane proteinsmembrane proteins are dispersed and individually inserted into theinserted into the phospholipid bilayerphospholipid bilayer of the plasmaof the plasma membranemembrane Phospholipid bilayer Hydrophilic region of protein Hydrophobic region of protein
  6. 6. Fluid Mosaic ModelFluid Mosaic Model A membrane is a fluid structurefluid structure with a “mosaic” of various proteins embedded in it when viewed from the top PhospholipidsPhospholipids can move laterallylaterally a small amount and can “flex” their tails Membrane proteinsMembrane proteins also move side to side or lateralllaterally making the membrane fluid
  7. 7. Freeze-fractureFreeze-fracture studies of the plasma membrane support the fluid mosaic model of membrane structure A cell is frozen and fractured with a knife. The fracture planefracture plane often follows the hydrophobic interior of a membraneoften follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layerstwo separated layers. The membrane proteins go wholly with one of the layersmembrane proteins go wholly with one of the layers.
  8. 8. The Fluidity of Membranes Phospholipids in the plasma membrane Can move within the bilayer two ways Lateral movement (~107 times per second) Flip-flop (~ once per month)
  9. 9. The type of hydrocarbon tailstype of hydrocarbon tails in phospholipids Affects the fluidity of the plasma membrane Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails The Fluidity of Membranes
  10. 10. The Fluidity of Membranes The steroid cholesterolsteroid cholesterol Has different effects on membrane fluidity at different temperatures 7.5 Cholesterol
  11. 11. Membrane Proteins and Their Functions A membrane is a collage of differentcollage of different proteins embeddedproteins embedded in the fluid matrix of the lipid bilayer Fibers of extracellular matrix (ECM)
  12. 12. Types of Membrane Proteins Integral proteinsIntegral proteins Penetrate the hydrophobic core of the lipid bilayer Are often transmembrane proteinstransmembrane proteins, completely spanning the membrane EXTRACELLULAR SIDE
  13. 13. Types of Membrane Proteins Peripheral proteinsPeripheral proteins Are appendages loosely bound to the surface of the membrane
  14. 14. 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
  15. 15. 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
  16. 16. The Role of Membrane Carbohydrates in Cell-Cell Recognition Cell-cell recognitionCell-cell recognition IIs a cell’s ability to distinguish one type of neighboring cell from another Membrane carbohydratesMembrane carbohydrates Interact with the surface molecules of other cells, facilitating cell-cell recognition
  17. 17. Synthesis and Sidedness of Membranes Membranes have distinct inside anddistinct inside and outside facesoutside faces This affects the movementaffects the movement of proteins synthesizedproteins synthesized in the endomembrane systemendomembrane system (Golgi and(Golgi and ER)ER)
  18. 18. Synthesis and Sidedness of Membranes Membrane proteins and lipidsMembrane proteins and lipids are made in the ERER andand Golgi apparatusGolgi apparatus ER
  19. 19. Membrane Permeability Membrane structurestructure results in selective permeabilityselective permeability A cell must exchange materialscell must exchange materials with its surroundingswith its surroundings, a process controlled by the plasma membrane
  20. 20. Permeability of the Lipid Bilayer Hydrophobic moleculesHydrophobic molecules Are lipid solublelipid soluble and can pass through the membrane rapidlyrapidly Polar moleculesPolar molecules Do NOT cross the membrane rapidlyrapidly
  21. 21. Transport Proteins Transport proteinsTransport proteins Allow passage of hydrophilichydrophilic substancessubstances across the membrane
  22. 22. Passive Transport Passive transportPassive transport is diffusion of a substance across a membrane with no energyno energy investment COCO22, H, H22O, and OO, and O22 easily diffuse across plasma membranes Diffusion of water is known as OsmosisOsmosis
  23. 23. Simple Diffusion DiffusionDiffusion Is the tendency for molecules of any substance to spread out evenlyspread out evenly into the available space Move from high to low concentrationhigh to low concentration DownDown the concentration gradient
  24. 24. Effects of Osmosis on Water Balance OsmosisOsmosis Is the movement of water across a semipermeable membrane Is affectedaffected by theby the concentration gradient ofconcentration gradient of dissolved substancesdissolved substances called thecalled the solution’ssolution’s tonicitytonicity
  25. 25. Water Balance of Cells Without Walls Tonicity Is the ability of a solution to cause a cell to gain or lose water Has a great impact on cellsimpact on cells without wallswithout walls
  26. 26. Three States of Tonicity
  27. 27. Isotonic Solutions If a solution is isotonicisotonic The concentration of solutesconcentration of solutes is the samesame as it is inside the cell There will be NO NETNO NET movement of WATER
  28. 28. Hypertonic Solution If a solution is hypertonichypertonic The concentration of solutesconcentration of solutes is greatergreater than it is inside the cell The cell will lose waterlose water (PLASMOLYSIS)(PLASMOLYSIS)
  29. 29. Hypotonic Solutions If a solution is hypotonichypotonic The concentration of solutesconcentration of solutes is lesless than it is inside the cell The cell will gain watergain water
  30. 30. Water Balance in Cells Without Walls Animal cell. An animal cell fares best in an isotonicisotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.
  31. 31. Water Balance of Cells with Walls Cell WallsCell Walls Help maintain water balance Turgor pressureTurgor pressure Is the pressure of water inside a plant cell pushing outward against the cell membrane If a plant cell is turgidturgid It is in a hypotonichypotonic environment It is very firm,firm, a healthy state in mosta healthy state in most plantsplants If a plant cell is flaccidflaccid It is in an isotonic or hypertonicisotonic or hypertonic environment
  32. 32. Water Balance in Cells with Walls Plant cell. Plant cells are turgid (firmturgid (firm) and generally healthiest in a hypotonic environmenthypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.
  33. 33. 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?How will the water molecules move? There will be no net movement of waterno net movement of water since the concentration of solute in each solution is equal
  34. 34. 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?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
  35. 35. 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 moleculesHow will the water molecules move?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
  36. 36. Facilitated Diffusion Facilitated diffusionFacilitated diffusion Is a type of PassivePassive Transport Aided by ProteinsProteins In facilitated diffusion Transport proteinsTransport proteins speed the movement of molecules across the plasma membrane
  37. 37. Facilitated Diffusion & Proteins Channel proteinsChannel 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.
  38. 38. Facilitated Diffusion & Proteins Carrier proteinsCarrier proteins Undergo a subtle change in shape that translocates the solute-binding site across the membrane A carrier proteincarrier protein alternates between two conformationsalternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either directioncan transport the solute in either direction, with the net movement being down the concentration gradientdown the concentration gradient of the solute.
  39. 39. Active Transport Active transport Uses energyUses energy to move solutes againstagainst their concentration gradients Requires energy, usually in the form of ATPATP
  40. 40. The sodium-potassium pumpsodium-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+ Na+ Na+ Na+ Na+ P ATP Na+ Na+ Na+ P ADP K+ K+ K+ K+ K+ K+ [Na+ ] high [K+ ] low
  41. 41. 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
  42. 42. Maintenance of Membrane Potential by Ion Pumps Membrane potentialMembrane potential Is the voltage difference across a membrane An electrochemical gradientelectrochemical gradient Is caused by the concentration electrical gradient of ions across a membrane An electrogenic pumpelectrogenic pump Is a transport protein that generates the voltage across a membrane
  43. 43. Proton Pump EXTRACELLULAR FLUID + H+ H+ H+ H+ H+ H+ Proton pump ATP CYTOPLASM + + + + – – – – – +
  44. 44. Cotransport CotransportCotransport Occurs when active transportactive transport of a specific solute indirectly drives the active transport of another solute Involves transport by a membranemembrane proteinprotein Driven by a concentration gradientconcentration gradient
  45. 45. Example of Cotransport Cotransport: active transport driven bydriven by a concentration gradienta concentration gradient
  46. 46. Bulk Transport Bulk transport across the plasma membrane occurs by exocytosisexocytosis and endocytosisand endocytosis Large proteins Cross the membrane by different mechanisms
  47. 47. Exocytosis & Endocytosis In exocytosisexocytosis Transport vesiclesTransport vesicles migrate to the plasma membrane, fuse with it, and release their contents In endocytosisendocytosis The cell takes in macromolecules by forming new vesicles fromforming new vesicles from the plasma membranethe plasma membrane
  48. 48. Endocytosis
  49. 49. Exocytosis
  50. 50. In phagocytosisphagocytosis, a cell engulfs a particle byengulfs a particle by Wrapping pseudopodiaWrapping pseudopodia around it and packaging it within a membrane- enclosed sac large enough to be classified as a vacuolevacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. Three Types of Endocytosis PHAGOCYTOSIS In pinocytosispinocytosis, the cell ““gulps” droplets ofgulps” droplets of extracellular fluidextracellular 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.
  51. 51. 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.