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Plasma Membrane.pptx

  1. Plasma Membrane Life Science 1st Semester
  2. • The plasma membrane is the boundary that separates the living cell from its surroundings
  3. Predominant constituent: phospholipids Membrane Structure Dispersed protein components
  4. Phospholipids- building blocks A balloon with three strings Amphipathic molecule
  5. Hydrocarbon ring structure of cholesterol plays a distinct role in determining membrane fluidity Membrane sterols Sterols=Steroid ring + Alcohol group
  6. Cholesterol molecules insert into the bilayer with their polar hydroxyl groups close to the hydrophilic head groups of the phospholipids.  The rigid hydrocarbon rings of cholesterol therefore interact with the regions of the fatty acid chains that are adjacent to the phospholipid head groups. This interaction decreases the mobility of the outer portions of the fatty acid chains, making this part of the membrane more rigid. Insertion of cholesterol interferes with interactions between fatty acid chains, thereby maintaining membrane fluidity at lower temperatures.
  7. • The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices • Peripheral proteins are bound to the surface of the membrane • Integral proteins penetrate the hydrophobic core • Integral proteins that span the membrane are called transmembrane proteins
  8. Membrane organization and properties described by: Sandwitch model proposed by Danielli‐Davson Unit membranes model of Robertson Fluid Mosaic Model by Singer and Nicolson 1972
  9. * ● FLUID- because individual phospholipids and proteins can move side-to-side within the layer, like it’s a liquid. ● MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above FLUID MOSAIC MODEL
  10. * Functions of Plasma Membrane ● Protective barrier ● Regulate transport in & out of cell ( a selective barrier to the passage of molecules selectively permeable) ● Allow cell recognition ● Provide anchoring sites for filaments of cytoskeleton
  11. * Functions of Plasma Membrane ● Provide a binding site for enzymes ● Interlocking surfaces bind cells together (junctions) ●Contains the cytoplasm (fluid in cell)
  12. Properties of the plasma membrane 1.Dynamic 2.Fluid 3.Asymmetric 4.Semipermeable
  13. 1. Dynamic Lateral movement occurs 107 times per second. Flip-flopping across the membrane is rare ( once per month). These studies have also shown that individual lipid molecules rotate very rapidly about their long axis and that their hydrocarbon chains are flexible.
  14. The fluidity of a lipid bilayer depends on -composition and temperature. A synthetic bilayer made from a single type of phospholipid changes from a liquid state to a two-dimensional rigid crystalline (or gel) state at a characteristic freezing point. This change of state is called a phase transition. 2. Fluid Membranes must be fluid to work properly; they are usually about as fluid as salad oil
  15. The temperature at which it occurs is lower (that is, the membrane becomes more difficult to freeze) if the hydrocarbon chains are short or have double bonds. A shorter chain length reduces the tendency of the hydrocarbon tails to interact with one another, and cis-double bonds produce kinks in the hydrocarbon chains that make them more difficult to pack together, so that the membrane remains fluid at lower temperatures. Bacteria, yeasts, and other organisms whose temperature fluctuates with that of their environment adjust the fatty acid composition of their membrane lipids to maintain a relatively constant fluidity. As the temperature falls, for instance, fatty acids with more cis-double bonds are synthesized, so the decrease in bilayer fluidity that would otherwise result from the drop in temperature is avoided. 2. Fluid Role of phospholipids
  16. Cholesterol tends to make lipid bilayers less fluid, at the high concentrations found in most eucaryotic plasma membranes It also prevents the hydrocarbon chains from coming together and crystallizing. In this way, it inhibits possible phase transitions 2. Fluid Role of Cholesterol
  17. 3. Asymmetry The Plasma Membrane Contains Lipid Rafts That Are Enriched in Sphingolipids, Cholesterol, and Some Membrane Proteins For some lipid molecules, such as the sphingolipids which tend to have long and saturated fatty hydrocarbon chains, the attractive forces can be just strong enough to hold the adjacent molecules together transiently in small microdomains. Such microdomains, or lipid rafts, can be thought of as transient phase separations in the fluid lipid bilayer where sphingolipids become concentrated.
  18. 3. Asymmetry Lipid asymmetry is functionally important. Many cytosolic proteins bind to specific lipid head groups found in the cytosolic monolayer of the lipid bilayer. The enzyme protein kinase C (PKC), for example, is activated in response to various extracellular signals. It binds to the cytosolic face of the plasma membrane, where phosphatidylserine is concentrated, and requires this negatively charged phospholipid for its activity.
  19. * Solubility ● Materials that are soluble in lipids can pass through the cell membrane easily
  20. Small molecules and larger hydrophobic molecules move through easily. e.g. O2, CO2, H2O 4.Semipermeable Membrane
  21. Synthesis and Sidedness of Membranes • Membranes have distinct inside and outside faces • The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus
  22. New phospholipid molecules are synthesized in the ER by membrane-bound enzymes which use substrates (fatty acids) available only on one side of the bilayer. Flipases transfer specific phospholipid molecules selectively so that different types become concentrated in the two halves. One sided insertion and selective flippases create an asymmetrical membrane Plasma Membrane Biosynthesis
  23. The Permeability of the Lipid Bilayer -Transport across membrane • Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly • Polar molecules, such as sugars, do not cross the membrane easily
  24. Transport Proteins • Transport proteins allow passage of hydrophilic substances across the membrane • Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel • Channel proteins called aquaporins facilitate the passage of water
  25. • Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane • A transport protein is specific for the substance it moves • Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease cystinuria
  26. Transport Can be active (energy requiring) or passive Three general classes of transport systems. Transporters differ in the number of solutes (substrates) transported and the direction in which each solute moves.
  27. Passive transport is diffusion of a substance across a membrane with no energy investment • Diffusion is the tendency for molecules to spread out evenly into the available space • Although each molecule moves randomly, diffusion of a population of molecules may be directional • At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other
  28. * Diffusion through a Membrane ● Cell membrane Solute moves DOWN concentration gradient (HIGH to LOW)
  29. • Substances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreases • No work must be done to move substances down the concentration gradient • The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen
  30. Effects of Osmosis on Water Balance • Osmosis is the diffusion of water across a selectively permeable membrane • Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides
  31. Figure 7.14 Lower concentration of solute (sugar) Higher concentration of solute Sugar molecule H2O Same concentration of solute Selectively permeable membrane Osmosis
  32. Water Balance of Cells Without Walls • Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water • Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane • Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water • Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water
  33. Figure 7.15 Hypotonic solution Osmosis Isotonic solution Hypertonic solution (a) Animal cell (b) Plant cell H2O H2O H2O H2O H2O H2O H2O H2O Cell wall Lysed Normal Shriveled Turgid (normal) Flaccid Plasmolyzed
  34. • Hypertonic or hypotonic environments create osmotic problems for organisms • Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments • The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
  35. Figure 7.16 Contractile vacuole 50 m
  36. Water Balance of Cells with Walls • Cell walls help maintain water balance • A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) • If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt
  37. • In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis
  38. Facilitated Diffusion: Passive Transport Aided by Proteins • In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane • Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane • Channel proteins include – Aquaporins, for facilitated diffusion of water – Ion channels that open or close in response to a stimulus (gated channels)
  39. Active transport uses energy to move solutes against their gradients • Facilitated diffusion is still passive because the solute moves down its concentration gradient, and the transport requires no energy • Some transport proteins, however, can move solutes against their concentration gradients
  40. The Need for Energy in Active Transport • Active transport moves substances against their concentration gradients • Active transport requires energy, usually in the form of ATP • Active transport is performed by specific proteins embedded in the membranes
  41. • Active transport allows cells to maintain concentration gradients that differ from their surroundings • The sodium-potassium pump is one type of active transport system
  42. Figure 7.18-1 EXTRACELLULAR FLUID [Na] high [K] low [Na] low [K] high CYTOPLASM Na Na Na 1
  43. Figure 7.18-2 EXTRACELLULAR FLUID [Na] high [K] low [Na] low [K] high CYTOPLASM Na Na Na 1 2 Na Na Na P ATP ADP
  44. Figure 7.18-3 EXTRACELLULAR FLUID [Na] high [K] low [Na] low [K] high CYTOPLASM Na Na Na 1 2 3 Na Na Na Na Na Na P P ATP ADP
  45. Figure 7.18-4 EXTRACELLULAR FLUID [Na] high [K] low [Na] low [K] high CYTOPLASM Na Na Na 1 2 3 4 Na Na Na Na Na Na K K P P P P i ATP ADP
  46. Figure 7.18-5 EXTRACELLULAR FLUID [Na] high [K] low [Na] low [K] high CYTOPLASM Na Na Na 1 2 3 4 5 Na Na Na Na Na Na K K K K P P P P i ATP ADP
  47. Figure 7.18-6 EXTRACELLULAR FLUID [Na] high [K] low [Na] low [K] high CYTOPLASM Na Na Na 1 2 3 4 5 6 Na Na Na Na Na Na K K K K K K P P P P i ATP ADP
  48. Figure 7.19 Passive transport Active transport Diffusion Facilitated diffusion ATP
  49. Cotransport: Coupled Transport by a Membrane Protein • Cotransport occurs when active transport of a solute indirectly drives transport of other solutes • Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell
  50. Figure 7.21 ATP H H H H H H H H Proton pump Sucrose-H cotransporter Sucrose Sucrose Diffusion of H        