Membrane pt.1
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Membrane pt.1

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Unit 4

Unit 4

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Membrane pt.1 Membrane pt.1 Presentation Transcript

  • Membranes Structure & Function ~Transport & Signaling
  • The Cell Membrane
    • Sometimes called the plasma membrane
    • Outer boundary of lipids and proteins
      • Formed of phospholipids
      • Proteins are embedded in the lipids
    • The membrane is approximately 5 nm thick
    • Holds the cell together - gives it shape
    • Regulates what enters and leaves the cell
  • The Phospholipid Bilayer
    • Biological membranes are a bilayer formed from phospholipids .
    • Phospholipids create a spherical, three dimensional shell around the cell.
    • They are often represented 2-dimensionally as:
  • Orientation of Phospholipids
    • Each phospholipid has a negatively charged phosphate head
      • These are hydrophilic
    • The heads point out : toward the environment and the interior of the cell
    • Each phospholipid has two tails that are highly hydrophobic hydrocarbon chains
    • The tails orient in , towards each other and away from the watery exterior of the cell
      • Create a hydrophobic interior part of the membrane
  •  
  • Membrane Polarity
  • The Fluid Mosaic
    • Lipid bilayers are fluid
      • Individual phospholipids diffuse throughout the 2-dimensional surface
    • Mosaic property of the membrane
      • proteins, cholesterol and other molecules are embedded in the phospholipids
    • Membrane proteins diffuse throughout the membrane
  •  
  • Membrane Fluidity
    • Several factors influence membrane fluidity:
    • Cholesterol
      • A necessary component of biological membranes
      • Breaks up Van der Waals forces and the close packing of phospholipid tails
      • Makes membrane more fluid
      • Regulating cholesterol regulates membrane fluidity
    • Ratio of saturated to unsaturated hydrocarbon chains in phospholipids
      • Impacts fluidity
      • Phospholipids with saturated hydrocarbon chains pack close together & form numerous Van der Waals bonds
  •  
  • The Membrane is Semi-permeable
    • The arrangement of phopholipids and embedded proteins makes the membrane semipermeable
    • Some molecules are allowed to pass freely (diffuse) through the membrane.
    • Other molecules cannot enter the cell
      • Virtually impermeable to large molecules
      • Molecules as small as charged ions need a special process to enter the cell
      • Quite permeable to lipid soluble low molecular weight molecules (like CO 2 & O 2 ).
  • Homeostasis
    • The process of maintaining a stable internal environment despite changing external conditions
    • The cells' environment is surrounded by fluids
    • The cytoplasm is mostly water
    • Most transport involves molecules in liquid state, in solution
      • solvent - water in most cell processes
      • solute
  • Diffusion
    • Molecules move from areas of higher concentration to areas of lower concentration
    • This process continues until the material is evenly distributed throughout the substance
    • A major means of molecular transport in the cell
    • Diffusion through membrane doesn't require energy
    • Increased temperature increases diffusion rate
    • Increased pressure increases diffusion rate
    • Limited by the diffusion rate of the molecule
    • Effective for some substances: e.g. water
  • The Concentration Gradient
    • Diffusion occurs down a concentration gradient
    • Concentration gradient
      • Difference in concentration of molecules of a substance from the highest to the lowest concentration
    • Moving from an area of high concentration to an area of low concentration is moving with the concentration gradient
    • The steeper the concentration gradient (greater difference between concentrations) the more rapid the diffusion.
  •  
  • Osmosis
    • Diffusion of water molecules through a selectively permeable membrane from an area of greater concentration to an area of lesser concentration
    • Water flows back and forth across the cell membrane until the concentration of water molecules is = on each side
    • When concentration is = on both sides: equilibrium
    • Concentration of water on each side of membrane determined by concentration of solutes in water
    • Requires no energy
  •  
  • Solute Concentration
    • The concentration of water on each side of membrane determined by concentration of solutes in water
    • Isotonic Solution
      • concentration of solutes inside = outside
      • rate of osmosis equal both ways
    • Hypotonic
      • concentration of solutes outside is less than inside
      • water moves from solution into cell
    • Hypertonic
      • concentration outside is greater than concentration inside
      • water flows out of cells
  • Water Balance
  • Cells in Hypotonic Solution
    • Hypotonic
      • concentration of solutes outside is less than inside
      • water moves from solution into cell
    • Osmotic Pressure
      • Pressure caused inside a cell or any sack by the passage of water in through osmosis
    • Freshwater plants
    • Turgor = pressure in cells from water flowing in
      • causes cell to be rigid
      • cell wall prevents bursting
    • Animal cells can't reach equilibrium in hypotonic solution because they lack a cell wall
      • Some cells have developed mechanisms to remove excess water before they burst
  • Contractile Vacuole
  • Cells in Hypertonic Solution
    • In hypertonic solution cells shrivel because water flows out of the cell
    • Drinking sea water
    • Road salt bad for plants
    • Some animals are specially adapted
      • Salmon 
  • Carrier Transport
    • Most molecules cannot move across the membrane freely
      • Require carriers
    • Carrier molecules are proteins in the cell membrane
      • transport large molecules or molecules that cannot dissolve in the lipids that make up the cell membrane
    •   2 types:
      • Facilitated Diffusion
      • Active Transport
  • Facilitated Diffusion
    • Like simple diffusion
    • Substances move with the concentration gradient
    • Carrier molecules speed up the movement of the diffusing substances
    • Protein channels
    • Allows charged molecules that couldn’t cross the membrane to diffuse freely in & out of the cell
    • Greatest use is small ions: Na + , K + , Cl -
    • Speed is limited by the number of protein channels, not the concentration gradient
  • Picturing Facilitated Diffusion
  • Active Transport
    • Also uses carrier molecules
    • Involves movement of materials against the concentration gradient
      • e.g. liver cells store glucose - have higher concentration of glucose than surrounding tissue, so active transport needed to move glucose in.
    • Requires energy
    • Two categories:
      • Primary & secondary
  • Primary Active Transport
    • Uses energy at the membrane protein itself to cause a conformational change
      • Energy from ATP hydrolysis
    • Shape change results in transport of molecule through the protein
    • Best known is Na + /K + pump
    • Antiport
      • Transports K + in and Na + out at the same time
  • The Na + /K + Pump – An Antiport
  • Secondary Active Transport
    • Uses energy to establish a gradient across the cell membrane
    • Utilizes the gradient to transport the desired molecule up its concentration gradient
  • Establishing a Gradient
  • The E.coli Lactose Symport
    • Symport
      • coupled transport in the same direction across a cell membrane
    • E. coli establishes a proton (H+) gradient across the cell membrane
    • Uses energy to pump protons out of the cell
    • Those protons are coupled to lactose at the lactose permease transmembrane protein
    • Lactose permease uses the energy of the protons moving down their concentration gradient to transport lactose into the cell
  • A Model Symport
  • The Lactose Symport
  • The Na + -Glucose Symport
    • Another secondary active transport system
    • Uses the Na + -K + pump as its first step
    • Establishes a strong Na + gradient across the cell membrane
    • Glucose-Na + symport protein uses the Na + gradient to transport glucose into the cell
  • The Na + -Glucose Symport
  • Na + -Glucose Symport in Gut Epithelium
    • Used in human gut epithelial cells
    • Cells take in glucose and Na + from the intestines
    • Transport them through the blood stream using Na + -glucose symports, glucose permease (a glucose facilitated diffusion protein) & the Na+/K+ pump
    • Epithelial cells are joined together by tight junctions
      • Prevent anything from leaking through from the intestines to the blood stream without being filtered by the epithelial cells
  • Na + -Glucose Symport in Gut Epithelium
  • Bulk Transport
    • Allows movement across the cell boundary without passing through the membrane
    • Materials that cannot pass through the membrane need to be transported into or out of the cell
      • e.g. droplets of fluid, particles of food, etc.
    •   Endocytosis
      • Bulk transport into the cell
      • Cell membrane encloses particle forming a pouch
      • Phagocytosis = feeding this way (amoebas, packman)
      • Pinocytosis = movement of liquids with solutes this way
    •   Exocytosis
      • the reverse - bulk transport out of cell
    • Require energy
  • Endocytosis