Lab 3 summary CC-BY-NC

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Summary of Lab 3 for BIOL 112 (Winter 2011). Week of Jan 24, 2011.

Cellular physiology: cell membrane transport.

1Introduction to cell membrane transport

  • Cell membrane regulates contents of cells
  • Differentially permeable - some things pass through more easily than others

2Passive transport

  • Brownian motion - random movement of particles suspended in fluid
    • Due to the kinetic energy of the particles of the fluid - they push it around on all sides, etc
    • Smaller ones move the most, because they're smaller and thus easier to push about
    • But on average, things stay at roughly the same position
  • Diffusion: movement of substance to place with lower concentration of the solute
    • Brought about kinetic energy of the molecules of the solution
    • Dynamic equilibrium: same number of molecules moving in any two directions
    • Slower in liquids than in gases (due to lower kinetic energy, and possibly less space as well)
    • Potassium permanganate in test tube to observe
  • Osmosis: diffusion of a solvent through a differentially permeable membrane
    • In cells, plasma membranes are hydrophobic
    • Must have a pore formed by a protein (aquaporin) to allow water through (in either direction)
    • Solutes generally cannot move across cell membrane; only water can, to restore equilibrium
    • Net flow of water: towards the more concentrated solution (cytoplasm or surrounding medium)
  • Hypotonic solution: lower concentration of solute outside than inside the cell
    • Cell will gain water, and swell
  • Hypertonic solution: higher concentration of solute outside than inside the cell
    • Water will leave the cell; the cell will tend to shrink
  • Isotonic or isosmotic - concentration same, no net gain or loss
  • Passive transport, so not controlled by cell, and no energy expended
  • Penetration of alcohol into beet cells: an experiment
    • When alcohols enter the cell membrane, they can damage it, resulting in betacyanin (red pigment) release
    • Three alcohols: methanol, ethanol, 1-propanol
    • The longer the carbon chain, the more soluble in fatty acids
    • Experiment: is entry easier for larger molecules (more fat soluble) or smaller molecules?
    • To see if alcohols get in through small holes in the cell membrane, or dissolve through a lipid cell membrane
    • 9 test tubes, 2M, 4M, 8M for each alcohol
    • Results: penetration increases with length (i.e. increasing fat solubility)
    • So propanol-1 penetrates the most easily at the lowest concentration, etc
    • This means that the cell membrane behaves like a thin layer of fat

3Active transport

  • Cell membrane contains enzyme systems, actively carry certain types of molecules in or out of a cell
  • Can go against a concentration gradient; requires work
  • Examples: movement of ions, conservation of glucose and amino acids in the kidneys
  • Requires metabolic energy - factors like temperature and oxygen influence respiration and thus rate of active transport
  • Experiment: transport of chloride ions into turnip cells
    • Batch 1: in potassium chloride, kept at room temperature
    • Batch 2: in potassium chloride, kept at 2 degrees Celsius
    • Batch 3: no potassium chloride (control)
    • Get some turnip pieces, boil them, then measure amount of chloride through titration with silver nitrate
    • We boiled the turnip first to destroy the cell membrane to release the potassium chloride into the water
      • Indicator: 25% potassium chromate, reddish brown colour will appear
      • Titration reaction: silver ion + chloride ion to form silver chloride, happens first
      • Indicator reaction: 2 silver ions + chromate ions to form silver chromate, AFTER the above happens
      • So we can tell how much potassium chloride was taken up the cell through this titration reaction
    • Results: chloride was transported into cell, against concentration gradient
    • Dependent on metabolic activity of cells - increases with temperature up until body temperature (proteins denature then)