Lab 3 summary

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)