Lab 6 summary CC-BY-NC

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Summary of Lab 6 for BIOL 112 (Winter 2011). Week of Feb 14, 2011. It was a lovely week.


1Introduction to photosynthesis

  • Unique to plants (and some bacteria and protists lol)
  • Pigments capture light energy, convert to chemical energy
    • Principal pigments: green chlorophylls
    • Two forms: chlorophyll a (blue-green) and b (yellow-green)
    • Primarily absorb blue and red light (narrow range of the light spectrum)
    • Accessory pigments: less efficient (carotenoids)
      • Absorb blue-green light
      • Examples: carotene, xanthophylls; can come in various colours
    • Pigments found in organelles called chloroplasts (where photosynthesis takes place)
  • The basis process: 6 carbon dioxide + 12 water --> sugar + 6 water + 6 oxygen (from original water molecule)
    • The water molecules are split by light
    • Measurement of the amount of oxygen evolved can indicate the rate of photosynthesis under different conditions

2Rate of photosynthesis under varying light conditions

  • Procedure:
    • Overhead lights turned off
    • Assemble manometer
    • Fill test tube with sodium carbonate solution (source of carbon dioxide)
    • Obtain sprig of Elodea, cut 1cm off broken end, place rest of sprig in test tube (should be no air in test tube)
    • Place test tube in rack, fill beaker with cold water, place in front of rack (heat sink, prevent heat from killing Elodea lol)
    • Make a light barrier around three sides of the test tube rack
    • Start the 30cm mark, leave for 15 minutes; record change in volume
    • Same for 5cm mark
    • Now, still at 5cm, repeat for three filters: green, red, blue (only the desired colour will pass through)
    • Make sure to normalise for 5 minutes to white light before moving to next colour filter
  • Results:
    • Distance: more oxygen evolved when plant is closer to light (greater light intensity)
    • Colour: green least, red and blue best
      • Chlorophyll a and b reflect green, but accessory pigments can absorb green etc
      • Absorb red and blue, but blue is better because it has a higher frequency, so more energy etc

3Ascending paper chromatography of plant pigments

  • Alcohol/acetone (?) extract of leaves provided (pigment solution)
  • Chlorophyll fluorescence: hold it up to light, intense red colour emitted
    • Substance absorbs radiant energy, immediately re-emits some of it (longer wavelength)
  • Process:
    • Use a glass slide, draw pigment onto the filter paper strip many times until dark
    • Add petroleum ether: acetone (a bit) to test tube; place paper in it (streak should be above)
    • Allow the solvent to move up the paper, 20 mins; pigments separate into 4 distinct bands
      • Top: carotenes (yellow-orange)
      • Xanthophylls (yellow)
      • Chlorophyll a (blue-green)
      • Chlorophyll b (yellow-green)
      • Also, brown bands of photo-degraded chlorophyll and translucent bands of lipid may be seen
    • Note: if we had used a pigment that was not soluble in the solvent, it wouldn't have moved at all
    • Also, if we used a different solvent, the bands would be different (because of different solubility in the solvent)
  • More soluble substances move up more (more soluble in the mobile phase etc)

4The necessity of light for chlorophyll formation

  • Chlorophyll formation almost always requires light
  • Demonstration: seedlings of peas, grown in either light or total darkness
  • Light inhibits growth of the stem in length, but stimulates growth of leaves
  • Those grown in the dark are longer, but no leaves, and
  • Red or blue light but not green light are effective for inducing greening
  • Some plants can form chlorophyll even without light
  • Chlorophyll formation also requires a certain temperature range and certain ions
  • A plant grown in the dark might lack chlorophyll because photochlorophyll can only become chlorophyll in the presence of light
    • The reaction is quick, so if we leave something in light for a short while, will quickly become green

5Formation of starch in photosynthesis

  • Geranium plants kept in darkness for 3 days, ensure that reserve food materials (mostly starch) were used up
  • In leaves, starch is readily degraded to sugar, which then is respired, producing chemical energy
  • After the starvation period, leaves partly covered with foil
  • Anyways, results:
    • Starch molecules: don't diffuse from light-receiving parts of leaves to covered parts
    • Because they are too large
    • In some plant parts, starch can be formed even in darkness ... from high concentrations of glucose; energy comes from glucose respiration