15 Photosynthesis Lab p. 125
produces glucose and polysaccharides such as starch.
- TESTING IF LIGHT IS NECESSARY TO PRODUCE STARCH
Observe a Coleus or Geranium plant that had been covered.
Note: do parts B and D first, then part A while waiting.
- Sketch a partially covered leaf.
- Boil the leaf to remove pigments other than chlorophyll.
- Place leaf in hot ethanol to extract the chlorophyll.
- Rinse leaf and place in petri dish.
- Soak the leaf in potassium iodide (I2KI) for 5 minutes, then rinse.
Dark brown areas of the leaf
indicate presence of starch.
- TESTING IF CARBON DIOXIDE IS ABSORBED DURING PHOTOSYNTHESIS p. 126
Carbon dioxide forms carbonic acid in water:
CO2 + H2O -> H2CO3
- Use Phenol Red as a pH indicator: red
in basic and neutral solutions, turning yellow
Dilute the Phenol Red 1/10 tap water or distilled water.
Make one batch for the class: add 50 ml Phenol Red to 450 ml of water.
- Make the solution acidic (yellow)
CO2 into the solution. This will provide a source of carbon for photosynthesis.
- Cut Elodea to make 2 sets of 5-cm and 10-cm lengths, place in screw-top test tubes.
- One set will be under lights, the other in a dark cabinet (wrap with aluminum foil).
- Treat for about 60 minutes, note color changes.
- TESTING IF OXYGEN IS GENERATED DURING PHOTOSYNTHESIS
Demonstration of gas-trapping system. Glowing splint indicates presence of oxygen.
- PIGMENT EXTRACTION p. 127
paper chromatography to extract chlorophylls, carotenoids and xanthophylls (a polar carotenoid).
- Grind spinach
NOTE: do not over-rinse; you only need a tiny, concentrated sample for the chromatography.
- Cut paper 2cm longer than test tubes.
- Use capillary pipet to place a spot of extract on the paper, about 1 cm from bottom.
- Mark the spot with a pencil.
- Place strip into test tube containing solvent
(acetone and toluene). The solution, with the suspended pigments, should move up by capillary action.
after about 30-60 minutes.
- ABSORPTION/ACTION SPECTRUM ANALYSIS p. 128
Use a band from chromatography paper or extract in a cuvette.
- Warm up Spectronic 20D for 15 minutes.
- See Appendix 3 p. 201
- Record absorbance from 400 to 700 nm.
- Remove cuvette.