Photosynthesis converts water and carbon dioxide to oxygen and sugar using energy from the sun. This process takes place in chloroplasts, located in the leaves of plants. Chloroplasts give leaves their green colour since they absorb red and blue light, thus reflecting green light. Carbon dioxide enters through stomates, small openings on the underside of leaves. Water is drawn up by the roots from the soil and into the leaves via the xylem. When no light is available, plants use stored sugars, in the form of starch, for food. In this experiment, you can determine that light is needed for photosynthesis to take place.
Destarch the Plant
Place a Geranium (Pelargonium), into a dark place for 24 to 48 hours. This will cause the plant to have to use its stored starch (a sugar) for energy since no sunlight is available for photosynthesis. Be sure to water the plant beforehand. After the allotted time, remove a single leaf and test it for starch before proceeding.
Test Leaf for Starch
Place the leaf in boiling water for 1-2 minutes and remove. Next, place the leaf in industrial methylated spirit (IMS) and heat gently for a few minutes. Finally, place the leaf in cold water for a few minutes, then add iodine. If the leaf turns purple, there is still starch present, and the plant must remain in the dark for more time. If the leaf turns yellow-orange-red in colour, the starch has been used up and the plant is ready for the rest of the experiment.
First, prepare a cover for a leaf. Obtain two pieces of black construction paper or aluminium foil and punch two holes in one side with a hole puncher or pencil. Then, remove the plant from the darkness and quickly cover a leaf with the piece of black construction paper or aluminium foil. The paper or foil should completely cover the leaf on both sides, with the top side of the leaf covered by the hole-punched paper.
Expose to Sunlight
Expose the plant to sunlight with the leaf covered as above for several days. Water the plant as usual.
Repeat Test and Results
Repeat the decolorizing and testing procedure given above on the leaf. Note that only the areas of the leaf that were left uncovered (where the holes were punched in the cover), have produced new sugars via photosynthesis, and are thus grey-blue-black, while the rest of the covered leaf shows no sugars made, and are thus yellow-orange-red.
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