In designing an experiment to find out whether light is required for photosynthesis
(a) what is the principle of the design
(b) what control would you use?
a) In an experiment to find out whether light is needed for photosynthesis, light should be excluded from a destarched leaf or part of a destarched leaf. After a few hours of sunlight the covered leaf and an exposed leaf (or simply the partly covered leaf) should be tested for starch. Only the parts which received light should go blue with iodine.
(b) The control is the leaf, or part of the leaf, which has not been exposed to light and does not contain starch. Alternatively, if the exclusion of light from the leaf is thought to be the experiment, the parts of the leaf exposed to light constitute the control.