A student investigated whether carbon dioxide is needed for a plant to make starch. Two identical destarched potted plants were each enclosed under a glass ...

This tests your grasp of a controlled photosynthesis experiment: the two jars are identical except for one factor, so any difference in the starch result must be caused by that factor.

(a) The independent variable is the one thing the student deliberately changes: the presence or absence of carbon dioxide in the air around the plant. [1]

(b) Both plants are destarched first (kept in the dark for a day or two) to remove the starch that was already in the leaves [1], so that any starch found at the end must have been made during the six hours of the investigation [1]. Without this step you could not tell new starch from old.

(c) The soda lime is there to absorb and remove the carbon dioxide from the air under that jar [1], creating the carbon dioxide free condition needed for the comparison.

(d) Predicted starch results:

(e) The plant supplied with carbon dioxide could photosynthesise, so it made starch [1]. The plant over the soda lime had no carbon dioxide [1], therefore it could not photosynthesise and made no starch [1]. This shows carbon dioxide is a raw material needed for the plant to make starch.

(f) The grease seals the jar and makes it airtight so gases cannot enter or leave [1]; this keeps each carbon dioxide condition pure for the whole six hours.

(g) To test a leaf for starch: boil the leaf in water and then in ethanol to remove the chlorophyll [1] (the ethanol is heated in a water bath, away from a naked flame, because it is flammable); then rinse the softened leaf, spread it out, add iodine solution and look for a blue-black colour [1], which shows starch is present.