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When green plants photosynthesise, the leaf produces simple sugars such as glucose, and sugar phosphates such as ribulose 1,5 biphosphate. These are transported to all leaf cells and other organs, and may be deposited as starch anywhere in the plant. The plant makes these products during the light independent stage of photosynthesis, and converts them to starch, other carbohydrates, lipids, and amino acids after the light independent stage and so light is not necessary.
This explains the presence of a small amount starch in the leaf cells left in the dark with glucose solution. Only a small amount is present because, starch can only be synthesised by using sugar phosphates and enzymic processes using starch phosphorylase enzymes. This would have been possible by using the small amounts of sugar phosphates already present in the leaves.
The presence of starch in the leaves left in glucose in the light is because the leaves have a plentiful supply of glucose, as well as being able to photosynthesise and produce their own simple sugars and phosphates. The reason for starch being present in the leaves left in water in the light, is that as there was plentiful light, the leaves had the capacity to photosynthesise, and produce their own glucose and other sugars and sugar phosphates, of which some was obviously converted to starch.
No starch was present in the leaves in the water in the dark because no light was present so no sugars could be produced through photosynthesis, neither was there any glucose available for the leaves to convert into starch. The potato results prove the idea that starch can only be synthesised by using sugar phosphates, because tube C was the only one which showed any sign of there being starch present, and tube C was the only one that contained Glucose 1-phosphate as well as the starch phosphorylase.
Starch is used by cells for storage (in a starch grain), as the molecules are big and cannot get out. It is also insoluble and this means that it stays where it is put and does not dissolve in water. The leaf experiment already showed that phosphates are more important for making starch than light, and this just reinforces the fact. Evaluation The experiment could have produced inaccurate results. The leaves were placed in a cupboard, for a dark place and it may have allowed a small amount of light to get through.
The temperature in the cupboard and under the daylight lamp may have also fluctuated, altering the results. Also, when pouring the de-ionised water into the beaker for the leaves to rest on, some air may have been absorbed. The plant samples used were kept in a bin liner overnight before the experiment was done, to destarch the plant. This may not have been long enough for the plant to use all its starch reserves, which would mean that starch could have been detected when it was really there all the time.