Why do plants convert glucose to starch in photosynthesis?

Glucose is soluble. If just glucose was stored in the plant cells then it could affect the way water moves in and out of the cells by osmosis by affecting the concentration gradient of solutes. Starch is insoluble in water so won't affect the way water moves in and out of the plant, meaning plants can store large amounts of starch in their cells.Starch molecules are very large so they cannot move out of the cell. This means they can act as energy stores. They can be converted back to glucoses needed to be used in respiration.