in the process of photosynthesis ,food A is prepared which gets converted into food D what are A and B why is A converted to B

In the process of photosynthesis the food prepared is Glucose which represents A and it gets converted to starch which represents B. A gets converted to B, because starch is not soluble in water and does not move away from the storage organs such as roots, stem etc. Starch does not affect the water concentration inside the cells.