Glucose and starch
The main product is glucose, and almost none of it stays as glucose for long.
The main product
The glucose formed in photosynthesis is stored temporarily within the leaves, as starch.
C6H12O6
Why a plant converts it so quickly
Glucose dissolves. A leaf cell full of dissolved glucose would draw water in and swell, and the sugar would leak away to wherever the concentration was lower. Starch does not dissolve, so it can be piled up inside a cell without disturbing anything. Grade 10 put it plainly: starch is the form in which plants store carbohydrate.
Both molecules are already familiar
Glucose, C6H12O6, is the monosaccharide from the chapter on the chemical basis of life, and starch is one of the three polysaccharides met there — hundreds of glucose units joined together. Photosynthesis is where the first of those molecules actually comes from.
Temporarily is the word to notice
Starch in a leaf is a holding arrangement, not a destination. It is broken down again within hours and sent elsewhere, which is the next page — and it is also why the leaf of a plant kept in the dark for two days contains no starch at all, which is what makes the experiments in this chapter possible.