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How do plants turn photosynthetic products into starch and sucrose?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — Photosynthesis Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

In plants, the primary product of photosynthesis is the intermediate molecule glyceraldehyde 3-phosphate (Gal3P). Plants convert this product into sucrose and starch through distinct processes based on the plant's metabolic needs and location:1

Sucrose Synthesis: Gal3P is exported from the chloroplasts into the cytoplasm of green cells. There, it is used for the biosynthesis of sucrose, which is then translocated to other parts of the plant to provide nourishment.12
Starch Synthesis: When photosynthetic carbon fixation exceeds the plant's immediate requirements, sugar phosphates accumulate in the cytosol. This accumulation triggers regulated enzymes within the chloroplast to increase starch synthesis. Starch is a polysaccharide formed by the condensation of multiple glucose units. Unlike sucrose, which is transported, starch is stored within the chloroplasts and can be broken down at night to provide energy for the plant's physiological needs.132