The conversion of glucose to starch in the leaf during the day principally

Analyze the question: During the day, leaves make glucose by photosynthesis. Some of it is converted to starch. The question asks for the main reason for this conversion.

Explain the concept:

• Glucose is soluble and osmotically active. If it built up in leaf cells, it would lower the water potential of the cell sap. Water would then enter the cells by osmosis, making them turgid and possibly causing them to burst.
• Starch is insoluble, large, and osmotically inactive. Converting glucose to starch removes the glucose from solution, so the cell's osmotic balance stays stable.
• The starch is also a temporary store. It is later changed back to soluble sugar (sucrose) and transported away at night. But this is a secondary benefit.

Go through the options:

• A. Enables photosynthesis: Wrong. Photosynthesis needs light, CO₂, water and chlorophyll. It doesn't need starch formation. If anything, removing glucose helps photosynthesis continue by preventing product build-up, but that is not the principal reason.
• B. Prevents osmotic problems: This fits. Insoluble starch does not change the cell's osmotic pressure.
• C. Enables the leaf to store the starch: This is circular. Storage is a result of the conversion, not its purpose. Leaves are also not the main storage organs of a plant, since storage happens mainly in roots, tubers and seeds.
• D. Enables glucose to be used up: Wrong. Converting glucose to starch is not "using up" glucose in respiration or growth. It only changes its form.

Answer: The conversion principally prevents the osmotic problems that would come from accumulating soluble glucose in the leaf cells.

Exam tip: Remember "soluble = osmotic effect, insoluble = no osmotic effect." Whenever a question asks why a plant stores sugar as starch, think "insoluble, so it doesn't affect water potential."