The first stable product of photosynthesis is

Analyze the question: It asks for the first stable product of photosynthesis, which is different from the final storage product or the transport form of sugar.

Explain the concept: Photosynthesis has two stages:

1. Light reaction (in the grana): water is split (photolysis), and ATP and NADPH are produced, releasing oxygen.
2. Dark reaction / Calvin cycle (in the stroma): CO₂ is fixed and reduced using ATP and NADPH.

In the Calvin cycle, CO₂ combines with RuBP, and the first stable compound formed is a 3-carbon molecule, 3-phosphoglycerate (PGA). This is then reduced to triose phosphate (G3P), which is used to build glucose. So in strict biochemical terms, the first stable product is PGA, but PGA is not among the options.

Working through the options:

• A. Starch: a storage product. Glucose units are joined together to form it later, so it is not the first.
• B. Fructose: a sugar formed along the way, but it is not the main first sugar product.
• C. Glucose: the first stable carbohydrate product that is made from the triose phosphates. This is the answer at WASSCE level.
• D. Sucrose: the transport form of sugar. It is made from glucose and fructose, so it comes later.

Sequence: CO₂ + H₂O → glucose → (converted to) sucrose for transport, or starch for storage.

At secondary school level, glucose is the expected answer, because PGA is not offered as an option and the other choices are all made from glucose.

Exam tip: Remember the order Glucose → Sucrose (transport) → Starch (storage). If a question asks for the "first" carbohydrate product, choose glucose. Starch is what you test for with iodine in a leaf, but it is made afterward from glucose.