Which of the following events does not occur in the light reaction stage of photosynthesis?

Analyze the question: We need the event that does NOT happen in the light stage (light-dependent reaction) of photosynthesis.

Concept: Photosynthesis has two stages:

1. Light reaction (in the grana of the chloroplast, needs light): • Chlorophyll absorbs light energy and becomes excited. • Photolysis of water occurs: 2H₂O → 4H⁺ + 4e⁻ + O₂. In the older WASSCE/NECO textbook version, water splits into hydrogen ions (H⁺) and hydroxyl ions (OH⁻), and the OH⁻ gives up its electrons to form water and oxygen. • ATP is formed (photophosphorylation), and NADP is reduced to NADPH using the hydrogen. • Oxygen is released as a by-product.

2. Dark reaction (in the stroma, does not need light directly): • CO₂ is fixed and reduced by the hydrogen (from NADPH) using ATP to form glucose (carbohydrate).

Checking each option:

• A. Chlorophyll absorbs sunlight: Occurs in the light stage. ✔
• B. Carbon (IV) oxide combines with hydrogen: This is the reduction of CO₂, which is the dark reaction (Calvin cycle). ✘ It does not occur in the light stage.
• C. Photolysis of water occurs: Occurs in the light stage. ✔
• D. Hydroxyl ions are produced: In the WASSCE syllabus model, water splits into H⁺ and OH⁻ during photolysis. ✔

**Answer: B.

Tip: Remember the split: Light stage = water splitting, ATP, O₂ released. Dark stage = CO₂ is used up to make sugar. Whenever you see CO₂ combining or glucose forming, think dark reaction.