The process that takes place at the light stage of photosynthesis is?

Step 1: Analyze the question The question asks which process happens in the light stage (light-dependent reaction) of photosynthesis.

Step 2: The concept Photosynthesis has two stages:

1. Light stage (light-dependent reaction) • Takes place in the grana of the chloroplast. • Needs light and chlorophyll. • Light energy splits water molecules (photolysis): 2H₂O → 4H⁺ + 4e⁻ + O₂ • The hydrogen ions are picked up by NADP to form NADPH. • ATP is made (photophosphorylation). • Oxygen is released as a by-product.

2. Dark stage (light-independent reaction, Calvin cycle) • Takes place in the stroma. • CO₂ is fixed and reduced to carbohydrate (glucose) using the ATP and NADPH from the light stage. • This involves the formation of phosphoglyceric acid (PGA).

Step 3: Check each option

• A. Splitting of water molecules to form hydrogen ions: This is photolysis, which happens in the light stage. ✔
• B. Reduction of carbon (IV) oxide to form carbohydrate: This is the dark stage. ✘
• C. Formation of two molecules of phosphoglyceric acid: This is CO₂ fixation in the dark stage (Calvin cycle). ✘
• D. Breaking down of ATP molecules to produce energy: ATP is formed in the light stage and used in the dark stage. ✘

Step 4: Answer

💡 Exam tip: Remember "Light = Lysis of water" (photolysis, producing O₂, H⁺ and ATP). "Dark = Deposits carbon" (CO₂ fixed into sugar). If an option mentions water splitting, oxygen release, or ATP formation, think light stage. If it mentions CO₂, glucose, or PGA, think dark stage.