ICSE Class 10 Biology: Photosynthesis — Complete Notes 2026

What is Photosynthesis?

• Process by which green plants synthesise glucose using sunlight, CO₂ and water
• Overall equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in presence of light and chlorophyll)
• Site: chloroplasts; specifically grana (light reactions) and stroma (dark reactions)

Structure of Chloroplast

• Double membrane-bound organelle; grana = stacked thylakoids; stroma = fluid-filled matrix
• Photosynthetic pigments in thylakoid membranes: chlorophyll a (blue-green), b (yellow-green), carotenoids
• Absorption spectrum: maximum at red (680 nm) and blue (430 nm) wavelengths

Light Reactions (Photo-stage)

• ATP and NADPH produced using light energy; water is photolysed → O₂ released
• Photosystem I and II; electron transport chain in thylakoid membrane
• O₂ produced is a by-product of water splitting, not CO₂ breakdown

Dark Reactions — Calvin Cycle

• CO₂ fixation: CO₂ + RuBP → 3PGA (3-carbon acid) catalysed by RuBisCO
• Uses ATP and NADPH from light reactions to reduce PGA to G3P (sugar precursor)
• 3 turns of cycle needed to produce 1 molecule of G3P (net gain)

Factors Affecting Photosynthesis

• Light intensity: increases rate up to light saturation point; carbon dioxide then becomes limiting
• CO₂ concentration: low CO₂ (0.04%) limits rate; increasing CO₂ increases rate
• Temperature: increases rate up to 35°C optimum; enzymes denature above 40°C

Experiments on Photosynthesis

• Variegated leaf: starch test shows only green (chlorophyll-containing) areas photosynthesise
• Oxygen evolution: Elodea bubbles; count bubbles per minute under different light intensities
• Starch test procedure: de-starch overnight, test leaf with iodine after 6 hours of light

Significance of Photosynthesis

• Primary source of food and oxygen for all life; base of food chain
• Locks carbon from atmosphere into organic compounds; carbon cycle regulation
• ICSE application question: why does a sealed room with a plant not maintain O₂ at night?