Photosynthesis is the process by which green plants make their own food using sunlight, water, and carbon dioxide. It is the source of nearly all energy in the food chain: making it one of the most important biological processes on Earth.
The equation
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂ In words: Carbon dioxide + Water + Light energy → Glucose + Oxygen Photosynthesis happens in the chloroplasts, specifically in the thylakoid membranes (light reactions) and the stroma (dark reactions/Calvin cycle).
What plants need for photosynthesis
• Chlorophyll: the green pigment that absorbs light. Absorbs red and blue light best; reflects green (hence plants look green). • Light: the energy source. More light = faster photosynthesis (up to a point). • Carbon dioxide: absorbed through stomata in leaves. • Water: absorbed through roots, transported up the xylem. • Suitable temperature: enzymes work best at 25-35°C.
Limiting factors
Even if one factor is in excess, photosynthesis is limited by the factor in shortest supply. • Low light intensity: the main limiting factor at dawn and dusk. • Low CO₂ concentration: common in greenhouses. • Low temperature: slows enzyme activity. Farmers use greenhouses to control these factors and boost crop yield.
Key points
- Equation: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂
- Happens in chloroplasts
- Chlorophyll absorbs red and blue light
- Limiting factors: light, CO₂, temperature
- Products: glucose (food) and oxygen (released)
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WAEC tip
WAEC often asks you to state factors that LIMIT photosynthesis and explain WHY each one does. Learn a one-sentence explanation for each factor.
Practise this
Describe an experiment to show that light is necessary for photosynthesis. Include the control, the variable, and how you would test for starch.
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