Overview
This lecture covers all essential aspects of IGCSE Biology Topic 6.1: Photosynthesis, including definitions, equations, roles of chlorophyll, uses of products, relevant experiments, and limiting factors.
Photosynthesis: Definition and Equations
- Photosynthesis is the process by which plants make carbohydrates using carbon dioxide, water, and light energy.
- The word equation: carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll).
- The balanced chemical equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light and chlorophyll).
Role of Chlorophyll and Raw Materials
- Chlorophyll is a green pigment in chloroplasts that captures light energy for photosynthesis.
- Carbon dioxide enters leaves through stomata; water is absorbed by roots and transported via xylem.
- Chlorophyll absorbs light energy to split water into hydrogen and oxygen.
- Oxygen is released; hydrogen combines with carbon dioxide to form glucose.
Uses and Storage of Carbohydrates in Plants
- Glucose is quickly converted to sucrose for transport in phloem to non-photosynthetic parts.
- Excess sugar is converted to starch for storage in chloroplasts, stems, roots, and tubers.
- Glucose also builds cellulose for cell walls and provides energy via respiration.
- Nectar, made from various carbohydrates, attracts insects for pollination.
Minerals Required for Photosynthesis
- Nitrogen ions help plants make amino acids and proteins.
- Magnesium is essential for producing chlorophyll.
Key Experiments in Photosynthesis
- Starch test: boil leaf, remove chlorophyll with ethanol, add iodine—blue-black indicates starch.
- Chlorophyll necessity: only green (chlorophyll-containing) areas of variegated leaves test positive for starch.
- Light necessity: foil-covered leaf parts do not produce starch; exposed parts do.
- Carbon dioxide necessity: plants without CO₂ do not produce starch.
- Oxygen production: aquatic plants produce oxygen bubbles during photosynthesis.
- Light & dark gas exchange: pondweed in light uses CO₂; in dark, releases CO₂.
- Light intensity: higher light leads to more oxygen bubbles (faster photosynthesis).
- Temperature: higher temperature increases photosynthesis until enzymes denature.
- CO₂ concentration: more CO₂ increases rate of photosynthesis.
Limiting Factors in Photosynthesis (Extended)
- Limiting factor: a necessary condition in short supply that limits photosynthesis (light, CO₂, or temperature).
- Increasing one factor raises photosynthesis rate until another factor becomes limiting.
- Farmers use greenhouses to optimize light, temperature, and CO₂ for crop growth.
Key Terms & Definitions
- Photosynthesis — process where plants make carbohydrates using light, CO₂, and water.
- Chlorophyll — green pigment capturing light energy in plants.
- Starch — storage form of carbohydrates in plants.
- Sucrose — carbohydrate transported within plants.
- Limiting factor — factor in short supply that restricts the rate of a process.
Action Items / Next Steps
- Review the photosynthesis experiments and practice interpreting results.
- Memorize both the word and balanced chemical equations for photosynthesis.
- Read Topic 6.2 on Leaf Structure for the next lesson.