What is photosynthesis? Equation and glucose uses
Bioenergetics • Photosynthesis
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Key concepts
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Definition and location of photosynthesis
Photosynthesis is a chemical reaction that stores energy from light in the bonds of glucose. The reaction occurs inside chloroplasts that contain the green pigment chlorophyll; chlorophyll absorbs light and transfers the energy to the reactions in the chloroplasts, so light is a required input. Because energy is absorbed from the environment, photosynthesis is classified as an endothermic reaction.
Word and symbol equations
The word equation for photosynthesis is: carbon dioxide + water → glucose + oxygen. The fully balanced symbol equation is: 6CO2 + 6H2O → C6H12O6 + 6O2, with light shown above the reaction arrow to indicate that light energy drives the process. Recognising the chemical symbols CO2, H2O, O2 and C6H12O6 is essential when representing the reaction. The light energy breaks and reforms bonds so products contain more stored chemical energy than reactants.
How light causes the reaction (cause → effect)
Light is absorbed by chlorophyll, which causes electrons and molecules in the chloroplast to gain energy. These high-energy electrons drive a series of controlled chemical steps that convert carbon dioxide and water into glucose; therefore increased light intensity increases the rate of photosynthesis until another factor becomes limiting. Without light, the energy supply is absent and the reaction cannot proceed.
Uses of glucose produced
Glucose is used immediately in respiration to release energy for cellular processes. Excess glucose becomes insoluble starch for storage in leaves, stems or storage organs; starch prevents osmotic loss of water while storing carbohydrate. Glucose is converted into fats or oils and stored in seeds and other tissues for long-term energy reserves. Glucose polymerises to form cellulose, which strengthens cell walls and supports growth. Finally, glucose combines with nitrate ions absorbed from the soil to synthesise amino acids; amino acids join to form proteins needed for enzymes, structure and growth.
Transport and allocation of glucose
Glucose is transported from leaves to other plant parts as soluble sugars (often sucrose) via phloem during translocation. When a tissue requires energy, translocated sugar undergoes respiration; when storage is required, sugar converts to starch or lipids in storage organs. Phloem transport links source tissues (leaves) to sinks (roots, fruits, seeds) and so allocates the products of photosynthesis where needed.
Limiting factors and definition
A limiting factor is any condition that reduces the rate of photosynthesis. The four main limiting factors are light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll (often affected by magnesium availability). If any one factor is in short supply, the rate cannot rise further even if other factors increase. Understanding limiting factors explains why photosynthesis varies with environment.
Key notes
Important points to keep in mind