Photosynthesis
Photosynthesis is the process by which green plants convert light energy into chemical energy stored in glucose. Learn the equation, limiting factors, and the required practical.
What is photosynthesis?
Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It takes place primarily in the chloroplasts of plant cells, using a green pigment called chlorophyll that absorbs light energy.
This process is fundamental to life on Earth — it produces the glucose that plants need for respiration, growth, and reproduction, and it releases oxygen as a by-product.
Photosynthesis is an endothermic reaction in which light energy is transferred to chemical energy. Carbon dioxide and water are converted into glucose and oxygen.
The photosynthesis equation
You must memorise both the word equation and the balanced symbol equation for your GCSE exam.
Word equation
carbon dioxide + water → glucose + oxygen
Balanced symbol equation
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Light energy is required for the reaction to proceed, and chlorophyll acts as a catalyst.
Where does photosynthesis happen?
Photosynthesis occurs in the mesophyll cells of leaves. These cells contain large numbers of chloroplasts. Leaves are adapted for efficient photosynthesis:
- Broad and flat shape — large surface area to absorb light
- Thin structure — short diffusion distance for gases
- Stomata on the underside — allow CO₂ in and O₂ out
- Network of veins — delivers water and carries away glucose
- Palisade mesophyll cells near the top — packed with chloroplasts
Limiting factors
A limiting factor is any factor that, when in short supply, slows down the rate of photosynthesis regardless of how much of the other factors are available.
1. Light intensity
As light intensity increases, the rate of photosynthesis increases — up to a point. Eventually, the graph plateaus because another factor becomes limiting.
2. Carbon dioxide concentration
Increasing CO₂ concentration increases the rate of photosynthesis until something else becomes limiting. In commercial greenhouses, farmers pump extra CO₂ to boost crop yields.
3. Temperature
Temperature affects the enzymes involved. As temperature increases, the rate increases due to greater kinetic energy. However, above 40–45°C, enzymes denature and the rate drops sharply.
Students often say plants "only photosynthesise during the day and only respire at night." In reality, plants respire all the time. During the day, the rate of photosynthesis usually exceeds the rate of respiration.
Uses of glucose
The glucose produced during photosynthesis has several uses:
- Respiration — broken down to release energy
- Making cellulose — strengthens cell walls
- Making amino acids — combines with nitrate ions for protein synthesis
- Making starch — insoluble storage molecule
- Making lipids — energy storage in seeds
| Use | What happens | Why it matters |
|---|---|---|
| Respiration | Glucose is broken down to release energy | Powers all life processes |
| Cellulose | Glucose molecules are linked into chains | Structural support for cell walls |
| Amino acids | Glucose + nitrate ions → amino acids | Growth and enzyme production |
| Starch storage | Glucose converted to insoluble starch | Energy reserve without affecting osmosis |
| Lipids | Glucose converted to fats and oils | Energy-dense storage in seeds |
The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration. At this point, there is no net gas exchange.
Chlorophyll absorbs red and blue light most effectively and reflects green light. This reflected green light is what makes leaves appear green.
Yes — chlorophyll absorbs light energy regardless of the source. This is why greenhouse growers use LED or fluorescent lights to extend growing hours.
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