What is photosynthesis?
Photosynthesis is the process by which green plants make their own food (glucose) using light energy. It takes place in chloroplasts, which contain the green pigment chlorophyll that absorbs light energy and transfers it to chemical energy in glucose.
Key idea
Word equation:
carbon dioxide + water → (light + chlorophyll) → glucose + oxygen
Balanced symbol equation:
6CO2 + 6H2O → C6H12O6 + 6O2
Raw materials and products
- Raw materials: carbon dioxide (from the air) and water (from the soil).
- Products: glucose (used for energy, growth and storage as starch) and oxygen (released as a waste gas).
- Energy source: light, usually from the Sun.
Limiting factors
The rate of photosynthesis is affected by light intensity, carbon dioxide concentration and temperature. Whichever is in shortest supply limits the rate. On a dull day, low light intensity is usually the limiting factor. On a bright day, carbon dioxide concentration or temperature may become the limiting factor instead.
The glucose made in photosynthesis is used in several ways: some is used at once in respiration to release energy; some is converted to starch for storage; some is used to make cellulose for cell walls; and some is combined with nitrogen (from nitrate ions) to make amino acids for proteins. In this way photosynthesis is the starting point for almost all the food and energy in living organisms.
Remember
- Photosynthesis takes in carbon dioxide and gives out oxygen.
- Chlorophyll absorbs light energy; it is not itself used up.
- Learn the balanced equation: 6CO2 + 6H2O → C6H12O6 + 6O2.