What is photosynthesis?
Photosynthesis is the process plants use to make glucose from carbon dioxide and water. It happens in chloroplasts, where chlorophyll absorbs light energy.
The glucose made by photosynthesis can be used in respiration, stored as starch, converted into cellulose, or used to make other useful substances.
Photosynthesis word equation
In exam answers, write the reactants on the left and the products on the right. Do not forget the plus signs between substances.
Photosynthesis symbol equation
The symbol equation shows the same reaction using chemical formulae. The large numbers balance the equation; the small numbers inside formulae must not be changed.
Why photosynthesis is endothermic
Photosynthesis is endothermic because energy is transferred from the environment to the plant. Light energy is absorbed by chlorophyll and used to make glucose.
Limiting factors of photosynthesis
- Low light intensity can slow the rate because less energy is available.
- Low carbon dioxide concentration can slow the rate because there is less reactant.
- Temperature affects enzyme-controlled reactions, so very high temperatures can reduce the rate.
- The amount of chlorophyll can limit how much light energy is absorbed.
Uses of glucose in plants
- Used in respiration to release energy.
- Converted into insoluble starch for storage.
- Used to make cellulose for cell walls.
- Combined with nitrate ions to make amino acids and proteins.
- Converted into lipids such as fats and oils for storage.
Required practical
A common required practical investigates how light intensity affects the rate of photosynthesis in pondweed. Students often measure bubbles of oxygen or the volume of oxygen produced over time.
Keep the control variables the same, such as temperature, carbon dioxide concentration, and the length of pondweed. Change only the distance from the lamp or light intensity.
Worked example
Question: Why does increasing carbon dioxide concentration increase the rate of photosynthesis up to a point?
Carbon dioxide is a reactant in photosynthesis, so increasing its concentration gives the plant more reactant to make glucose. The rate rises until another factor, such as light intensity or temperature, becomes limiting.