Photosynthesis · GCSE Biology

The photosynthesis reaction

Photosynthesis happens in chloroplasts. Chlorophyll absorbs light energy, which is used to make glucose.
Word equation: carbon dioxide + water → glucose + oxygen (light is needed).
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Photosynthesis is endothermic: energy is transferred from the environment (as light) to the chloroplasts.

The rate of photosynthesis is affected by temperature, light intensity, carbon dioxide concentration and the amount of chlorophyll.
A limiting factor is the factor in shortest supply, which stops the rate increasing.
Raising light intensity or CO₂ concentration increases the rate until another factor becomes limiting, so the graph levels off.
Temperature increases the rate up to an optimum; above it, the enzymes involved are denatured and the rate falls.
(Higher) Light intensity is inversely proportional to the square of the distance from the light source: light intensity ∝ 1 ÷ distance².

Farmers use greenhouses to control conditions: heaters for temperature, lamps for light, and paraffin heaters or gas to raise CO₂.
(Higher) Growers must balance the cost of extra heat, light or CO₂ against the value of the extra crop, so rates are not always kept at the maximum.

Some glucose is used in respiration to release energy.
Glucose is converted into insoluble starch for storage, and into fats or oils for storage (especially in seeds).
Glucose is used to make cellulose, which strengthens cell walls.
Glucose is combined with nitrate ions from the soil to make amino acids, which are used to make proteins.

Put a piece of pondweed in a boiling tube of water with sodium hydrogencarbonate (to supply CO₂).
Place a lamp at a measured distance and count the bubbles of oxygen (or collect the gas in a gas syringe) in a set time.
Repeat at different distances. Keep temperature and CO₂ constant; a beaker of water between lamp and tube can absorb heat.
As the lamp moves closer, light intensity increases and more oxygen is produced, until another factor becomes limiting.

Photosynthesis: The process in which plants use light energy to make glucose from carbon dioxide and water.
Chlorophyll: The green pigment in chloroplasts that absorbs light.
Endothermic: A reaction that takes in energy from the surroundings.
Limiting factor: The factor in shortest supply that stops the rate of a process from increasing.
Light intensity: How bright the light is; it falls as distance from the source increases.
Inverse square law: Light intensity is proportional to 1 ÷ distance².
Starch: An insoluble carbohydrate that plants use to store glucose.
Cellulose: A carbohydrate made from glucose that strengthens plant cell walls.
Nitrate ions: Mineral ions absorbed from soil and used to make amino acids and proteins.