Students should be able to explain the effects of temperature, light…

The idea

Photosynthesis rate depends on several interacting factors. Light intensity supplies energy for the reaction, so raising it increases the rate until another factor becomes limiting. Carbon dioxide is a reactant, so increasing its concentration raises the rate until limited elsewhere. Temperature affects enzyme activity: the rate rises with temperature up to an optimum, then falls rapidly as enzymes denature. Chlorophyll amount determines how much light energy can be absorbed, so more chlorophyll supports a faster rate. A practical example is pondweed exposed to a lamp: moving the lamp closer increases light intensity and oxygen bubbles per minute, but eventually the rate levels off.

What earns the marks

• Light intensity: as light intensity increases, the rate of photosynthesis increases because more energy is supplied, until another factor becomes limiting.
• Carbon dioxide concentration: increasing carbon dioxide concentration increases the rate because it is a reactant used to make glucose, until another factor becomes limiting.
• Temperature: the rate increases with temperature up to an optimum because enzyme activity increases, then decreases at higher temperatures because enzymes denature.
• Chlorophyll amount: a greater amount of chlorophyll allows more light energy to be absorbed, so the rate of photosynthesis increases.
• Limiting factor idea: at any point the factor in shortest supply limits the rate, so changing other factors may have little effect until that factor is relieved.

Common mistakes and how to put them right

• Saying the rate keeps rising indefinitely with light intensity.

  Put it right: It plateaus when another factor becomes limiting.

• Stating that high temperature increases the rate without limit.

  Put it right: Above the optimum enzymes denature and the rate falls.

• Confusing chlorophyll amount with light intensity.

  Put it right: Chlorophyll is the pigment amount in the chloroplast, while light intensity is the external light supply.

Exam tips

• Link each factor to its biological role: light and chlorophyll for energy absorption, carbon dioxide as a reactant, temperature for enzyme activity.
• Use the phrase 'until another factor becomes limiting' when describing a plateau on a graph.
• When explaining temperature, refer to enzymes and denaturation rather than saying heat 'kills' the plant.