Learning focus
Develop accurate biological explanations, interpret plant diagrams and data, and connect practical evidence with theory.
Light intensity
At low light intensity, light is limiting and increasing it increases the rate. At higher light, the graph levels off because another factor, often carbon dioxide concentration or temperature, becomes limiting.
Carbon dioxide concentration
Increasing carbon dioxide raises the rate until chloroplast reactions are limited by light, temperature or enzyme capacity. Enrichment is useful only when other conditions are adequate.
Temperature
As temperature rises, enzyme-controlled reactions accelerate because particles have more kinetic energy and successful collisions are more frequent. Above an optimum, enzymes denature and rate falls.
Limiting factor
A limiting factor is the factor in shortest effective supply that restricts rate. Improving a non-limiting factor produces little or no increase. The limiting factor can change with time of day or season.
Agricultural application
Greenhouses may use artificial lighting, heating and carbon-dioxide enrichment. The gain in crop yield must be balanced against energy and equipment costs. Controlling one factor is wasteful if another remains limiting.
Practical or data skill
Interpret two curves measured at different carbon-dioxide concentrations. Identify the limiting factor in each region and explain the plateau.
Examination tip
A plateau means another factor is limiting; it does not mean photosynthesis has stopped.
Review questions and suggested answers
Question 1
Why does a light-intensity graph level off?
Suggested answer
Another factor becomes limiting.
Question 2
Why can very high temperature reduce rate?
Suggested answer
Photosynthetic enzymes denature and active sites change shape.
Question 3
What is a limiting factor?
Suggested answer
A factor in shortest effective supply that restricts the rate of a process.
