Limiting factors in photosynthesis

What is a limiting factor?

A factor that limits the rate of a physical process by being in short supply.

An increase in a limiting factor increases the rate of the process.

What does the law of limiting factors state?

“When a chemical process is affected by more than one factor, its rate is limited by the factor which is nearest its minimum value. If its quantity is changed it will directly affect a process.”

What does photosynthesis require?

• Energy in the form of light (usually from the sun)

• Chlorophyll and other pigments to absorb light energy

• Water to combine with carbon dioxide to form organic compounds

• A suitable temperature to provide optimum conditions for the enzymes that control photosynthetic reactions

  • Carbon dioxide which will be fixed in the Calvin cycle

  • Enzymes such as Rubisco to catalyse reactions.

What are the main limiting factors of photosynthesis?

• Light intensity

• Carbon dioxide concentration

• Temperature

A shortage of any of these factors will limit the maximum rate at which photosynthesis can take place.

Describe and explain the graph showing how light intensity affects the rate of photosynthesis.

As the light intensity increases, the rate of photosynthesis increases until it plateaus.

Region A: light intensity is low and prevents photosynthesis reaching a maximum rate – as light intensity increases so does the rate. This shows that light intensity is the limiting factor.

Region B: the amount of photosynthetic pigment or another factor such as carbon dioxide concentration or rubisco concentration, becomes the limiting factor.

Region C: has reached max rate of photosynthesis (E).

Region D: is the saturation point – an increase in light intensity causes no further increase in the rate of photosynthesis.

Describe and explain the graph showing how carbon dioxide concentration affects the rate of photosynthesis.

As the carbon dioxide increases, the rate of photosynthesis increases until it plateaus.

• At low CO₂ levels, photosynthesis is limited because enough CO₂ is not available . As the concentration increases, more CO₂ is available for the enzyme Rubisco to fix during the Calvin cycle, leading to an increased rate of photosynthesis.

• At higher CO₂ concentrations, the rate of photosynthesis continues to rise but eventually plateaus. This occurs when other factors (such as light intensity or temperature) become limiting, meaning photosynthesis cannot increase further despite higher CO₂ levels.

Describe and explain the graph showing how temperature affects the rate of photosynthesis.

As the temperature increases, the rate of photosynthesis increases up to an optimum. Past the optimum temperature, the rate decreases.

Increased temperature increases the rate of photosynthesis because the kinetic energy of the molecules involved increases.

Above a particular temperature, the enzymes (for example, rubisco in the Calvin cycle) denature, and the rate of photosynthesis decreases.