Limiting factors in Photosynthesis

• Discuss the limiting factors in photosynthesis with reference to carbon dioxide concentration, light intensity and temperature.

Law of limiting factors

• The limiting factor for a metabolic process is the factor that is present at the lowest or least favourable value.

Carbon dioxide concentration

• Average carbon dioxide content of the atmosphere is 0.03-0.04%

• Increase in CO2 up to 0.5% will increase the rate of photosynthesis

• BUT optimum is 0.1%

0.1 0.5 Carbon dioxide conc

Rateof photosynthesis

With reference to the graph:

• What makes 0.1% the optimum figure?

• Why is there little point in recording rates above the 0.5% CO2 concentration?

Light intensity

• Experiments carried out by Blackmann

• At low light intensities, the rate of photosynthesis increases as light increases

• Beyond 20 kilolux the rate levels off

Effect of light

• Allows opening of stomata so CO2 can enter

• Absorbed by chlorophyll excites electrons• Splits water molecules photolysis

Light intensity and the compensation point

Effect of temperature

• Mostly affects enzyme controlled reactions of Calvin cycle

• At high light intensity, increase in temperature increase in rate.

• At low light intensity, little change.• Above 30oC photosynthesis is

reduced at both high and low light intensity- can you suggest why?

Effect of temp. and light togetherRate of photosynthesis

Temperature/oC

High light intensity

Low light intensity

• What temperature is likely to be the optimum temperature at high light intensity?

• What is likely to be the temperature at which photosynthesis stops?

• Explain why this happens.

Now do questions on p67

Graphs and quiz

• http://www.cix.co.uk/~argus/Dreambio/fertilisers%20and%20crops/limiting%20factors.htm