Go to 3.8 Assessment statements

8.2.8  Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide. (Michael)

http://www.bbc.co.uk/schools/gcsebitesize/science/images/photosyn_1.gif

THE CONCEPT OF LIMITNG FACTORS

Light intensity, carbon dioxide concentration and temperature are three factors that can determine the rate of photosynthesis. If the level of one of these factros is chaged, the rate of photosynthesis changes. Usually, only changes to one of the factros will affect the rate of photosynthesis in a plant at a particualr time. This is the factor that is nearest to its minimum and is called the limiting factor. changing the limiting factor increases or decreases the rate, but changes to the other factors have no effect. This is because photosynthesis is a complex process involving many steps. The overall rate of photosynthesis in a plant is determined by the rate of whichever step is proceeding most slowly at a particular time. This is called the rate-limiting step. The three limiting factros affect different rate-limiting steps.

The effect of Light intensity

At low light intensities, there is a shortage of the products of the light-dependent reactions - NADPH and ATP. The rate-limiting step in the Calvin cylce is the point whre glycerate 3-phosphoate is reduced. At high light intensities some other factor is limintg . Unless a plant is heavily shaded, or the sun is rising or setting, light intensity is not usually the limiting factor.

The effect of CO2 concentration

At low and medium CO2 concentrations, the rate-limiting step in the Calvin cycle is the point where CO2 is fixed to produce glycerate 3-phosphate. RuBP and NADPH accumulate. At high CO2 concentrations some other factor is limiting. Because the level of of carbon dioxide in the atmosphere is never very high, carbon dioxide concentration is often the limting factor.

The effect of temperature

At low temperatures, all of the enzymes that catalyse the reactions of the Calvin cycle work slowly. NADPH accumulates. At intermediate temperatures, some other facotr is limitng. At high temperatures, RuBP carboxylase does not work effectively, so the rate-limiting step in the Calvin cycle is the point whre CO2 is fixed. NADPH accumulates.

http://www.bryoecol.mtu.edu/chapters/11-3PhotoLimits.pdf