6. What happens at the light saturation point?

As light intensity is increased, eventually an intensity is reached above which light no longer is the factor limiting the overall rate of photosynthesis. We say that a process is 'saturated' for a particular reactant when adding more of it does not increase the rate, just as a sponge becomes saturated with water when it can't hold any more. At the light saturation point, increasing the light no longer causes an increase in photosynthesis.


Above the light saturation point, the light-dependent reactions are producing more ATP and NADPH than can be used by the light-independent reactions for CO2 fixation.

What limits the rate of photosynthesis above the light saturation point? Under normal conditions, availability of CO2 is the limiting factor. The concentration of CO2 in the atmosphere is relatively low. At approximately 0.035%, the amount of CO2 in the atmosphere is much less than that of O2 or N2, which are approximately 20% and 70%, respectively.


Burning of fossil fuels and forest destruction has caused CO2 in the atmosphere to increase approximately 25% (from 0.029% to 0.035%) in the last one hundred years. Many scientists fear that elevated atmospheric CO2, a major contributor to global warming, will lead to dramatic changes in global climate patterns. Some plant physiologists have pointed out that elevated temperature and CO2 levels will increase photosynthesis rates and possibly increase crop yields.

How would we expect elevated CO2 concentrations to affect the light saturation curve of a plant? In the diagram below the red line is the original response curve. (select answer by clicking on letter)

Click here for explanation of answer.


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