The rate of O2 production by the light reactions varies with the intensity of light because light is required as the energy sour
ce for O2 formation. Thus, lower light levels generally mean a lower rate of O2 production. In addition, lower light levels also affect the rate of CO2 uptake by the Calvin cycle. This is because the Calvin cycle needs the ATP and NADPH produced by the light reactions. In this way, the Calvin cycle depends on the light reactions.
But is the inverse true as well? Do the light reactions depend on the Calvin cycle?
Suppose that the concentration of CO2 available for the Calvin cycle decreased by 50% (because the stomata closed to conserve water).
Which statement correctly describes how O2 production would be affected? (Assume that the light intensity does not change.)
a. The rate of O2 production would decrease because the rate of ADP and NADP+ production by the Calvin cycle would decrease.
b. The rate of O2 production would decrease because the rate of G3P production by the Calvin cycle would decrease.
c. The rate of O2 production would remain the same because the light intensity did not change.
d. The rate of O2 production would remain the same because the light reactions are independent of the Calvin cycle.
Yes, the light reactions also depend upon the Calvin cycle. Calvin cycle refers to a phenomenon that is used by the plants and algae to convert carbon dioxide from the atmosphere into sugar, the food needed by the autotrophs in order to grow. The plants rely upon the Calvin cycle for food and energy.
In the given case, the rate of oxygen production would get diminish as the rate of ATP and NADP+ generated by the Calvin cycle diminishes. Of all the outcomes of the Calvin cycle, ADP and NADP+ are the only ones that get utilized by light reactions.
The ADP and NADP+ are used up by the light reactions to fuel their reactions. This illustrates that if the rate of ADP and NADP+ generated by the Calvin cycle diminishes the production of oxygen by the light reactions also diminishes.
