The rate of O₂ production by the light reactions varies with the intensity of light because light is required as the energy source 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?

While the Calvin cycle produces ADP and NADP+, both of which are photophosphorylated by the light-dependent reactions, there is another carbon cycle, called PHOTORESPIRATION, in which the enzyme RuBisCO (RuBP oxygenase-carboxylase), OXYGENATES RuBP (ribulose-1,5-bisphosphate), instead of the CARBOXYLATION OF RuBP (the normal Calvin Cycle), releasing 3CO2 and spending ATP and NADPH (produced by the light-dependent reactions), thus, providing the ADP and NADP+ photophosphorylated by the light-dependent reactions.

PHOTORESPIRATION results in a NET LOSS of fixated carbon, and is activated by certain conditions like HIGH TEMPERATURE and LOWER CO2 AVAILABILITY (due to CLOSED STOMATA as a response to droughts). However, the main functions of this system are not clear. It may be involved in nitrate assimilation, free radicals regulation, plant growth, homeostasis of plant cells, among others.