Question

The Calvin cycle “dark” reactions, which fix CO2, do not function in the dark, what are the likely reasons for this. How are these reactions regulated by light?

Answer 1

Photosynthesis proceeds in two parts that is the light reaction and dark reaction. Light reaction transform light energy into ATP and NADPH. The dark reaction uses the ATP and NADPH to reduce carbon atoms from their fully oxidized state like carbon dioxide to a more reduced state as hexose.

The Calvin cycle operates during the day whereas carbohydrate degradation to yield energy happens at night.

The first step in the Calvin cycle is the fixation of CO2, in this process CO2 molecule condenses with Ribulose-1,5-bisphosphate ( Rubisco) to form an unstable compound which readily hydrolyzes to give 3-phosphoglycerate.
Rubisco requires a divalent metal ion ( Mg2+) for activity.

The light reaction leads to a change in stroma by increasing Mg2+ concentration and pH increment. These changes contribute to the activation of the Calvin cycle enzyme.
The activity of Rubisco or Ribulose-1,5-bisphosphate increases on illumination because light facilitates the carbamate formation necessary for enzyme activity.

In the presence of light proton is pumped into the thylakoid space. Mg2+ ion from the thylakoid space is released into the stroma to compensate for the influx of protons. CO2 adds to the deprotonated form of lysine 201 to rubisco, Mg2+ ion binds to the carbamate ( Carbamate formation is favored at alkaline pH) to generate the active form of the enzyme. Thus light leads to the generation of regulatory signals as well as ATP and NADPH.

Reduced thioredoxin formed by the light driven transfer of electrons from ferridoxin activates several enzyme of Calvin cycle.

It is evident for enzyme to get activated light is necessary.