do you know the mechanism of photosynthesis? it's not just a mere 6CO2 + 6H2O -> C6H12O6 + 6O2..
photosynthesis reaction is divided into two parts:
1. the light reaction, whereby UV from the sun is converted to chemical potential energy in the form of excited electrons
2. the dark reaction, whereby CO2 is absorbed. the dark reaction is known as calvin cycle.
both the light and dark reactions occur independent of each other. the dark reaction consist of many sub-steps, enzymes and different sub-strate.
energy from the excited electron is converted into ATP at many sub-steps. Some of the ATPs formed is used to fuel the conversion of carbon dioxide to glucose.
the calvin cycle consist of carbon fixation (where CO2 is absorbed into the plant), PGA conversion and PGAL conversion. if you increase CO2 concentration, more CO2 will be absorbed, thus PGA and PGAL conversion occur at increased rate, so rate of sugar production increase. sugar production requires energy. the energy comes from sunlight (whose energy is harnessed via the light reaction). if there isnt sufficient energy available for reaction, then PGA & PGAL conversion occurs at a lower rate, thus sugar production rate decreases as well.
I hope this is sufficient to answer your question. anyway, i move your post to chemical biology.