In discussion, we analyzed the initial and final states of a toy model of photosynthesis: the reaction co2 + h2o ↔ h2co + o2.

the binding of the carbon in the organic molecule (formaldehyde) has effectively traded a strong c=o double bond for a weaker o=o double bond, so the reaction is endothermic from left to right. (the right side has a higher chemical energy so you have to put energy in to get there.)

although that tells the overall result, the actual mechanism of photosynthesis is more complicated. discovering how this works is one of the great stories of figuring out what is happening in a complex biological process.