How passive are biochemical reactions like photosynthesis? Is it as simple as increasing one of the products gets you more reactants? What drives it all?

I'm wondering about the sensitivity of soil respiration to rain events, I started thinking about how the rate of plant root respiration is linked to the rate of photosynthesis. So working backwards through the carbon cycle in plants I arrive at the standard photosynthesis equation. Assuming the concentration of CO2 and intensity of light are kept constant, it seems, by the basic stoichiometry maths, that if you give a plant more water it will produce more sugar and O2.

Then I started to think about the degree to which this reaction can be controlled by the plant. If you have CO2 and water and give them (the right amount/type of) energy, they will "want" to react, right? Whether it be in a plant or a beaker. The plant may control it indirectly by say closing stomata or reducing water uptake - but those would themselves be the result of other chemical reactions. As in: if this happens, do this; if the levels of this substance exceed this much, do that; if there is a decline in this stuff, take these steps; etc. Like a mindless computer program.

So i'm approaching a view that a plant (and other organisms) is just a jumble of chemical reactions in an apparently interactive relationship, the result of which we call a "plant".

If that's the case, moving to the chemistry part of biochem, what drives reactions? Why does CO2 readily react with H2O? Something about valency, stable orbits - I may be on the wrong track but I sense it's fundamentally due to either entropy or enthalpy. In which case, does the description of reactions as "passive" hold?