Also, consider that under low CO2 concentrations, the plant needs to use mechanisms that concentrate carbon, or extract carbon from carbonates. Both of these require more energy, thus you get less sugar produced per quantity of electrons hitting the plant.
Thanks mattinmd, that's a really useful post, and this is the piece of understanding I was missing.
Originally Posted by mattinmd
If you start digging into a biology text you'll get more into the specific reactions of photosynthesis, but that's a very "looking at the trees not the forest" perspective.
This is exactly the problem I had, I tried to gather the information from the wikipedia page to refresh my understanding, but I got so bogged down in details that I was missing the basics.