Photosynthesis - why 6 CO2 &cH20?

This is a very basic question from someone who should know better, but has never really stopped to enquire after this basic fact of the photosynthesis equation. But it suddenly struck me that I have always taken the equation for granted, and now I want to find out precisely *why *it is always the exact figure of 6 parts CO2, etc? I confess I don't have much idea of what this equation looks like in practise, ie what 6CO2 would appear like as it entered the plant, so I am unable to visualize the equation in action quantitatively, only its results. So, what happens if it were 5 parts CO2, would you end up with no reaction?