The main design of this experiment is based on the requirement to have CO2 present for photosynthesis. Remember, there are two parts of photosynthesis, the light reaction and the dark reaction. However, many students believe (incorrectly) that the dark reactions can occur in the dark. No, not so! The dark reactions are better known as "light-independent" reactions. In other words, they don't DIRECTLY use light, but indirectly, they of course, require light. So, the light reactions convert light energy into ATP and NADPH so that these two molecules can be used to convert CO2 into glucose in the dark reactions. But remember, without light, the ATP and NADPH will run out, and the dark reactions won't take place either.
So, this experiment is a nice ...