2.40 understand that respiration continues during the day and night, but that the net exchange of carbon dioxide and oxygen depends on the intensity of light

REMEMBER

Respiration is constant. It continues at the same level during day and night.

Photosynthesis varies depending on light intensity

Click here to see this on a graph.

This means that the net exchange of carbon dioxide and oxygen varies aswell.

If there is less photosynthesis it means that there will be less intake of CO2 and less production of oxygen. This means more CO2

If there is more photosynthesis it means there will be more intake of CO2 but more production of oxygen. This means more oxygen.

So... 

If the light intensity is greater (e.g. day) then there will be more CO2 intake and vice versa.