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

Respiration is a continuous process in living things, so won't stop at any time. But photosynthesis depends on light and so will stop in the dark. This means that in the night carbon dioxide will be being given out by respiration but not taken in for photosynthesis, so the net exchange of carbon has an increased out put. In the same way at night oxygen will not be being given out as there is no photosynthesis.