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

Photosynthesis happens when light is available (as it takes energy from the light source). However, plants must respire all the time, else they would die. In the day (when light intensity is strong) plants make more O2 from photosynthesis than they use in respiration, so in daytime they release this O2 (by diffusion), they also use up more CO2 than they produce, so they take in CO2 (by diffusion). At nighttime (when light intensity is low) plants only respire. This is because there is not enough light for photosynthesis (this means they take in oxygen and release CO2).