A. Oxygen – Carbon dioxide
B. Carbon dioxide – Oxygen
C. Oxygen – Carbon monoxide
D. None of theseCarbon monoxide – Oxygen

During daylight hours, plants take in carbon dioxide and release oxygen through photosynthesis, and at night only about half that carbon is then released through respiration. However, plants still remain a net carbon sink, meaning they absorb more than they emit.

During daylight, plants are undergoing a process known as photosynthesis. In that the plants take up carbon dioxide and in the presence of light use it to manufacture sugars. From these sugars they can manufacture all other organic molecules. A by-product, actually more a waste product, of this reaction is oxygen, which is released by plants.

Now, consider this. All living things need to respire and the general equation for respiration says that glucose and oxygen give carbon dioxide and water. Plants are also living and do respire. Only at day time, they have sunlight and drive their energies into making food for themselves from carbon dioxide. At night, there is no sun. So, photosynthesis cannot work. However, the plants do have to respire. So, they do just that and we see that at night they are taking in oxygen and giving off carbon dioxide just like us!