Photosynthesis and respiration are two different processes.
Photosynthesis takes place in light which is available only during the daytime. In this process, plants take in carbon dioxide and give out oxygen. However, at night, in the absence of sunlight, photosynthesis does not occur. Hence, there is no release of oxygen during night.
Respiration is a continuous process which takes place during the day and also at night. In this process, plants take in oxygen and give out carbon dioxide. Hence, there is a continuous release of carbon dioxide during the day time and night time. However, during daytime, the waste carbon dioxide released during respiration is utilised by the plants for photosynthesis.
