Plants also do respiration

We know that during daylight (or artificial light, except green) plants carry out photosynthesis and release OXYGEN.  Like animals, plants also require energy to live and grow.  This energy comes from  glucose during respiration.  Respiration uses oxygen and give up carbon dioxide.  To demonstrate this, we can do the following:

Enclosed plant, with different levels of O2 and CO2

 

The graph shows that the level of oxygen rises in the daytime.  This is due to photosynthesis producing more oxygen than the plant's respiration using it up.  The level of oxygen falls during night due to the plant's respiration using it up and no more is being produced as there is no photosynthesis happening.  The amount of oxygen rises gradually, over the period of time.

 

The level of CO₂ falls during day time due to photosynthesis using it up.  However, it rises at night because of respiration.  On the whole the level of CO₂ falls since the plant use up more CO₂ than it produces.