Respiration in plants:

During respiration, plants take in oxygen and release carbon-di-oxide, just as animal also do. Gaseous exchange with air in atmosphere takes place in plants with stomata.

Photosynthesis: 

During photosynthesis, carbon-di-oxide from the air and water from the soil react in the presence of sunlight to produce food. Plants release oxygen during photosynthesis which is much more than the oxygen consumed by the plants during respiration.

Respiration in animats:

When we breath in air, the oxygen present in the air reacts chemically with digested food within the body to produce carbon-di-oxide gas. The inhaled air contains more oxygen while the exhaled air contains more carbon-di-oxide.

So plants sent out more oxygen during the photosynthesis. Animals sent out more carbondi-oxide during respiration. Plants take carbondi-oxide for photosynthesis. Animals take oxygen during respiration. So plants and animals maintain the balance of oxygen and carbon-dioxide in air.