Why does the amount of oxygen in the air not get depleted?

Vast amount of oxygen present in the air is consumed during the respiration of billions of plants and animals, which in turn release the vast amount of carbon dioxide and water vapor. Furthermore, large amount of oxygen is consumed during the process of combustion of fuels. In spite of the fact that oxygen is being continuously consumed, its amount in air remains same. Why?

During the day time, the carbon dioxide diffuses into the pores of green leaves. Here, it reacts with water, in the presence of chlorophyll to form oxygen and glucose. The oxygen is released in the air. The glucose is converted into sugar and then starch by a very complex process. As the volume of oxygen released is equal to the volume of carbon dioxide absorbed, therefore, amount of oxygen in the air does not get depleted. This chemical process taking place in the green leaves is called photosynthesis, it is defined as under :

Definition: The process due to which the plant leaves absorb carbon dioxide gas from atmosphere and then in the presence of water, chlorophyll and sunlight, convert it to glucose (sugar) and oxygen is called photosynthesis.

It is interesting to note that photosynthesis is a reverse chemical process of respiration. During respiration, it is the glucose and oxygen which react chemically to form carbon dioxide, water and release heat energy. However, during photosynthesis, the carbon dioxide and water react chemically by the absorption of light energy to form glucose and oxygen.