define respiration?

Respiration is the bodily process of inhalation and exhalation. It is the process of taking in oxygen and releasing carbon dioxide. The process involves the consumption of oxygen and liberation of carbon dioxide and water.

The oxygen inhaled is used to burn/oxidize/break down the food (glucose). This reaction produces energy required for all activities. Water and carbon dioxide are by-products of this reaction. This process occurs inside the mitochondria and is called cellular respiration. It is exactly opposite to the process of photosynthesis. It can be represented as:

Thus, cellular respiration is the process by which energy is released from the breakdown of organic substances (glucose).