All parts of the body (muscles, brain, heart, and liver) need energy to work. This energy comes from the food we eat. Food is the fuel for the body
Food is the fuel for the body. The mitochondria are the converters; they convert the fuel into useable energy. When food is digested, or broken down into its smallest molecules and nutrients, and air is taken in, or inspired, the smallest molecules and nutrients cross into the bloodstream. These molecules and nutrients include things such as glucose (a sugar molecule derived from carbohydrates) and oxygen.
Just as fire burns oxygen and gives off carbon dioxide and water, mitochondria act like furnaces when they convert glucose into adenosine triphosphate (ATP): They “burn” (use) oxygen and give off carbon dioxide and water. Because the process uses oxygen, it is said to be aerobic (as in aerobic exercise).
This chemical process of respiration occurs in every cell, so it is called aerobic cellular respiration. The steps that occur in this process are described by the Krebs cycle (also called the tricarboxylic acid [TCA] cycle). The Krebs cycle is a cornerstone to understanding how cells function.
Do not confuse respiration with breathing. Breathing is just a part of respiration. Breathing actually is the act of inspiring and expiring; respiration is the exchange of oxygen and carbon dioxide between cells and the atmosphere. So, people respire, but it happens at the cellular level.