How Cell Mitochondria Convert Food into Useable Energy
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.
Use food only as a fuel. Otherwise, you will consume more fuel than is necessary to make the machine called your body function. And you know what happens to the excess fuel? It gets stored for later (the machine stays prepared for times of low fuel intake) as fat.
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.
Aerobic cellular respiration can be diagrammed like this, with each step breaking down the products in the step preceding it:
Food (ingested) + Air (inhaled)
Carbohydrate + Oxygen and Nitrogen
Glucose + Oxygen (final products of digestion and inhalation)
ATP (energy) + Carbon Dioxide (exhaled) + Water (exhaled and excreted)
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.