Cellular respiration

Cellular respiration is the set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from food into energy that can be used to carry out life processes. For the human body, usable energy comes in the form of a molecule called adenosine triphosphate (ATP). Cellular respiration is then the way in which the cells of the human body turn nutrients into ATP; this process can be broken down into four core steps:


 * 1) Glycolysis: the first step of cellular respiration is the breakdown of glucose into energy that the body can utilize.
 * 2) Oxidative decarboxylation of pyruvate: this step is done by an enzyme called the Pyruvate Dehydrogenase Complex (PDC).
 * 3) Citric acid cycle (Krebs cycle or TCA cycle): is a series of chemical reactions that take stored energy (proteins, fats, carbs) and convert them into ATP.
 * 4) Oxidative phosphorylation: the last step of cellular respiration uses a combination of several enzymes and oxygen to create a large quantity of ATP from the energy-dense molecules that were broken down in the previous steps.

Fermentation
Fermentation occurs when the cell does not have adequate oxygen to undergo cellular respiration. Fermentation is an alternative process to allow for glycolysis to continue even if there isn't oxygen present.

Learn more

 * Wikipedia - Cellular respiration
 * Cellular Respiration by Bozeman Science
 * ATP & Respiration: Crash Course Biology
 * Introduction to cellular respiration | Cellular respiration | Biology | Khan Academy