The cellular respiration is the set of biochemical reactions why particular organic compounds are completely degraded by oxidation to become inorganic substances, a process that provides useful energy by the cell (mainly in the form of ATP).
As seen in the introduction to the topic, there are two types of cellular respiration:
- Aerobic respiration. The final electron acceptor is molecular oxygen, which is reduced to water. It is carried out by the vast majority of living beings, and they are called aerobic organisms.
- Anaerobic respiration. The final electron acceptor is an inorganic molecule other than oxygen. It is a very common type of metabolism in many microorganisms, especially prokaryotes. It should not be confused with fermentation, also an anaerobic process, but in which nothing like an electron transport chain is involved.
We will focus on aerobic cellular respiration, by which the pyruvic acid obtained in glycolysis is completely oxidized to CO2 and H2O, in the presence of oxygen. Cellular respiration occurs in two successive stages:
- The Krebs cycle: It takes place in the mitochondrial matrix, which contains all the enzymes, water, phosphates and other molecules necessary for respiration. It consists of eight chemical reactions in which, from acetyl-coenzyme A from oxidation reactions of different molecules (especially pyruvic acid, fatty acids and amino acids), ATP, CO2 and other molecules with reducing power are obtained ( NADH and FADH2).
- The electron transport chain: the molecules with reducing power obtained in the Krebs cycle, NADH and FADH2, enter the electron transport chain or respiratory chain, where the electrons go from a reduced molecule to an oxidized one until they reach to the final acceptor, which is oxygen, which will be reduced to form water. The energy obtained in this process, called oxidative phosphorylation, is invested in the synthesis of ATP and is explained by Mitchell's chemosmotic hypothesis. For every NADH that enters the chain, three ATP, and for each FADH2 two ATPs.
NADH → 3 ATP
FADH 2 → 2 ATP
The respiratory chain occurs in the mitochondrial ridges, where specific enzymes are found, which are grouped in such a way as to facilitate energetic coupling and electron transfer. For this process, the presence of oxygen in the cell is essential.