Cellular respiration processes refer to the metabolic ability of the body to release energy to sustain life from nutrients supplied from outside the body. In relation to the human body, this means that the food we consume is transformed into ATP energy, which is subsequently used for homeostasis and the maintenance of normal body function. In fact, cellular respiration refers to energy metabolism within the cell consisting of three phases, namely glycolysis, the Krebs cycle, and oxidative phosphorylation (Wakim & Grewal, 2021). Under anaerobic conditions, cells can use fermentation processes to convert organics into energy units.
The first step in cellular respiration is glycolysis, which takes place predominantly in the cytoplasm of the cell. This process results in the sequential formation of several metabolites from a glucose molecule — which enters the cell as a result of digestive transformations, — including fructose-6-phosphate, 3-phosphoglyceraldehyde, 3-phosphoglycerate, and two pyruvate molecules as the final product (Wakim & Grewal, 2021). As a consequence of glycolysis, two molecules of ATP and two molecules of NADH are formed from one molecule of glucose. In the Krebs cycle, the second stage of cellular respiration, processes take place in the mitochondrial matrix; as a result of preliminary transformations, pyruvate is oxidized to acetyl-CoA. Acetyl-CoA is used in the initial steps of the Krebs cycle by combining with oxaloacetic acid, resulting in the formation of citric acid. Through a series of successive conversions, the cycle is eventually completed by the formation of 8 NADH and 2 FADH2 from pyruvate; accordingly, two turns of the Krebs cycle are required for the complete conversion of glucose. Finally, the last stage of cellular respiration is oxidative phosphorylation, which takes place on the membrane of the mitochondrial cristae. This is the most profitable step because it produces 34 ATP molecules (Wakim & Grewal, 2021). Oxidative phosphorylation is also commonly referred to as the electron-transport chain since the key conversion processes there are carried out by electron transfer between complex proteins and cofactors and the corresponding proton gradient between the membrane. The previously formed NADH and FADH2 are oxidized at this stage, which leads to energy synthesis in the form of ATP. Thus, with the help of these three reactions, the body produces energy, the primary source of which is glucose.
Reference
Wakim, S., & Grewal, M. (2021). Cellular respiration. Cellular Respiration. LibreTexts.