Cellular respiration is the actual biochemical process of enzymatic breakdown of respiratory substrate inside the cells. The energy stored in the chemical bonds of glucose are broken down with the help of oxygen and energy is released for various chemical reactions in the body. Respiration is of two types-aerobic and anaerobic.
Aerobic is the respiration which takes place in the presence of oxygen and anaerobic is the one which takes place in the absence of oxygen.
Aerobic respiration is the usual mode of respiration in all higher organisms and most of the lower organisms. The reason is that it yields maximum amount of energy. Aerobic respiration occurs in two steps-Glycolysis and Krebs cycle.
It is the first step of respiration which is common to both aerobic and anaerobic modes of respiration and occurs in the cytoplasm. Respiratory substrate is double phosphorylated before it undergoes lysis to produce 3 carbon compound, glyceraldehyde phosphate.NADH2 and ATP are produced when glyceraldehyde is changed to pyruvate.
Pyruvate passes into mitochondria. It undergoes oxidative decarboxylation to produce acetyl CoA, CO2 and NADH2.Acetyl CoA enters Krebs cycle. Here 2 decarboxylations,4 dehydrogenations and one phosphorylation or ATP synthesis occurs.
NADH2 and FADH2 liberate electrons and hydrogen ions. They are used in building up ATP molecules and activating oxygen molecules to combine with hydrogen to form water. Synthesis of ATP from ADP and inorganic phosphate with the help of electron generated energy during oxidation of reduced coenzymes (NADH2 and FADH2) is called oxidative phosphorylation. Oxygen is used at the end of Krebs cycle for combining with hydrogen and the process is called terminal oxidation.