The process of cellular respiration occurs through three stages:

(1) First stage of cellular respiration:

First stage of cellular respiration is glycolysis. It occurs in the cytoplasm of cell. In this stage, glucose is degraded and generates energy. There are two types of glycolysis present, one is aerobic glycolysis and another is anaerobic glycolysis.

In aerobic glycolytic pathway, glucose becomes oxidized in the presence of oxygen, and one molecule of glucose yields two molecules of ATP, two molecules of pyruvate (also called pyruvic acid) and two molecules of NADH. One molecule of NADH provides three molecules of ATP via the electron transport chain. Therefore, in aerobic glycolytic pathway, one molecule of glucose generates 8 molecules of ATP. Pyruvate, the end product of aerobic glycolysis enters into the mitochondria where it converted into Acetyl CoA and produces two molecules of NADH by oxidative decarboxylation. Acetyl CoA takes part in second stage of cellular respiration for ATP generation and NADH provides ATP via the electron transport chain.

In anaerobic glycolytic pathway, glucose becomes oxidized without participation of oxygen, and each molecule of glucose provides two molecules of ATP and two molecules of lactate. Lactate is diffuses into the blood circulation and taken up by the liver where it reconverted into glucose. Though anaerobic glycolysis provides only two molecules of ATP, it is a valuable source of ATP under several conditions, including in cells deprived of sufficient oxygen such as in hypoxic state, in shock and during heavy exercise, or in cells that lack mitochondria.

(2) Second stage of cellular respiration:

The second stage of cellular respiration occurs in the mitochondrial matrix of cell and takes part through the Citric acid cycle (also called Krebs cycle or TCA cycle). Oxygen is essential in this stage. Acetyl Co-A is the substrate of Citric acid cycle. Two molecules of Acetyl Co-A are produced in first stage of cellular respiration by breakdown of one molecule of glucose. In second stage, these two molecules of Acetyl Co-A are oxidized to carbon dioxide with release of two molecules of ATP, six molecules of NADH and two molecules of FADH₂. One NADH provides three ATP and one FADH₂ provides two ATP via the electron transport chain. Therefore, in second stage of cellular respiration, two molecules of Acetyl Co-A provide total 24 molecules of ATP.

(3) Third stage of cellular respiration:

The third stage of cellular respiration occurs via the electron transport chain. This chain is located in the inner mitochondrial membrane of cell. In this stage, NADH and FADH₂ donate electrons. These electrons are passed along the electron transport chain from one carrier to another. Eventually the electrons combine with oxygen and hydrogen ions to form water. Free energy is released in this stage as electrons are passed along the electron transport chain from one carrier to another. One molecule of NADH releases three ATP and one molecule of FADH₂ releases two ATP via the electron transport chain.

Diagram of three stages of cellular respiration:

Metabolic fuels in different stages of cellular respiration:

In general, the food molecule glucose (derived from carbohydrates) is used as metabolic fuel in different stages cellular respiration, but amino acids (derived from proteins) and fatty acids (derived from fats) can also be used as metabolic fuels when stored glucose in our body become depleted.

Fats are stored in the adipose tissue as triglycerides. When the principle fuel molecule glucose is depleted and energy needed, triglycerides are degraded into glycerol and fatty acids. Glycerol is converted into dihydroxyacetone phosphate (one of intermediates of glycolysis) and enters into the first stage of cellular respiration. Fatty acids are oxidized by the process of β-oxidation producing acetyl CoA that enters into second stage of cellular respiration (citric acid cycle).

Amino acids are used as metabolic fuel only if proteins intake are high, or if glucose and fat reserves are depleted. Major numbers of amino acids are converted into acetyl CoA and ammonia by the process of transamination and subsequent deamination. This Acetyl CoA can be enters into the second stage of cellular respiration (citric acid cycle), and ammonia is excreted through urine as waste product.