What is catabolism?

Catabolism is the degradative processes of metabolism. It concerned with the breakdown of complex food molecules to simple ones, with a concomitant release of free energy. For example, carbohydrates are complex molecules, breakdown into simple molecules of carbon dioxide and water with concomitant release free energy as a form of ATP through a series of catabolic reactions.

The purpose of catabolism is to capture chemical energy in the form of ATP (adenosine triphosphate) from the breakdown of nutrient molecules from the food we eat. Generated ATP (adenosine triphosphate) through catabolic reactions utilizes the cells in one of two ways:

Energy can be used for anabolic reactions to make up our bodies. or

Energy can be used by the cells for energy consuming activities of body including movement of muscle, nerve impulse conduction, active transport, and so on. This type of energy usage is called burning calories where chemical energy is converted into either heat energy or mechanical energy.

Catabolic reactions in our bodies are divided into three stages. In the first stage, our eaten food molecules such as carbohydrate, lipid, and protein are breakdown into their smaller components (carbohydrate breakdown into glucose, lipid into fatty acids, and protein into amino acids) outside cells. In the second stage, these smaller components of food molecules glucose, amino acids, and fatty acids are taken up by the cells and further broken down into even smaller molecules acetyl-CoA. Some energy is generated as ATP in this stage. Finally, the acetyl CoA is oxidised to water and carbon dioxide, and reduces co-enzymes, NADH and FADH2 in TCA cycle. These reduced co-enzymes are re-oxidized via the electron transport chain and generated large amount of energy as ATP.

Major catabolic reactions in our bodies:

Carbohydrate catabolism:

Glycolysis – Glucose is the major fuel of body. It is catabolized to pyruvate (in aerobic condition) or lactate (in anaerobic condition) and generates small amount of energy as ATP by the process of glycolysis.
Glycogenolysis – Sometimes the metabolic fuel of glucose arise from the liver or muscle glycogen by the process of glycogenolysis.
Pentose phosphate pathway – It is an alternative pathyway for glucose catabolism. Here glucose is directly oxidized and supplies energy as NADPH.
Citric acid cycle – Pyruvate, the end product of glycolysis catabolize to Acetyl CoA that enters into the citric acid cycle, and broken down into carbon dioxide and water, and produces large amount of energy.
Lipid catabolism:

Beta-oxidation of fatty acid – fatty acids are converted into molecules of Acetyl CoA in a process called beta-oxidation of fatty acid.
Hydrolysis of triglyceride – Triglyceride is hydrolysis into glycerol and free fatty acid.
Protein catabolism:

Catabolism of body protein to amino acids – our body proteins are converted to amino acids via a series of catabolic reactions.
Transamination and Deamination – α-amino group is removed from the amino acids by transamination and deamination, forming ammonia and corresponding α-ketoacid.
Catabolism of carbon skeleton of amino acid – Carbon skeleton of amino acid is catabolised into seven common intermediate products such as pyruvate, fumarate, oxaloacetate, α-ketoglutarate, acetyl CoA, acetoacetyl CoA and succinyl CoA.