Living organisms require energy to perform various biological processes. The energy is obtained from food through various pathways. The three major pathways of breakdown of food in living organisms are:


Glycolysis is the breakdown of glucose into pyruvate. The process occurs in the cytoplasm of the cell and does not require oxygen. The pathway includes the following steps:

• Glucose is converted into glucose-6-phosphate by hexokinase enzyme.


• Glucose-6-phosphate is converted to fructose-6-phosphate by isomerase enzyme.


• Fructose-6-phosphate is converted to 2 molecules of glyceraldehyde-3-phosphate by aldolase enzyme.


• Glyceraldehyde-3-phosphate is converted to 1,3-bisphosphoglycerate by dehydrogenase enzyme.


• 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate by phosphoglycerate kinase enzyme.


• 3-phosphoglycerate is converted to pyruvate by pyruvate kinase enzyme.

The citric acid cycle is the process of breaking down carbohydrates, fats, and proteins into carbon dioxide and water. The cycle occurs in the mitochondria of the cell and requires oxygen. The pathway includes the following steps:

• Acetyl-CoA combines with oxaloacetate to form citrate.


• Citrate is converted to isocitrate by aconitase enzyme.


• Isocitrate is converted to alpha-ketoglutarate by isocitrate dehydrogenase enzyme.


• Alpha-ketoglutarate is converted to succinyl-CoA by alpha-ketoglutarate dehydrogenase enzyme.


• Succinyl-CoA is converted to succinate by succinyl-CoA synthetase enzyme.


• Succinate is converted to fumarate by succinate dehydrogenase enzyme.


• Fumarate is converted to malate by fumarase enzyme.


• Malate is converted to oxaloacetate by malate dehydrogenase enzyme.

Oxidative phosphorylation is the process of generating ATP from the electron transport chain. The process occurs in the mitochondria of the cell and requires oxygen. The pathway includes the following steps:

• Electrons from NADH and FADH2 are passed through the electron transport chain to oxygen.


• As electrons move through the electron transport chain, protons are pumped from the mitochondrial matrix to the intermembrane space.


• Protons move from the intermembrane space to the mitochondrial matrix through ATP synthase enzyme.


• As protons move through ATP synthase enzyme, ADP is converted to ATP.

In conclusion, glycolysis, citric acid cycle, and oxidative phosphorylation are the three major pathways of breakdown of food in living organisms. These pathways are essential for