Metabolism is a process that plays a vital role in our body to maintain life. It is identified into two kinds.

1. Constructive – also called anabolism. This is the process wherein complex substances were formed from simpler ones.
2. Destructive – also called catabolism. Unlike constructive metabolism, this is the process where complex substances are broken down to produce and release energy

The series of chemical reaction in our body cell is called a metabolic pathway. In each of the pathway that a chemical passes, the chemical is being modified. There can be a lot of metabolic pathways in one cell. This is because of the numerous chemicals that are involved. Listed below are just some of the considered most important of the metabolic pathways:

• Glycolysis - is the first metabolic pathway that was discovered. This metabolic pathway converts glucose into pyruvate. Pyruvate is the main intersection of several metabolic pathways.

• Citric acid cycle – is also called as the Krebs’ cycle. It was named after its discoverer, Sir Hans Adolf Krebs. This cycle is involved in the conversion of protein, fats and carbohydrates to generate usable energy.

• Oxidative Phosphorylation – the metabolic pathway that uses the energy being released by the oxidation of nutrients is called oxidative phosphorylation. It is widespread and is a very effective way to release energy compared to other process.

• Pentose phosphate pathway – Pentose phosphate pathway is the metabolic pathway that generates Nicotinamide adenine dinucleotide phosphate (NADPH). This NADPH is responsible for generating free radicals in immune system.

• Urea cycle – is the series of chemical reaction that takes place in the liver and results in the production of urea. Urea is a crystal like compound that is excreted in urine.

• Fatty acid oxidation (-oxidation) – is where fatty acids are broken down in order to release energy. Fatty acids is important the same as carbohydrates and protein in contributing energy in our body. Our body can store excess glucose easily as fats.

• Gluconeogenesis – is the metabolic pathway that results in the production of glucose in the body.

• Anaerobic respiration – is another way of producing energy without the use of oxygen in the process. The process is mainly used by bacterias that lives in environment without oxygen.

• HMG-CoA reductase – also called as mevalonate pathway. It is important in the production of dimethylallyl pyrosphate (DMAPP) and Isopentenyl pyrophosphate (IPP), which acts as the basis for the production of molecules that is used in other processes.

• Porphyrin synthesis – is the metabolic pathway in which D-aminolevulinic acid (dALA) is formed. This is thru the reaction of amino acid glycine with succinyl-CoA from the metabolic pathway citric acid cycle.

From the above-mentioned metabolic pathway, Glycolysis, citric acid cycle, oxidative phosporylation and anaerobic respiration occur in all living organisms. While Fatty acid oxidation, Gluconeogenesis, pentose phosphate, urea cycle, HMG-CoA reductase pathway, and porphyrin synthesis occurs in most if not in all living organisms.