Which of the following biomolecules is common to respiration...

The organic substances which are oxidised to produce energy during respiration are called respiratory substrates. Carbohydrates are the main respiratory substrates as our food contains 65-70% carbohydrates. The most preferred substrate is glucose as it is the most available carbohydrate. Fats are used as the respiratory substrate in deficiency of carbohydrates. Proteins are rarely used as a respiratory substrate. (only in deficiency of carbohydrates and fats).When carbohydrates and fats are used as respiratory substrates, the process is called floating respiration.

When glucose is used as a respiratory substrate, it undergoes glycolysis. First glucose is converted to glucose-6-phosphate with the help of ATP and the enzyme hexokinase. Then through a series of reactions, it produces two molecules of pyruvic acid.

Pyruvic acid (pyruvate) undergoes oxidative decarboxylation in mitochondria to form acetyl co A in the presence of enzyme Pyruvate dehydrogenase and magnesium ions as co-factor. The hydrogen released during this reaction is accepted by coenzyme NAD⁺(Nicotinamide Adenine Dinucleotide) and it is reduced to form NADH₂. This reaction is called link reaction or transition reaction or gateway reaction of aerobic respiration.

The acetyl co-A produced in the mitochondria undergoes TCA cycle to produce 3 molecules of NADH₂, 1 molecule of FADH₂, and 2 molecules of carbon dioxide

When fats are used as respiratory substrates, they are broken down into fatty acids and glycerol. Fatty acids undergo beta-oxidation to produce many acetyl co-A and thus enter TCA cycle. Glycerol which is also a breakdown product of fats, gets converted into Dihydroxy acetone phosphate (DHAP). DHAP isomerises to 3-PGAL and enters the glycolytic pathway.

When proteins are used as respiratory substrate, the process is called protoplasmic respiration. Proteins are broken down into amino acids. These amino acids undergo various chemical reactions are converted into pyruvic acid. This is converted to acetyl co-A which enters the TCA cycle.

Thus it is evident that, whatever may be the respiratory substrate it will ultimately enter the TCA cycle for generating energy.