Biology · Respiration in Plants · NEET
Plants can use fats and proteins as respiratory substrates too, not only carbohydrates like glucose. But these big molecules are never fed straight into glycolysis. Each is first broken into small units that enter the respiratory pathway at a specific point. So glucose is the usual fuel, but fats and proteins are backup fuels that feed the same machinery.
A fat is first split into glycerol and fatty acids. The glycerol part is changed to PGAL (3-phosphoglyceraldehyde) and joins glycolysis. The fatty acid part is broken down into 2-carbon pieces to form acetyl CoA, which enters the Krebs cycle. So fats have two entry doors: glycerol via glycolysis, fatty acids via acetyl CoA.
Proteins are first broken into amino acids. Before an amino acid can be respired, its amino group (–NH2) is removed in a step called deamination. The remaining carbon skeleton then enters the pathway as pyruvic acid, acetyl CoA, or one of the Krebs cycle acids (like alpha-ketoglutaric acid), depending on which amino acid it is.
Acetyl CoA. Carbohydrates give pyruvate, which becomes acetyl CoA. Fatty acids are broken directly to acetyl CoA. Many amino acids also converge at acetyl CoA. This is why NEET keeps asking for the 'common biomolecule' — the answer is acetyl CoA, the meeting point of all three substrate types.
Amphibolic means it does both. The respiratory pathway usually breaks molecules down (catabolism) to release energy. But its intermediates (like acetyl CoA and Krebs acids) can also be pulled out to build fatty acids, amino acids, and other molecules (anabolism). Because it works in both directions, it is called an amphibolic pathway, not just catabolic.
No. This is a common mix-up. Glycerol (the backbone of a fat) enters early, at glycolysis, after being converted to PGAL (linked through dihydroxyacetone phosphate). The fatty acid tails enter later, as acetyl CoA into the Krebs cycle. Same fat molecule, two different entry points.
Fatty acids are connected with the respiratory pathway through:
Which of the following biomolecules is common to respiration-mediated breakdown of fats, carbohydrates and proteins?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Acetyl CoA. All three substrate types converge at acetyl CoA, which then enters the Krebs cycle. This is the most frequently asked point in NEET on this topic.
A fat is split into glycerol and fatty acids. Glycerol is converted to PGAL and enters glycolysis; fatty acids are broken down to acetyl CoA and enter the Krebs cycle.
Deamination is the removal of the amino group (–NH2) from an amino acid. After deamination, the remaining carbon skeleton enters respiration as pyruvate, acetyl CoA, or a Krebs cycle acid.
Because it both breaks down molecules (catabolism, to release energy) and supplies intermediates that are used to build fats and proteins (anabolism). Since it serves both, it is amphibolic rather than purely catabolic.
Glycerol is converted to PGAL (via dihydroxyacetone phosphate) and enters at the glycolysis stage, not at acetyl CoA.
