Biology · Respiration in Plants · NEET
Traditionally respiration was called catabolic because it breaks down substrates to release energy. But the same intermediates (like acetyl CoA) can be withdrawn to synthesise new molecules, which is anabolism. Because it involves both catabolism and anabolism, NCERT calls the respiratory pathway amphibolic, not just catabolic.
'Amphi' means both. An amphibolic pathway is one that works in both directions of metabolism: it can break molecules down (catabolic) and its intermediates can be used to build molecules up (anabolic). Respiration is the classic NEET example.
NCERT explains that the same compounds which enter the respiratory pathway to be broken down for energy are the very compounds that can be withdrawn from it for synthesis. For example, fatty acids are broken to acetyl CoA for energy; and acetyl CoA can be withdrawn to synthesise fatty acids. Which direction happens depends on the cell's needs.
When the cell needs fat, acetyl CoA is withdrawn from the respiratory pathway to build fatty acids. When it needs energy, fatty acids are instead degraded to acetyl CoA and fed in. The same intermediate serves both directions, which is exactly why the pathway is amphibolic.
Fats break into glycerol (enters as PGAL) and fatty acids (enter as acetyl CoA); proteins are broken to amino acids that, after deamination, enter as pyruvate, acetyl CoA or Krebs-cycle intermediates. These same entry points are also exit points for building fats and amino acids, which is why the pathway is amphibolic.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because it does both catabolism (breaking substrates down for energy) and anabolism (its intermediates are withdrawn to synthesise fats, proteins and other compounds). Doing both makes it amphibolic.
Acetyl CoA. It is produced when fatty acids are broken down for energy, and it is also withdrawn to synthesise fatty acids. The same intermediate serves breakdown and building.
The whole respiratory pathway, including glycolysis and the Krebs cycle, is amphibolic because their intermediates can be both broken down for energy and withdrawn for biosynthesis of amino acids, fats and nucleotides.
Catabolic means only breakdown with energy release. Amphibolic means the pathway does both breakdown (catabolism) and building (anabolism). Respiration is amphibolic, not just catabolic.
Yes. NCERT has a dedicated section (12.6 Amphibolic Pathway) and a direct question 'Discuss the respiratory pathway is an amphibolic pathway,' so it is directly examinable for NEET.
