Respiration in Plants - Amphibolic pathways

Respiration is traditionally considered a catabolic process because it involves the breakdown of substrates like glucose to release energy in the form of $ATP$.

The respiratory pathway is more accurately described as an Amphibolic pathway because it involves both catabolism (breakdown) and anabolism (synthesis).

Intermediates of the Krebs cycle are withdrawn for the synthesis of other biomolecules. For example, when the organism needs to synthesize fatty acids, $Acetyl\,CoA$ is withdrawn from the respiratory pathway.

Different substrates enter the pathway at different stages: Fats are broken down into glycerol (entering as $PGAL$ or $3-phosphoglyceraldehyde$) and fatty acids (entering as $Acetyl\,CoA$). Proteins are broken down by proteases into amino acids, which enter at various stages like $Pyruvate$, $Acetyl\,CoA$, or within the $TCA$ cycle.

The breakdown of respiratory substrates to release energy is catabolism, while the withdrawal of intermediates from the same pathway to synthesize proteins or fats is anabolism.

Respiratory Quotient ($RQ$): The ratio of the volume of $CO_2$ evolved to the volume of $O_2$ consumed during respiration. It depends on the type of respiratory substrate used.

For carbohydrates, the $RQ$ is $1.0$ because equal amounts of $CO_2$ and $O_2$ are evolved and consumed respectively.

For fats, the $RQ$ is less than $1$ (approximately $0.7$ for tripalmitin) because fats are oxygen-poor and require more external $O_2$ for oxidation.

For proteins, the $RQ$ is approximately $0.9$.