Describe the metabolic processes of the respiratory pathway.

(N/A) Glucose is the favored substrate for respiration.
All carbohydrates are usually first converted into glucose before they are used for respiration.
Other substrates can also be respired,but they do not enter the respiratory pathway at the first step.
As shown in the figure,fats must first be broken down into glycerol and fatty acids.
If fatty acids are to be respired,they are first degraded to acetyl $CoA$ and then enter the pathway.
Glycerol enters the pathway after being converted to $PGAL$ (phosphoglyceraldehyde).
Proteins are degraded by proteases into individual amino acids,which,depending on their structure,enter the pathway at various stages within the Krebs' cycle or as pyruvate or acetyl $CoA$.
Since respiration involves the breakdown of substrates,it has traditionally been considered a catabolic process,and the respiratory pathway as a catabolic pathway.
However,the respiratory pathway is also involved in the synthesis of fatty acids,where acetyl $CoA$ is withdrawn from the pathway.
Similarly,during the synthesis of proteins,respiratory intermediates form the link.
Breaking down processes within the living organism is catabolism,and synthesis is anabolism.
Because the respiratory pathway is involved in both anabolism and catabolism,it is more appropriate to consider the respiratory pathway as an amphibolic pathway rather than just a catabolic one.