Describe the amphibolic pathway.

(N/A) Glucose is the favoured 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 enter the pathway.
Glycerol enters the pathway after being converted to $PGAL$.
Proteins are degraded by proteases,and the individual amino acids,depending on their structure,enter the pathway at some stage within the Krebs' cycle or as pyruvate or acetyl $CoA$.
Since respiration involves the breakdown of substrates,the respiratory process has traditionally been considered a catabolic process and the respiratory pathway as a catabolic pathway.
However,various compounds are withdrawn from the respiratory pathway for the synthesis of these substrates. For example,when an organism needs to synthesize fatty acids,acetyl $CoA$ is withdrawn from the respiratory pathway.
Hence,the respiratory pathway is involved in both the breakdown (catabolism) and synthesis (anabolism) of fatty acids and proteins.
Because the respiratory pathway is involved in both anabolism and catabolism,it is better to consider it an amphibolic pathway rather than just a catabolic one.