Amphibolic pathway

Glucose is the favoured respiratory substrate, and other carbohydrates are usually converted to glucose first. Other substrates can be respired, but they join the pathway at later points, not at the first step.

Fats are first split into glycerol and fatty acids. Fatty acids are broken down to acetyl CoA before entering; glycerol enters after being converted to PGAL.

Proteins are broken down by proteases into amino acids, which after deamination enter according to their structure: as pyruvate, as acetyl CoA, or at some stage within Krebs' cycle.

The same intermediates are withdrawn from the pathway when the cell builds these molecules; for example, acetyl CoA is taken out of the pathway to synthesise fatty acids.

Catabolism is breaking down and anabolism is building up. Since the respiratory pathway serves both, it is better called an amphibolic pathway than a purely catabolic one.