The common respiratory substrate is glucose because only glucose molecule enters into the first step of respiration. Other carbohydrates like starch should get converted into glucose and will be oxidised during respiration. Substrates like fats should be converted into fatty acids and glycerol. Glycerol will enter the pathway only at the middle of glycolysis after the formation of glyceraldehyde 3-phosphate. Fatty acids will be converted to acetyl CoA and enter the pathway. Proteins will be degraded by the enzyme proteases and get converted to amino acids. These acetyl CoA enters the Kreb's cycle.
Breaking down processes within the living organisms is said to be catabolism and synthesis is said to be anabolism. Process of respiration is considered to be a catabolic process because it involves breakdown of substrates and liberation of energy. But respiratory substrates form a link between anabolic and catabolic process. During respiration, fatty acids are breakdown to acetyl CoA, but during formation of fatty acids again this acetyl CoA is being utilised from the respiratory pathway. Since the respiratory pathway is involved in both anabolism and catabolism, we can infer that respiratory pathway is an Amphibolic pathway.