Amphibolic pathway : Respiratory pathway is involved in both anabolism and catabolism, thus the respiratory pathway is an amphibolic pathway rather than as a catabolic one.

Respiration involves the breakdown of substrates. The respiratory process has traditionally been considered a catabolic process and the respiratory pathway as a catabolic pathway.

The intermediates of the respiratory pathway are withdrawn and used for the synthesis of the other substrates.
Fatty acids are usually broken down to acetyl-CoA before entering the respiratory pathway. here it is used as a substrate. But when the organism needs to synthesise fatty acids, acetyl CoA is withdrawn from the respiratory pathway for the synthesis of fatty acids. Hence, the respiratory pathway has both synthesis and breakdown of fatty acids. 

Similarly, respiratory intermediates, are the link in the synthesis and breakdown of proteins too. Thus, it is amphibolic pathway.

The assumptions made during the calculation of ATP are :

(a) The reaction like glycolysis, TCA cycle, and ETS, are assumed to occur in a sequential and orderly pathway.

(b) NADH produced during the process of glycolysis enters into mitochondria to undergo oxidative phosphorylation.

(c) Glucose is assumed to be the only substrate

(d) It is assumed that no other molecule enters the pathway at the intermediate stages.

(e) The intermediates produced during respiration are not utilized in any other process.

C₆H₁₂O₆+ 6O₂→ 6CO₂ + 6H₂O+ 684 K. cal.

Most common respiratory substrate is glucose, because it gets easily oxidised.

1. Cellular respiration uses oxygen while fermentation does not require oxygen.
2. Cellular respiration produces 38 ATP, while fermentation produces only 2 ATP.
3. Cellular respiration is more efficient than fermentation in the generation of ATP.
4. In aerobic respiration the glucose is completely broken down into carbondioxide and water. While in fermentation the glucose is partialy broken down into carbon dioxide and ethanol/lactic acid. 

The main steps in aerobic respiration are as follows :

1. Glycolysis.

2. Oxidative decarboxylation of pyruvic acid to acetyl coenzme-A.

3. Tricarboxy acid cycle or Krebs cycle.

4. Electron transport system.

5. Chemiosmotic ATP synthesis.