Carbohydrates are usually first converted into glucose before they are used for respiration. Fats are broken down into glycerol and fatty acids first. If fatty acids were to be respired they would first be degraded to acetyl CoA and enter the pathway. Glycerol would enter the pathway after being converted to 3-phosphoglyceraldehyde (PGAL). The proteins are degraded by proteases to individual amino acids (after deamination) and depending on their structure enter the pathway within the Krebs’ cycle or as pyruvate or acetyl CoA. Thus, acetyl CoA is the common metabolite of all the three (carbohydrates, proteins and fats).