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 then they do not enter the respiratory pathway at the first step.
Since respiration involves breakdown as well as synthesis of substrates, the respiratory process involves both catabolism. That is why respiratory pathway is considered to be an amphibolic pathway rather than as a catabolic one.
