What are the assumptions made during the calculation of net gain of ATP?
For theoretical purposes we make certain assumptions about the pathways in aerobic respiration i.e., about glycolysis, TCA Cycle and ETS. We assume that these pathways take place one after the other whereas in reality they take place simultaneously. We start with glucose as the primary respiratory substrate. But respiratory substrates can be obtained from proteins and fats which enter the pathway at respective stages. There are a lot of intermediates formed in all these reactions which have different reactions in different processes of body. They are generated and withdrawn from the pathways as per the need. All these pathways have enzymes bringing about the reactions. So in practicality, we have to take into account the substrates used, intermediates formed and utilized, factors controlling enzyme rate. But for theoretical calculation, we make certain assumptions which help us calculate ATP formed in the end of cellular respiration.
