Final answer:
The exact number of ATP produced in cellular respiration can vary due to differences in transport chain efficiency, shuttle systems in mitochondria, and energy used in transporting metabolic intermediates, as well as the cell's regulatory needs.
Step-by-step explanation:
The exact number of ATP produced in cellular respiration is not definitively known due to several reasons. First, different species have variations in the efficiency of electron transport chain complexes, which affects the proton gradient and the number of hydrogen ions pumped across the mitochondrial membrane. Secondly, the shuttle systems that transport electrons from cytoplasmic NADH into the mitochondria can use different carriers (NAD* or FAD*), and FAD* carriers result in fewer ATP molecules due to starting at a lower energy point in the electron transport chain (Complex II). Lastly, the actual yield of ATP can be lower than the theoretical maximum because some energy in eukaryotic cells is used to transport intermediates from the cytoplasm into the mitochondria. Additionally, cellular respiration is regulated according to the cell's needs, which includes diverting intermediates for other biosynthetic pathways, further affecting ATP yield.