Final answer:
ATP production would be severely hindered without NADH and FADH2 as these molecules are essential for the electron transport chain. Glycolysis would slow down or stop without the regeneration of NAD+, and cells would be forced to rely on inefficient anaerobic pathways.
Step-by-step explanation:
If NADH and FADH2 were not available, ATP production would be significantly impaired. These two molecules are crucial for the electron transport chain (ETC), which is the main source of ATP in aerobic respiration. In their absence, NAD+ would not be regenerated from NADH, causing the second half of glycolysis to slow down or stop. This would mean that neither pyruvate nor citric acid would continue to be produced at the rates needed to feed into the Krebs cycle and onward to the ETC.
Without the oxidation of NADH and FADH2 by oxygen in the ETC, ATP synthesis would be dramatically reduced. This could force the cell to rely on less efficient anaerobic pathways like fermentation, which do not produce ATP from NADH, thus leading to much lower energy yields. Consequently, none of the processes depending on these reactions, such as proton pumping across membranes or the passage of electrons through the ETC, would occur. The entire energy economy of the cell would be compromised.