Why are there different numbers of ATP molecules produced per glucose in cellular respiration?

The variation in the number of ATP molecules produced per glucose molecule during cellular respiration arises due to differences in the efficiency of ATP generation at different stages of the process. In glycolysis, one molecule of glucose is broken down into two molecules of pyruvate. During this process, a small amount of ATP is directly synthesized, resulting in a net gain of two ATP molecules per glucose molecule. In the citric acid cycle, acetyl-CoA is further broken down, leading to the production of two more ATP molecules (or the energetically equivalent GTP) per glucose molecule. The majority of ATP production occurs during oxidative phosphorylation, with each NADH and FADH2 molecule contributing to the synthesis of additional ATP molecules. The precise number of ATP molecules generated per NADH or FADH2 molecule can vary, with estimates typically ranging from approximately 2.5 to 3 ATP molecules per NADH and 1.5 to 2 ATP molecules per FADH2. 

Different types of cells produce varying numbers of ATP molecules per glucose molecule during cellular respiration, depending on the specific shuttle mechanisms they utilize. For example, cardiac muscle cells and liver cells utilize the malate-aspartate shuttle and these cells can produce up to 32 ATP molecules per glucose molecule through oxidative phosphorylation. In contrast, skeletal muscle cells utilize the glycerol 3-phosphate shuttle, which is less efficient than the malate-aspartate shuttle. Thus, skeletal muscle cells typically produce around 30 ATP molecules per glucose molecule through oxidative phosphorylation.