Early estimates suggested that the oxidation of glucose via aerobic respiration would produce 38 ATP. Based on what you know of the theoretical yields of ATP from cellular respiration, show how this total was determined. Why do biologists now think this amount of ATP per molecule of glucose is not achieved in cells?

Verified step by step guidance

1. Step 1: Understand the stages of cellular respiration. Cellular respiration consists of glycolysis, the Krebs cycle, and oxidative phosphorylation (including the electron transport chain). Each stage contributes to ATP production.
2. Step 2: Calculate ATP from glycolysis. Glycolysis converts one glucose molecule into two pyruvate molecules, producing a net gain of 2 ATP and 2 NADH. The 2 NADH can potentially produce 6 ATP via oxidative phosphorylation (assuming 3 ATP per NADH).
3. Step 3: Calculate ATP from the Krebs cycle. Each acetyl-CoA entering the Krebs cycle generates 1 ATP (or GTP), 3 NADH, and 1 FADH2. Since two acetyl-CoA molecules are produced from one glucose, this results in 2 ATP, 6 NADH, and 2 FADH2. The 6 NADH can produce 18 ATP and the 2 FADH2 can produce 4 ATP via oxidative phosphorylation.
4. Step 4: Sum the ATP totals. Adding the ATP from glycolysis (2 ATP + 6 ATP from NADH), the Krebs cycle (2 ATP + 18 ATP from NADH + 4 ATP from FADH2), and the ATP directly produced from oxidative phosphorylation of the NADH and FADH2 generated in these processes, the theoretical maximum yield would be 38 ATP (2+6+2+18+4+6 from oxidative phosphorylation of the NADH from glycolysis).
5. Step 5: Understand why actual ATP yield is lower. Biologists now recognize that the theoretical maximum is seldom reached due to losses like the cost of transporting pyruvate into mitochondria, and the use of the proton gradient for purposes other than ATP synthesis (like heat production and transport of other ions). Additionally, the ratio of ATP per NADH and FADH2 can vary depending on the cell type and conditions, often being lower than the theoretical 3 and 2, respectively.