Biology · Respiration in Plants · NEET
Because the number 38 is reached only by assuming four ideal conditions that are never fully true inside a living cell. NCERT clearly says these calculations 'can remain only a theoretical exercise.' The pathways actually run at the same time, intermediates are borrowed for other work, and ATP is spent as it is made. So 38 is a maximum on paper, not the real yield.
(1) There is a sequential, orderly pathway, one substrate forming the next, with glycolysis, TCA cycle and ETS following one after another. (2) The NADH made in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation. (3) None of the intermediates in the pathway are used to make any other compound. (4) Only glucose is respired, no other substrate enters at any intermediate stage. Learn these four; NEET can ask them word for word.
The difference is the glycolytic NADH shuttle. Assumption 2 says the 2 NADH made in the cytoplasm during glycolysis enter the mitochondria and give full 3 ATP each. If a cell uses the glycerol-phosphate shuttle, that NADH is passed to FADH2 and gives about 2 ATP each, so you lose 2 ATP and reach 36. NCERT counts full value, so its number is 38.
Assumption 1 (orderly, one-after-another pathway) and assumption 3 (no intermediate withdrawn) fail most clearly. NCERT says all pathways work at the same time, and substrates enter and leave as needed. Because the pathway is amphibolic, intermediates like acetyl CoA or oxaloacetate are constantly pulled out to build fats and amino acids, so full ATP is never captured.
No. Assumption 4 is false because fats, proteins and other sugars also enter respiration at different points. Fats break to acetyl CoA, proteins give amino acids that feed in as keto acids. Since many substrates are being respired together, a clean 'per glucose' number cannot be measured in a living system.
No, it is still useful. NCERT says the exercise helps us 'appreciate the beauty and efficiency of the living system' in extracting and storing energy. It gives an upper limit and shows how much more efficient aerobic respiration is than fermentation (2 ATP). Just remember it is a ceiling, not the exact real value.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Four. Orderly sequential pathway, glycolytic NADH transferred into mitochondria, no intermediate withdrawn, and only glucose respired.
No. It is a theoretical maximum under ideal assumptions. In a living cell the true yield is lower and variable because pathways run together and intermediates are used elsewhere.
Because intermediates are withdrawn to build other molecules (fats, amino acids), which is exactly why assumption 3 fails. Being amphibolic means it does both breakdown and building.
Follow NCERT: net gain of 38 ATP per glucose, and add that this is a theoretical figure valid only under the stated assumptions.
