The respiratory balance sheet: 38 ATP

The respiratory balance sheet

In plain words: think of it as a household budget drawn up on paper. You list every rupee that comes in at each stage, add them up, and get a neat total. The real household never runs quite like the plan, but the plan shows where the money comes from. The balance sheet does the same for ATP: it adds up the ATP made at each stage of respiration and arrives at 38 ATP per glucose, a figure that holds only under a few tidy assumptions.

Assumptions in counting ATP

NCERT says the calculation can be made only on four assumptions (page 161):

1. There is a sequential, orderly pathway, with one substrate forming the next, and with glycolysis, the TCA cycle and the 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 is used to make any other compound.
4. Only glucose is respired: no alternative substrates enter the pathway at any intermediate stage.

NCERT then adds that “this kind of assumptions are not really valid in a living system” (page 161):

• all pathways work simultaneously, not one after another;
• substrates enter and leave the pathways as and when necessary;
• ATP is used as and when needed;
• enzymatic rates are controlled by multiple means.

Transport clarification: NCERT’s second assumption is an accounting shorthand. Cytosolic NADH does not freely cross the inner mitochondrial membrane; its reducing equivalents reach the respiratory chain through transfer routes whose entry points affect ATP yield (Alberts and others, Molecular Biology of the Cell, “The Mitochondrion”).

Yet the exercise is still worth doing, NCERT says, “to appreciate the beauty and efficiency of the living system in extraction and storing energy” (page 161).

Net gain of 38 ATP

NCERT concludes: “Hence, there can be a net gain of 38 ATP molecules during aerobic respiration of one molecule of glucose” (page 161). The 38 comes from adding up the earlier sections, using NCERT’s values of 3 ATP per NADH and 2 ATP per FADH_₂ oxidised in the ETS (page 160):

• Glycolysis: a net 2 ATP made directly, and 2 NADH + H^⁺ (NCERT page 156); by assumption 2 these NADH are oxidised in the mitochondria, giving 2 × 3 = 6 ATP.
• Pyruvate to acetyl CoA: two NADH from the two pyruvic acid of one glucose (page 158), giving 2 × 3 = 6 ATP.
• Krebs’ cycle: per pyruvate, three NADH + H^⁺, one FADH_₂ and one GTP, which gives one ATP (page 159). For two pyruvate: 2 ATP + 6 × 3 + 2 × 2 = 24 ATP.

Beyond the NCERT text, the real yield is lower than 38. Measured values are about 2.5 ATP per NADH and 1.5 per FADH_₂ (Hinkle, Biochimica et Biophysica Acta, 2005), which turn the same count into 4 + 10 × 2.5 + 2 × 1.5 = 32 ATP. NCERT itself warns that its count rests on assumptions that are not really valid in a living system: all the pathways work at the same time, substrates enter and are withdrawn as needed, ATP is used as and when needed, and enzyme rates are controlled in many ways (page 161). This 32 is still an idealised calculation. If the two cytosolic reducing equivalents instead enter at the ubiquinone level and each yields 1.5 ATP, the total is 30 ATP. These common 30–32 estimates are not universal yields, especially across plants and microbes with different respiratory routes. Use 38 when a question explicitly requests NCERT’s model or specifies its 3/2 assumptions.

Fermentation and aerobic respiration compared

NCERT compares fermentation with aerobic respiration on page 162. Its slow/vigorous wording is a qualitative textbook description, not a universal comparison of reaction rates: fermentation can support rapid glycolytic flux.

So aerobic respiration extracts far more ATP from each glucose: by NCERT’s count, 38 against 2 (Example 2).

Solved examples

Common mistakes to avoid

Quick revision summary

• The ATP count per glucose is only a theoretical exercise, made on four assumptions: an orderly sequential pathway, glycolytic NADH oxidised in the mitochondria, no intermediates drawn off, and glucose as the only substrate.
• In a living system, pathways run simultaneously, substrates enter and leave, ATP is used as needed, and enzyme rates are controlled in many ways.
• With 3 ATP per NADH and 2 per FADH_₂: glycolysis 8, pyruvate → acetyl CoA 6, Krebs’ cycle 24, a net 38 ATP per glucose.
• Fermentation breaks glucose down partly and nets 2 ATP through glycolysis; aerobic respiration breaks it down fully to CO_₂ and H_₂O and makes many more ATP.

Questions students ask

Why do newer textbooks give fewer than 38 ATP per glucose?

Newer textbooks give fewer ATP per glucose because they use measured values, which are lower than NCERT’s: NCERT’s 38 uses 3 ATP per NADH and 2 per FADH_₂ (page 160), while measurements, beyond the NCERT text, give about 2.5 and 1.5 (Hinkle, Biochimica et Biophysica Acta, 2005), so the same 10 NADH and 2 FADH_₂ give 4 + 10 × 2.5 + 2 × 1.5 = 32 ATP. NCERT itself warns that its count rests on assumptions that do not hold in a living cell, where substrates are withdrawn from the pathways as and when necessary (NCERT page 161). A different shuttle can lower this idealised count to 30; actual coupling and substrate use also matter. Use 38 when the question explicitly uses NCERT’s assumptions.

Why is NCERT’s 38 ATP called a net gain, and what is the gross yield?

NCERT’s 38 ATP per glucose is a net gain because the 2 ATP that glycolysis uses at its start, to phosphorylate glucose and then fructose-6-phosphate, have already been subtracted, the deduction NCERT asks for when it counts the ATP of glycolysis (NCERT pages 156–157 and 161). By the same count, which NCERT does not write out, the gross yield of one glucose in aerobic respiration is 38 + 2 = 40 ATP. The same subtraction is behind fermentation’s net gain of only two ATP per glucose (NCERT page 162).

Why does aerobic respiration make so much more ATP than fermentation?

Aerobic respiration gives more ATP per glucose than these fermentation pathways because, with O_₂ as the final hydrogen acceptor, the NADH + H^⁺ and FADH_₂ formed as glucose is broken down are oxidised through the electron transport system, which drives the synthesis of most of the ATP (NCERT page 160). Without O_₂, the cell reoxidises its NADH + H^⁺ to NAD^⁺ by turning pyruvic acid into ethanol and CO_₂ or into lactic acid, which makes no ATP (page 157), so only the net 2 ATP of glycolysis remain (page 162). Most of the energy of glucose stays locked in the ethanol or lactic acid: less than seven per cent of it is released (page 157).

Do prokaryotes get 38 ATP from one glucose and eukaryotes only 36?

NCERT gives one figure, a net gain of 38 ATP per glucose, and does not treat prokaryotes and eukaryotes separately; its count assumes that the NADH made in glycolysis is transferred into the mitochondria and gives 3 ATP each (NCERT pages 160–161). Beyond the NCERT text, some older textbooks give eukaryotes 36, counting the 2 glycolytic NADH at 2 ATP each, as if they were FADH_₂, and prokaryotes 38, since they have no mitochondria for that NADH to enter (NCERT Chapter 8, page 90). Both figures are model-dependent; there is no universal prokaryote/eukaryote 38/36 rule. Common modern eukaryotic estimates are about 30–32 depending on transfer of cytosolic reducing equivalents, and actual yields vary further.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. What is the net gain of ATP from the aerobic respiration of one glucose molecule, according to NCERT?

   Show answerHide answer

   38 ATP (NCERT page 161).

2. Q2. How many NADH does one glucose yield in all, counting glycolysis, the conversion of pyruvate to acetyl CoA and the Krebs’ cycle?

   Show answerHide answer

   2 + 2 + 6 = 10 NADH (NCERT pages 156–159).

3. Q3. According to NCERT’s qualitative wording, in which process is NADH oxidised to NAD^⁺ “rather slowly”?

   Show answerHide answer

   In fermentation; in aerobic respiration the reaction is very vigorous (NCERT page 162).