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Botany · Respiration in Plants

Respiratory Balance Sheet (ATP Yield)

Section 12.5 of NCERT Class 11 Biology presents the respiratory balance sheet as an accounting of maximum ATP produced when one glucose molecule is completely oxidised through glycolysis, pyruvate oxidation, the Krebs cycle, and the electron transport system. NCERT explicitly frames this as a theoretical exercise — the numbers depend on assumptions that rarely hold in a living cell. NEET examines both the stage-wise yield and the caveat that conditions in vivo can reduce the actual output.

NCERT grounding

NCERT Class 11 Biology, Chapter 12 — Respiration in Plants, Section 12.5 is titled "The Respiratory Balance Sheet." The text states: "It is possible to make calculations of the net gain of ATP for every glucose molecule oxidised; but in reality this can remain only a theoretical exercise." It then lists four assumptions on which the calculation rests and concludes that a net gain of 38 ATP molecules can occur during aerobic respiration of one glucose molecule. The balance sheet synthesises the outputs of all four metabolic stages — glycolysis, pyruvate oxidation, the TCA cycle, and the electron transport system (ETS).

"Hence, there can be a net gain of 38 ATP molecules during aerobic respiration of one molecule of glucose."

NCERT Class 11 Biology, Chapter 12 — The Respiratory Balance Sheet

The four stages of ATP production

Complete aerobic oxidation of glucose proceeds through four sequential stages. Each stage contributes to the ATP tally either through substrate-level phosphorylation (direct transfer of a phosphate group to ADP without the electron transport chain) or through the production of reduced coenzymes (NADH and FADH2) that are subsequently oxidised in the ETS to drive oxidative phosphorylation.

Aerobic respiration — four ATP-contributing stages

One glucose molecule (6C)
  1. Stage 1

    Glycolysis

    Cytoplasm. Glucose → 2 pyruvate. Net 2 ATP (substrate-level) + 2 NADH.

    EMP pathway
  2. Stage 2

    Pyruvate Oxidation

    Mitochondrial matrix. 2 pyruvate → 2 acetyl-CoA + 2 CO2 + 2 NADH.

    ×2 per glucose
  3. Stage 3

    Krebs Cycle

    Matrix. 2 turns: 6 NADH + 2 FADH2 + 2 GTP (= 2 ATP substrate-level).

    TCA / citric acid cycle
  4. Stage 4

    ETS + Oxidative Phosphorylation

    Inner mitochondrial membrane. NADH → 3 ATP each; FADH2 → 2 ATP each.

    34 ATP from coenzymes

Stage-by-stage balance sheet

The table below aggregates ATP equivalents from each stage, following the NCERT convention (NADH = 3 ATP; FADH2 = 2 ATP; 1 GTP = 1 ATP).

Stage Location Direct ATP NADH produced FADH2 produced ATP from coenzymes Stage total
Glycolysis Cytoplasm 2 ATP (net) 2 NADH — 2 × 3 = 6 ATP 8 ATP
Pyruvate oxidation (×2) Mitochondrial matrix — 2 NADH — 2 × 3 = 6 ATP 6 ATP
Krebs cycle (×2 turns) Mitochondrial matrix 2 GTP = 2 ATP 6 NADH 2 FADH2 6×3 + 2×2 = 22 ATP 24 ATP
Grand total 4 ATP 10 NADH 2 FADH2 34 ATP 38 ATP
38

Theoretical maximum ATP per glucose

NCERT's stated net gain. Composed of 4 ATP by substrate-level phosphorylation and 34 ATP by oxidative phosphorylation via the ETS. NCERT calls this a theoretical maximum that is valid only under idealised assumptions.

Electron carriers: NADH and FADH2

The bulk of ATP comes not from substrate-level reactions but from the reoxidation of electron carriers in the electron transport system. Understanding why NADH and FADH2 yield different ATP quantities is essential for calculating stage-wise totals.

NADH vs FADH2 — ETS entry points and ATP yield

NADH

3 ATP

per molecule (NCERT convention)

  • Donates electrons to Complex I (NADH dehydrogenase)
  • Electrons pass through Complexes I, III, and IV
  • Three proton-pumping sites traversed
  • Mitochondrial NADH: direct entry at high energy level
  • Cytoplasmic NADH (from glycolysis): must be shuttled in; some energy cost may apply
VS

FADH2

2 ATP

per molecule (NCERT convention)

  • Donates electrons to Complex II (succinate dehydrogenase)
  • Electrons enter at ubiquinone, bypassing Complex I
  • Only two proton-pumping sites traversed (Complex III and IV)
  • Lower entry energy level = fewer protons pumped = less ATP
  • Produced exclusively in the Krebs cycle (succinate → fumarate step)

According to NCERT: "Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while that of one molecule of FADH2 produces 2 molecules of ATP." This P/O ratio difference arises because FADH2 enters the electron transport chain at a lower energy level, downstream of Complex I, so fewer protons are translocated per electron pair and less ATP is synthesised by Complex V (ATP synthase).

Glycolysis: the net 2 ATP in detail

Glycolysis consumes 2 ATP in activation steps (glucose → glucose-6-phosphate and fructose-6-phosphate → fructose-1,6-bisphosphate) and produces 4 ATP by substrate-level phosphorylation (two per triose, at the BPGA → 3-PGA and PEP → pyruvate steps). The net gain is therefore 4 − 2 = 2 ATP per glucose.

Additionally, 2 NADH are produced when PGAL is oxidised to 1,3-bisphosphoglycerate. These cytoplasmic NADH contribute 6 ATP in the NCERT table (2 × 3), but NIOS notes that if the shuttle cost of transporting them into the mitochondria is counted, the effective yield drops, reducing the total to 36 ATP.

Pyruvate oxidation: the link reaction

Each pyruvate molecule is oxidatively decarboxylated by the pyruvate dehydrogenase complex in the mitochondrial matrix. The net for both molecules: 2 CO2 released, 2 NADH formed, 2 acetyl-CoA generated. No ATP is produced directly at this stage; all energy is captured in the 2 NADH (= 6 ATP via ETS).

Krebs cycle: one GTP per turn (NEET 2020)

Each turn of the Krebs cycle produces: 3 NADH (at isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, and malate dehydrogenase steps), 1 FADH2 (at succinate dehydrogenase), and 1 GTP (at succinyl-CoA synthetase — the sole substrate-level phosphorylation in the cycle). Two turns per glucose give: 6 NADH, 2 FADH2, and 2 GTP (= 2 ATP directly). The 6 NADH contribute 18 ATP and the 2 FADH2 contribute 4 ATP via ETS, for a Krebs-cycle contribution of 24 ATP in total.

Figure 1 — ATP flow diagram Respiratory Balance Sheet — ATP yield by stage Glycolysis 8 ATP 2 net + 2 NADH Pyruvate Oxidation 6 ATP 2 NADH × 3 Krebs Cycle 24 ATP 6 NADH + 2 FADH2 + 2 GTP TOTAL 38 ATP theoretical maximum ATP Yield per Glucose — NCERT Stage-by-Stage

Figure 1. Comparative ATP contributions from glycolysis (8 ATP), pyruvate oxidation (6 ATP), and the Krebs cycle (24 ATP), totalling 38 ATP under NCERT's theoretical assumptions. The Krebs cycle dominates the balance sheet, accounting for 63% of the total yield.

NCERT caution on the balance sheet

NCERT explicitly identifies four assumptions on which the 38-ATP figure rests — assumptions that are not valid in a living cell:

The four NCERT assumptions — all must hold simultaneously for 38 ATP to be the actual yield; in vivo, none of them hold strictly.

Sequential pathway

Glycolysis, TCA cycle, and ETS must function one after another, with each substrate forming the next. In living cells, all pathways operate simultaneously.

NADH shuttle into mitochondria

Cytoplasmic NADH from glycolysis must be transferred into the mitochondria without energy cost. In reality, shuttle mechanisms may consume ATP, reducing the total (some accounts give 36 instead of 38).

NIOS: may give 36 ATP

No diversion of intermediates

None of the intermediates (acetyl-CoA, oxaloacetate, alpha-ketoglutarate, etc.) must be withdrawn for biosynthesis. In practice, these compounds feed into amino acid, fatty acid, and nucleotide synthesis.

Only glucose as substrate

No alternative substrates (fats, proteins, organic acids) must enter at intermediate stages. In vivo, multiple substrates enter simultaneously at various points in the respiratory pathway.

NEET Trap

36 ATP or 38 ATP? Both numbers appear in study material.

NIOS explicitly states that some biologists use 36 ATP because the 2 cytoplasmic NADH molecules produced during glycolysis must be shuttled into the mitochondria, consuming 2 ATP in the process (38 − 2 = 36). This applies to eukaryotes. In prokaryotes (no mitochondria, no shuttle cost), 38 is correct. NCERT Class 11 uses 38 ATP as the stated net gain. If a NEET question quotes the NCERT text, answer 38. If the question specifies eukaryotes with mitochondrial shuttle cost, 36 may be valid. Read the question carefully — most official keys follow NCERT's 38.

Rule: For NEET purposes, follow the NCERT figure of 38 ATP unless the question specifically asks about the shuttle-cost scenario. The NCERT caution is about conditions, not about changing the number.

Worked examples

Worked example 1

Calculate the ATP yield from the complete aerobic oxidation of one glucose molecule, showing contributions from each stage.

Glycolysis: 2 ATP (substrate-level) + 2 NADH × 3 = 6 ATP from ETS = 8 ATP. Pyruvate oxidation (×2): 2 NADH × 3 = 6 ATP from ETS. Krebs cycle (×2 turns): 2 GTP (= 2 ATP, substrate-level) + 6 NADH × 3 = 18 ATP + 2 FADH2 × 2 = 4 ATP = 24 ATP total from Krebs. Grand total: 8 + 6 + 24 = 38 ATP. Substrate-level total: 2 + 2 = 4 ATP. Oxidative phosphorylation total: 6 + 6 + 18 + 4 = 34 ATP.

Worked example 2

A student says glycolysis produces 4 ATP per glucose. Is this correct? What is the net ATP from glycolysis and why?

The student is quoting gross ATP production. Glycolysis does produce 4 ATP by substrate-level phosphorylation (2 in the BPGA → 3-PGA step and 2 in the PEP → pyruvate step). However, the pathway also consumes 2 ATP in its activation phase (glucose → glucose-6-phosphate and fructose-6-phosphate → fructose-1,6-bisphosphate). Therefore, the net gain is 4 − 2 = 2 ATP. This 2-net-ATP figure is the value tested in NEET (see NEET 2022 Q.127).

Worked example 3

In how many steps of the entire aerobic respiratory pathway is substrate-level phosphorylation carried out?

Substrate-level phosphorylation (SLP) occurs at three points in the overall pathway (counting per glucose, not per turn of the cycle): (1) BPGA → 3-PGA in glycolysis (producing 2 ATP for 2 trioses), (2) PEP → pyruvate in glycolysis (producing 2 ATP for 2 trioses), and (3) succinyl-CoA → succinate in the Krebs cycle (producing 1 GTP per turn, so 2 GTP for 2 turns per glucose). Total direct ATP/GTP = 4 + 2 = 4 ATP equivalents from SLP. The remaining 34 ATP come from oxidative phosphorylation in the ETS.

Common confusion & NEET traps

Substrate-level vs oxidative phosphorylation — what NEET tests

Substrate-Level Phosphorylation

4 ATP

per glucose (2 glycolysis + 2 Krebs)

  • Direct transfer of phosphate from substrate to ADP
  • Does not require the electron transport chain
  • Occurs in glycolysis (×2) and Krebs cycle (×2 as GTP)
  • Can occur in anaerobic conditions
  • Krebs: exactly 1 GTP per turn (NEET 2020 PYQ)
VS

Oxidative Phosphorylation

34 ATP

per glucose (via ETS + ATP synthase)

  • Driven by proton gradient across inner mitochondrial membrane
  • Requires functional electron transport chain and O2
  • Converts NADH (→ 3 ATP) and FADH2 (→ 2 ATP)
  • Accounts for ~89% of total ATP from aerobic respiration
  • Blocked by cyanide, CO (inhibit Complex IV)

NEET PYQ Snapshot — Respiratory Balance Sheet (ATP Yield)

Real NEET questions testing ATP accounting, substrate-level phosphorylation, and the glycolysis net gain.

NEET 2022 · Q.127

Net gain of ATP molecules when each molecule of glucose is converted to two molecules of pyruvic acid is:

  1. 8
  2. 4
  3. 2
  4. 6
Answer: (3) 2 ATP

Why: Glycolysis (glucose → 2 pyruvic acid) occurs in the cytoplasm. Gross ATP production = 4 (2 from BPGA → 3-PGA, 2 from PEP → pyruvate). ATP consumed in activation = 2 (glucose → G6P and F6P → F1,6-BP). Net = 4 − 2 = 2 ATP. The NADH produced (2 molecules) is not counted here because it requires the ETS (aerobic conditions) to yield ATP — the question asks only about the conversion to pyruvic acid.

NEET 2020 · Q.36

The number of substrate-level phosphorylations in one turn of the citric acid cycle is:

  1. Two
  2. Zero
  3. Three
  4. One
Answer: (4) One

Why: In one turn of the Krebs (citric acid) cycle, substrate-level phosphorylation occurs at exactly one step: the conversion of succinyl-CoA to succinic acid, catalysed by succinyl-CoA synthetase, producing one molecule of GTP (equivalent to ATP). All other energy-capturing steps in the cycle (three NADH, one FADH2) involve oxidative phosphorylation, not substrate-level phosphorylation. Since two turns occur per glucose, the total from this step is 2 GTP per glucose.

Concept — Balance Sheet Assumptions

NCERT states that the calculation of net gain of ATP during aerobic respiration is a "theoretical exercise" because:

  1. Oxygen is not always available in cells
  2. All pathways in the living cell work simultaneously; substrates are diverted; ATP is utilised continuously
  3. Only 36 ATP are produced, not 38
  4. The Krebs cycle does not always complete all turns
Answer: (2)

Why: NCERT specifically states that the assumptions underlying the balance sheet — sequential pathway functioning, NADH transfer into mitochondria, no intermediate diversion, only glucose as substrate — "are not really valid in a living system; all pathways work simultaneously and do not take place one after another; substrates enter the pathways and are withdrawn from it as and when necessary; ATP is utilised as and when needed; enzymatic rates are controlled by multiple means." Options 3 and 4 misrepresent NCERT's position.

FAQs — Respiratory Balance Sheet (ATP Yield)

Frequently asked questions on the NCERT respiratory balance sheet, ATP accounting, and NEET exam conventions.

What is the net ATP gain from glycolysis per glucose molecule?

The net gain from glycolysis is 2 ATP per glucose molecule. Four ATP are produced by substrate-level phosphorylation, but 2 ATP are consumed in the activation steps (phosphorylation of glucose and fructose-6-phosphate), leaving a net gain of 2 ATP. Additionally, 2 NADH are produced in the cytoplasm.

How many substrate-level phosphorylations occur in one turn of the Krebs cycle?

One substrate-level phosphorylation occurs per turn of the Krebs cycle — the conversion of succinyl-CoA to succinic acid produces one GTP, which is equivalent to one ATP. Since two turns occur per glucose, a total of 2 GTP (= 2 ATP) are produced from the Krebs cycle by substrate-level phosphorylation.

Why does NCERT call the respiratory balance sheet a "theoretical exercise"?

NCERT states that the balance sheet can remain only a theoretical exercise because it assumes a sequential, orderly pathway where glycolysis, TCA cycle, and ETS function one after another; that NADH from glycolysis is transferred into mitochondria without cost; that no intermediates are diverted for biosynthesis; and that only glucose is respired. In living systems, all pathways work simultaneously, substrates are withdrawn as needed, and ATP is utilised continuously, making a fixed yield unrealistic.

How many NADH molecules are produced per glucose during the entire aerobic respiration pathway?

A total of 10 NADH are produced per glucose: 2 NADH from glycolysis (cytoplasmic), 2 NADH from pyruvate oxidation (mitochondrial matrix), and 6 NADH from two turns of the Krebs cycle (3 per turn, mitochondrial matrix). Additionally, 2 FADH2 are produced from the Krebs cycle.

What is the difference in ATP yield between NADH and FADH2 in the electron transport system?

According to NCERT (older P/O ratio accounting), oxidation of one NADH yields 3 ATP, while oxidation of one FADH2 yields 2 ATP. FADH2 enters the electron transport chain at a lower energy level (at ubiquinone via Complex II), bypassing Complex I, which means fewer protons are pumped and less ATP is generated per molecule compared to NADH.

What is the grand total of ATP produced during aerobic respiration of one glucose molecule according to NCERT?

According to NCERT Class 11 Biology, the net gain is 38 ATP per glucose molecule during complete aerobic respiration. This includes 2 ATP from glycolysis (substrate-level), 2 ATP from the Krebs cycle (substrate-level), and 34 ATP from oxidative phosphorylation via the electron transport system (10 NADH × 3 ATP + 2 FADH2 × 2 ATP). NCERT explicitly notes this is a theoretical maximum.

Why does NIOS mention 36 ATP instead of 38 ATP for some calculations?

NIOS notes that some biologists count 36 ATP instead of 38 because the 2 cytoplasmic NADH produced during glycolysis must be transported into the mitochondria against a concentration gradient, consuming 2 ATP in the process. This shuttle cost reduces the total from 38 to 36. However, in prokaryotes (no mitochondria), 38 ATP is accepted because no transport cost is incurred. NCERT itself uses 38 as the net gain figure.

Related Topics
Respiration in Plants Back to the full chapter notes — all subtopics. Glycolysis Ten-step EMP pathway; produces 2 net ATP and 2 NADH per glucose. Krebs Cycle TCA cycle: 2 turns per glucose, generating the bulk of reduced coenzymes. Electron Transport System Complexes I–IV, proton gradient, and ATP synthase — 34 of the 38 ATP. Oxidative Phosphorylation Chemiosmosis, F0-F1 ATP synthase, and the P/O ratio explained. Fermentation Anaerobic alternative: only 2 ATP net, without ETS involvement. Amphibolic Pathway Why the respiratory pathway is both catabolic and anabolic. Do Plants Breathe? Gas exchange in plants — stomata, lenticels, and diffusion.