The Respiratory Balance Sheet
The respiratory balance sheet is a theoretical summary of the total ATP produced during the complete aerobic oxidation of one molecule of glucose. It integrates all stages of respiration—glycolysis, link reaction, Krebs cycle, and Electron Transport System (ETS)—to show how energy is released step by step.
Important: This balance sheet represents an ideal situation and is mainly used for conceptual clarity and exam calculations.
Assumptions Used in the Respiratory Balance Sheet
To simplify calculations, the following assumptions are made:
- Respiration occurs in a sequential and uninterrupted manner.
- All NADH and FADH₂ produced are fully utilised in the ETS.
- No respiratory intermediates are diverted for biosynthesis.
- Only glucose is used as the respiratory substrate.
- NADH yields 3 ATP and FADH₂ yields 2 ATP (NCERT convention).
Net ATP Calculation (Per Glucose Molecule)
Glycolysis (Cytoplasm)
- ATP produced: 4
- ATP consumed: 2
- Net ATP: 2 ATP (substrate-level phosphorylation)
- NADH produced: 2 → 6 ATP (via ETS)
Total from Glycolysis = 8 ATP
Link Reaction (Mitochondrial Matrix)
- NADH produced: 2 → 6 ATP
Total from Link Reaction = 6 ATP
Krebs Cycle (2 Turns)
- ATP (as GTP): 2
- NADH: 6 → 18 ATP
- FADH₂: 2 → 4 ATP
Total from Krebs Cycle = 24 ATP
Grand Total ATP Yield
- Glycolysis: 8 ATP
- Link Reaction: 6 ATP
- Krebs Cycle: 24 ATP
Total = 38 ATP per glucose molecule
Why 36 or 38 ATP?
The 2 NADH produced during glycolysis are formed in the cytoplasm and must be transported into mitochondria using shuttle systems:
Malate–Aspartate Shuttle (liver, kidney, heart):
Each NADH → 3 ATP → Total = 38 ATP
Glycerol Phosphate Shuttle (brain, muscle):
Each NADH → 2 ATP → Total = 36 ATP
Hence, the ATP yield is often stated as 36–38 ATP.
Practical Reality
In real cells, the ATP yield is slightly lower due to:
- Use of ATP in transport of metabolites
- Proton leakage across inner mitochondrial membrane
- Variable efficiency of shuttle systems
Therefore, the respiratory balance sheet is a theoretical maximum, not an exact cellular count.
Memory Capsules – Respiratory Balance Sheet
One Glucose → Complete Oxidation → 36–38 ATP
Stage-wise ATP Contribution:
- Glycolysis → 8 ATP
- Link Reaction → 6 ATP
- Krebs Cycle → 24 ATP
Coenzymes to Remember:
- Total NADH = 10 → 30 ATP
- Total FADH₂ = 2 → 4 ATP
Why Variation?
- Cytoplasmic NADH needs shuttle
- Shuttle decides 36 vs 38 ATP
Exam Tip: Respiratory balance sheet is theoretical, not actual cellular yield.
💡 Questions and Answers
Q1. What is the respiratory balance sheet?
A1: The respiratory balance sheet is a theoretical calculation showing the total ATP produced when one glucose molecule is completely oxidised during aerobic respiration.
Q2. Why is the actual ATP yield less than the theoretical value?
A2: Because some ATP is used in transport of intermediates, some energy is lost due to proton leakage, and shuttle systems are not perfectly efficient.
Q3. Which stage of respiration produces maximum ATP indirectly?
A3: The Electron Transport System (ETS) produces the maximum ATP indirectly using NADH and FADH₂.
Q4. Why do textbooks mention both 36 and 38 ATP?
A4: Because the ATP yield depends on the shuttle system used to transport cytoplasmic NADH into mitochondria—malate shuttle gives 38 ATP, glycerol phosphate shuttle gives 36 ATP.
Q5. Is the respiratory balance sheet an exact representation of cellular respiration?
A5: No. It is an idealised model used for understanding and exams. Actual ATP yield in living cells is usually lower.