Question
Compare aerobic and anaerobic respiration in terms of ATP yield, products formed, and conditions required. Where does each type occur in the cell?
Solution — Step by Step
Aerobic respiration is the breakdown of glucose in the presence of oxygen to produce ATP. Anaerobic respiration (or fermentation) breaks down glucose in the absence of oxygen. Both start with glycolysis in the cytoplasm — that’s their common ground. The paths diverge after glycolysis depending on oxygen availability.
After glycolysis (cytoplasm), the pyruvate produced enters the mitochondria. There it undergoes the Krebs cycle (matrix) and then the electron transport chain (inner mitochondrial membrane).
Total ATP yield from one molecule of glucose via aerobic respiration: approximately 36–38 ATP (the exact number varies by textbook; NCERT uses 36 ATP).
Products: and (and heat).
When oxygen is absent, pyruvate cannot enter the mitochondria for the Krebs cycle. Instead, two different pathways operate:
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Lactic acid fermentation (in animal muscle cells, RBCs, some bacteria): Pyruvate → Lactic acid. No released. Net ATP yield: 2 ATP.
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Alcoholic fermentation (in yeast, some plant tissues): Pyruvate → Ethanol + . Net ATP yield: 2 ATP.
The key purpose of both pathways is to regenerate NAD⁺ so glycolysis can continue — not primarily to make ATP.
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen required | Yes | No |
| Location | Cytoplasm + Mitochondria | Cytoplasm only |
| ATP yield | 36–38 ATP | 2 ATP |
| Products | CO₂ + H₂O | Lactic acid OR Ethanol + CO₂ |
| Completeness | Complete oxidation | Incomplete oxidation |
| Organisms | Most eukaryotes | Yeast, muscle cells under stress, some bacteria |
The 2 ATP from glycolysis is all anaerobic respiration achieves. Aerobic respiration produces the extra 34–36 ATP through the electron transport chain — electrons from NADH and FADH₂ are passed down the chain, releasing energy that pumps H⁺ ions across the inner mitochondrial membrane, driving ATP synthase. This chemiosmotic process is extraordinarily efficient. Anaerobic respiration simply cannot access this machinery without oxygen as the final electron acceptor.
Why This Works
The efficiency gap (2 vs 36–38 ATP) comes down to one thing: whether glucose is completely oxidised. In aerobic respiration, all 6 carbons end up as CO₂ — maximum energy is extracted. In anaerobic respiration, the 6-carbon glucose only partially breaks down (to 3-carbon lactic acid or 2-carbon ethanol), leaving most chemical energy locked in those molecules.
This is why athletes who run fast (anaerobic) tire quickly and feel muscle burn (lactic acid accumulation), while slow joggers (aerobic) can sustain activity for hours.
Alternative Method — Energy Accounting
Think of glucose as a currency note. Aerobic respiration “exchanges” the full value (36–38 ATP coins). Anaerobic respiration only manages to extract 2 coins and leaves the rest locked in the products. The “exchange counter” is the mitochondria — without oxygen to keep it running, you’re stuck with street-corner change.
Common Mistake
The biggest error is claiming that anaerobic respiration produces “no ATP at all.” It produces 2 ATP (from glycolysis). Also, do not say fermentation is the same as anaerobic respiration — fermentation is specifically the regeneration of NAD⁺ that follows glycolysis in the absence of oxygen. In NEET MCQs, these distinctions matter.
Another common slip: stating that lactic acid fermentation releases CO₂. It does not. Only alcoholic fermentation releases CO₂.
For board exams, a well-drawn comparison table like the one in Step 4 will fetch you full marks on a 3-mark comparison question. Memorise the 6 rows — especially ATP yield (2 vs 36–38) and products.