Question
Compare aerobic and anaerobic respiration. Where does each occur in the body, and why does one produce so much more energy than the other?
This is a favourite NCERT Class 7 question — it appears in almost every school exam as a table-based “differentiate between” question. Let’s build a proper understanding so you can answer any variation of it.
Solution — Step by Step
Respiration is the process of breaking down glucose to release energy (ATP) that cells can use. The word “aerobic” just means “with air” (specifically oxygen), and “anaerobic” means “without air.”
In aerobic respiration, glucose is completely broken down using oxygen. The reaction happens in two stages — first in the cytoplasm, then inside the mitochondria.
The mitochondria do the heavy lifting — this is why we call them the “powerhouse of the cell.”
When oxygen is absent or limited, cells switch to anaerobic respiration. Glucose is only partially broken down, so far less energy is released — just 2 ATP.
In yeast and some bacteria, the end products are ethanol and CO₂ (fermentation). In our muscle cells during intense exercise, the end product is lactic acid.
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen needed? | Yes | No |
| Location | Cytoplasm + Mitochondria | Cytoplasm only |
| ATP produced | 38 | 2 |
| End products | CO₂ + H₂O | Lactic acid (muscles) / Ethanol + CO₂ (yeast) |
| Glucose breakdown | Complete | Incomplete |
| Examples | Normal cell activity | Sprinting, yeast fermentation |
When you sprint 100m, your muscles demand energy faster than your lungs can supply oxygen. Cells switch to anaerobic respiration — fast but inefficient. The lactic acid buildup is exactly what causes that burning feeling in your legs. Once you rest and oxygen returns, the lactic acid is broken down.
Why This Works
The giant difference in ATP output (38 vs. 2) comes down to how completely glucose is broken down. Aerobic respiration runs glucose through the full Krebs cycle inside the mitochondria, extracting almost all the chemical energy stored in the molecule.
Anaerobic respiration stops early — after just the glycolysis stage in the cytoplasm — because the next steps require oxygen to proceed. The remaining energy is still locked inside the lactic acid or ethanol molecules, which is why they are considered “waste” products from the cell’s perspective.
This is also why organisms with mitochondria (eukaryotes) can sustain longer, more energy-intensive activities than bacteria that rely on anaerobic pathways.
A quick memory trick: “Aero = Airport = needs air = more energy”. The more oxygen you bring in, the more complete the burn, the more ATP you get.
Alternative Method
If the exam asks you to explain rather than differentiate, use the fuel analogy:
Aerobic respiration is like burning wood completely in a fire with plenty of oxygen — you get maximum heat and the wood is fully used up (CO₂ + H₂O as ash and smoke).
Anaerobic respiration is like smothering that same fire — the wood only partially burns, you get less heat, and you’re left with charred wood (lactic acid/ethanol) that still has energy trapped inside it.
This analogy works well in long-answer questions to show the examiner you understand the concept, not just memorised it.
Common Mistake
Many students write that anaerobic respiration produces “no CO₂.” This is wrong for muscle cells — lactic acid fermentation produces no CO₂. But in yeast (alcoholic fermentation), CO₂ is produced. In board exams, always specify which organism you mean. Writing a blanket “anaerobic = no CO₂” will lose you marks if the question mentions yeast.
The other common slip: students say anaerobic respiration happens “in the mitochondria.” No — because there’s no oxygen, the process cannot enter the mitochondria. It stays in the cytoplasm only. The mitochondria literally have no role in anaerobic respiration.