Question
Compare aerobic and anaerobic respiration. Describe the pathways involved and the net ATP yield from each.
(NEET, CBSE Class 11 — Respiration in Organisms)
Solution — Step by Step
Both aerobic and anaerobic respiration begin with glycolysis in the cytoplasm. One glucose (6C) is split into two molecules of pyruvate (3C). Net yield: 2 ATP + 2 NADH. No oxygen is required for this step.
In the presence of O, pyruvate enters the mitochondria, is converted to acetyl CoA (producing CO and NADH), and enters the Krebs cycle. The Krebs cycle produces more NADH, FADH, and GTP. Finally, NADH and FADH donate electrons to the ETC on the inner mitochondrial membrane, generating most of the ATP via oxidative phosphorylation.
Without O, pyruvate is converted to either ethanol + CO (alcoholic fermentation, by yeast) or lactic acid (lactic acid fermentation, by muscles and some bacteria). No further ATP is produced beyond glycolysis. The purpose of fermentation is to regenerate NAD so glycolysis can continue.
- Aerobic: 36-38 ATP per glucose (glycolysis: 2, Krebs: 2 GTP, ETC: 32-34)
- Anaerobic: Only 2 ATP per glucose (from glycolysis alone)
Aerobic respiration is about 18 times more efficient than anaerobic.
graph TD
A["Glucose"] -->|"Glycolysis (cytoplasm)<br/>2 ATP, 2 NADH"| B["Pyruvate"]
B -->|"O₂ present<br/>Aerobic"| C["Acetyl CoA → Krebs Cycle<br/>→ ETC → 36-38 ATP total"]
B -->|"No O₂<br/>Anaerobic"| D{"Fermentation"}
D -->|"Yeast"| E["Ethanol + CO₂<br/>2 ATP total"]
D -->|"Muscle"| F["Lactic acid<br/>2 ATP total"]Why This Works
The massive difference in ATP yield comes from the electron transport chain, which operates only with oxygen as the final electron acceptor. In anaerobic conditions, the cell cannot run the ETC, so all the energy stored in NADH is wasted (the NADH is used to regenerate NAD in fermentation, not for ATP synthesis).
This is why aerobic organisms are more active and can sustain higher metabolic rates — they extract almost 20 times more energy from the same glucose molecule.
Alternative Method — Track the NADH
Aerobic: 10 NADH + 2 FADH per glucose all go to ETC, producing ~34 ATP. Anaerobic: 2 NADH are consumed in fermentation, producing 0 ATP. The NADH difference explains the efficiency gap.
For NEET: glycolysis is the only common step. Everything after pyruvate diverges based on oxygen availability. Also remember — glycolysis occurs in the cytoplasm, Krebs cycle in the mitochondrial matrix, and ETC on the inner mitochondrial membrane.
Common Mistake
Students say anaerobic respiration produces “no ATP.” Wrong — it produces 2 ATP (from glycolysis). Fermentation itself does not make ATP, but it allows glycolysis to continue by regenerating NAD. Without fermentation, glycolysis would stop because all NAD would be consumed.