Question
Compare aerobic and anaerobic respiration with respect to pathways, end products, location, and ATP yield.
Solution — Step by Step
flowchart TD
A[Glucose] --> B[Glycolysis - Cytoplasm]
B --> C{O2 available?}
C -->|Yes - Aerobic| D[Pyruvate enters Mitochondria]
D --> E[Krebs Cycle + ETC]
E --> F[CO2 + H2O + 36-38 ATP]
C -->|No - Anaerobic| G{Organism type?}
G -->|Yeast| H[Ethanol + CO2 + 2 ATP]
G -->|Muscle cells| I[Lactic acid + 2 ATP]Both aerobic and anaerobic respiration begin with glycolysis in the cytoplasm. Glucose (6C) is broken down to 2 molecules of pyruvate (3C). Net gain: 2 ATP + 2 NADH. No oxygen is needed for this step.
Pyruvate enters the mitochondria, is converted to acetyl CoA (releasing CO and NADH), enters the Krebs cycle (producing NADH, FADH, GTP, CO), and the reduced coenzymes feed into the electron transport chain (ETC) on the inner mitochondrial membrane. O is the final electron acceptor, forming HO. Total yield: 36-38 ATP per glucose.
Without O, pyruvate cannot enter the mitochondria. Instead, it undergoes fermentation in the cytoplasm. In yeast: pyruvate is decarboxylated to acetaldehyde, then reduced to ethanol + CO (alcoholic fermentation). In animal muscle cells: pyruvate is directly reduced to lactic acid (lactic acid fermentation). Both pathways regenerate NAD so glycolysis can continue. Total yield: only 2 ATP per glucose.
Why This Works
| Feature | Aerobic | Anaerobic |
|---|---|---|
| O required | Yes | No |
| Location | Cytoplasm + Mitochondria | Cytoplasm only |
| End products | CO + HO | Ethanol + CO OR Lactic acid |
| ATP yield | 36-38 per glucose | 2 per glucose |
| Complete oxidation? | Yes | No (organic end products still have energy) |
| Organisms | Most eukaryotes | Yeast, some bacteria, muscle cells (temporarily) |
Aerobic respiration is 18-19 times more efficient because glucose is completely oxidised to CO and HO. In anaerobic respiration, the organic end products (ethanol or lactic acid) still contain significant chemical energy — it is incomplete oxidation.
Common Mistake
Students say “anaerobic respiration happens only in microorganisms.” Human muscle cells also undergo lactic acid fermentation during intense exercise when O supply cannot keep up with demand. This causes muscle fatigue and cramps. The lactic acid is later transported to the liver and converted back to pyruvate (Cori cycle) when O is available.