Biology · Respiration in Plants · NEET
| Oxygen needed | Not needed (anaerobic) | Needed (aerobic) |
| Site | Cytoplasm only | Cytoplasm + mitochondrion |
| Breakdown of glucose | Incomplete | Complete |
| End products | Ethanol + CO2 or lactic acid | CO2 + water |
| Energy released | Less than 7% of glucose energy | Almost all glucose energy |
| Net ATP per glucose | About 2 ATP | About 36 ATP |
In NCERT Class 11 they are treated as the same idea: both break glucose without oxygen. Fermentation is the incomplete oxidation of glucose under anaerobic conditions, giving ethanol (in yeast) or lactic acid (in our muscles). So for NEET, treat fermentation as the common example of anaerobic respiration.
In fermentation glucose is only partly broken down. The end products (ethanol or lactic acid) still hold most of the energy of glucose. So less than 7% of the energy in glucose is released, and not even all of that becomes ATP. In aerobic respiration glucose is fully oxidised to CO2 and water, so almost all the energy is set free, giving about 36 ATP.
It depends on the type. Alcoholic fermentation (yeast) produces CO2 along with ethanol. Lactic acid fermentation (our muscles, some bacteria) makes only lactic acid and does NOT release CO2. Aerobic respiration always releases CO2 (from the Krebs cycle).
Both start with glycolysis, which gives a net gain of only 2 ATP. Fermentation stops there, so its net yield stays at 2 ATP per glucose. Aerobic respiration continues through the Krebs cycle and ETS to make about 36 ATP per glucose. This huge gap is why oxygen-based respiration is far more efficient.
Yes. Glycolysis (the EMP pathway) is common to both and happens in the cytoplasm, splitting glucose into two pyruvic acid molecules. After glycolysis the paths differ: fermentation converts pyruvate to ethanol or lactic acid in the cytoplasm, while aerobic respiration sends pyruvate into the mitochondrion for the Krebs cycle and ETS.
Glycolysis needs NAD+ to keep running. In fermentation, converting pyruvate to ethanol or lactic acid uses up NADH and gives back NAD+, so glycolysis can continue making a small amount of ATP even without oxygen. In aerobic respiration, oxygen and the ETS regenerate NAD+ instead.
What amount of energy is released from glucose during lactic acid fermentation?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Aerobic respiration by far. Fermentation nets only about 2 ATP per glucose (from glycolysis), while aerobic respiration nets about 36 ATP per glucose.
Fermentation: ethanol + CO2 (yeast) or lactic acid (muscle/bacteria). Aerobic respiration: CO2 + water (complete oxidation).
Fermentation occurs entirely in the cytoplasm. Aerobic respiration begins in the cytoplasm (glycolysis) but the Krebs cycle and ETS occur inside the mitochondrion.
NCERT notes that fermentation is hazardous because it builds up either acid or alcohol, which are harmful if they accumulate.
Pyruvic acid decarboxylase and alcohol dehydrogenase convert pyruvic acid to CO2 and ethanol in yeast.
