Fermentation releases very little energy because glucose is only partly broken down (incomplete oxidation). It gives a net gain of just 2 ATP per glucose, and less than 7% of the energy stored in glucose is released. Memory hook: "Half-eaten glucose = tiny reward" — the C-H bonds are still locked inside lactic acid or ethanol, so most energy is never released.
Fermentation stops after glycolysis: glucose is only partly broken down to lactic acid or ethanol, giving a net of just 2 ATP and releasing less than 7% of glucose energy because the Krebs cycle and ETS are skipped.
Your doubts, answered
Why does fermentation release less than 7% of glucose energy?
Because glucose is only partly (incompletely) oxidised. In fermentation, glucose is broken only up to pyruvic acid, which is then turned into lactic acid or ethanol. Most of the energy stays trapped inside the C-H and C-C bonds of these end products. So less than 7% of the energy in glucose is released, and not even all of that is trapped as ATP. NCERT states this exact figure, and NEET 2022 asked it directly.
Why is the net ATP gain in fermentation only 2?
Glycolysis makes 4 ATP by substrate-level phosphorylation but spends 2 ATP in the early (investment) phase to activate glucose. So the net gain is 4 minus 2 = 2 ATP per glucose. Fermentation does not run the Krebs cycle or ETS, so no extra ATP is added after glycolysis. That is why the net gain stays at just 2 ATP.
Where does the rest of the energy go if it is not released?
It stays locked inside the end products. Lactic acid and ethanol still hold most of the C-H bonds of the original glucose. Since these molecules are never broken down further in fermentation, the energy in their bonds is never released. This is why yeast cells and muscle cells that ferment must use a huge amount of glucose to get a small amount of ATP.
Does fermentation use oxygen, the Krebs cycle, or the ETS?
No. Fermentation is anaerobic (no oxygen). It uses only glycolysis, then converts pyruvate to lactic acid or ethanol to regenerate NAD+. It skips pyruvate oxidation, the Krebs cycle, and the electron transport system. Because the ETS is where most ATP is normally made, skipping it is the main reason fermentation yields so little energy.
Why must NADH be reoxidised in fermentation, and how does that limit energy?
Glycolysis needs a steady supply of NAD+ to keep running. In fermentation, the only way to get NAD+ back is to hand the hydrogen from NADH to pyruvate, forming lactic acid or ethanol. This regenerates NAD+ but wastes the energy-rich electrons that would have gone to the ETS to make many ATP. So the recycling step keeps glycolysis alive but throws away the biggest energy source.
⚠️ The NEET trap ✗ Fermentation gives 2 ATP because it only makes 2 ATP in glycolysis. ✓ Glycolysis makes 4 ATP (gross) but spends 2 ATP first, so the NET gain is 2 ATP. Fermentation adds no ATP after glycolysis. 🧠 NTA loves gross-vs-net. Remember: 4 made minus 2 spent equals 2 net. If an option says 'four' or 'six', it is counting gross or ignoring the ATP invested.
Real NEET questions
NEET 2022
What amount of energy is released from glucose during lactic acid fermentation?
A · Approximately 15%
B · More than 18%
C · About 10%
D · Less than 7% ✓
Solution: In fermentation (lactic acid or alcoholic) only incomplete oxidation of glucose occurs, so less than 7% of the energy stored in glucose is released, and not even all of that is trapped as ATP. The other options overstate the small yield of anaerobic breakdown. (NCERT: less than seven per cent of the energy in glucose is released.)
NEET 2022
What is the net gain of ATP when each molecule of glucose is converted to two molecules of pyruvic acid?
A · Four
B · Six
C · Two ✓
D · Eight
Solution: Glycolysis converts one glucose to two pyruvate, producing 4 ATP by substrate-level phosphorylation but consuming 2 ATP in the preparatory phase, giving a net gain of 2 ATP. 'Four' and 'Six' count gross production or ignore the ATP invested, so they are wrong.
Solved Respiration in Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
How many ATP does fermentation produce per glucose?
A net gain of only 2 ATP per glucose molecule, all from glycolysis by substrate-level phosphorylation.
What percentage of glucose energy is released in fermentation?
Less than 7% of the energy stored in glucose is released, because glucose is only incompletely oxidised.
Why is aerobic respiration more efficient than fermentation?
Aerobic respiration fully oxidises glucose using the Krebs cycle and ETS, yielding about 36 to 38 ATP, while fermentation stops at pyruvate and gives only 2 ATP.
Is the energy released in lactic acid and alcoholic fermentation the same?
Yes, both release less than 7% of glucose energy and both give a net of 2 ATP, because both stop after glycolysis.
