Biology · Respiration in Plants · NEET
No, but glycolysis is the first part of it. Every cell first does glycolysis to split one glucose into two pyruvic acid molecules. Fermentation is what happens AFTER glycolysis when there is no oxygen: the pyruvate is changed into lactic acid or into ethanol and CO2. So glycolysis is a shared starting step, and fermentation is the extra anaerobic finishing step. For NEET, remember the NCERT line: glycolysis gives pyruvate, and its fate depends on oxygen.
Fermentation happens without oxygen, so it is anaerobic. Because it does not use the mitochondria, all of it takes place in the cytoplasm (the same place as glycolysis). This is a common trap: students think respiration always uses mitochondria, but fermentation and glycolysis are fully cytoplasmic. NCERT says fermentation occurs in many prokaryotes, unicellular eukaryotes like yeast, and even in germinating seeds.
Because glucose is only broken down half-way (incomplete oxidation). Glycolysis makes 4 ATP but uses 2 ATP in the early investment steps, so the net gain is only 2 ATP per glucose. The conversion of pyruvate to lactic acid or ethanol makes NO extra ATP. NCERT states plainly that in fermentation there is a net gain of only two molecules of ATP for each glucose degraded to pyruvic acid. Less than 7% of the energy in glucose is released, and not even all of that is trapped as ATP.
NADH is used to regenerate NAD+ so glycolysis can keep running. In glycolysis, NAD+ picks up hydrogen to become NADH+H+. If there is no oxygen, the ETS cannot re-oxidise this NADH. So in fermentation the NADH+H+ acts as the reducing agent: it gives its hydrogen to pyruvate (making lactic acid) or to acetaldehyde (making ethanol), and turns back into NAD+. This recycled NAD+ lets glycolysis continue. NCERT: NADH+H+ is reoxidised to NAD+ in both processes.
They are closely linked and NCERT often treats fermentation as the anaerobic route, but be careful. Fermentation is one type of anaerobic breakdown where the final hydrogen acceptor is an organic molecule (pyruvate or acetaldehyde), NOT an inorganic molecule and NOT oxygen. Some bacteria do true anaerobic respiration using inorganic acceptors like nitrate. For NEET at Class 11 level, fermentation = the anaerobic incomplete oxidation of glucose to lactic acid or ethanol.
What amount of energy is released from glucose during lactic acid fermentation?
What is the net gain of ATP when each molecule of glucose is converted to two molecules of pyruvic acid?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the breaking down of glucose without oxygen. Glucose is split into pyruvic acid, and then pyruvic acid is turned into lactic acid or into ethanol and CO2. It gives only a little energy (net 2 ATP).
Lactic acid fermentation, which makes lactic acid (in our skeletal muscle during heavy exercise and in some bacteria), and alcoholic fermentation, which makes ethanol and CO2 (in yeast).
Pyruvic acid decarboxylase and alcohol dehydrogenase convert pyruvic acid into CO2 and ethanol. In lactic acid fermentation, lactate dehydrogenase converts pyruvic acid to lactic acid.
There is a net gain of only 2 ATP per glucose molecule, all from glycolysis. Converting pyruvate to lactic acid or ethanol produces no extra ATP.
Because the end products, acid or alcohol, can build up and are harmful to the cell. Also, NADH is re-oxidised to NAD+ slowly, so the process is inefficient compared with aerobic respiration.
