Biology · Respiration in Plants · NEET
It is anaerobic (no oxygen used). Fermentation is a type of anaerobic respiration. Because oxygen is absent, pyruvate cannot enter the mitochondria for the Krebs cycle or ETS, so it is broken down incompletely in the cytoplasm into lactic acid.
The real purpose is to recycle NAD+. Glycolysis uses up NAD+ (it becomes NADH). If NAD+ is not regenerated, glycolysis stops and no ATP is made. By reducing pyruvate to lactic acid, the cell hands the hydrogen from NADH back, so NADH turns into NAD+ again and glycolysis can continue. Lactic acid is basically a waste product; the NAD+ recycling is the point.
Lactate dehydrogenase (LDH). It catalyses the transfer of hydrogen (reducing power) from NADH to pyruvic acid, forming lactic acid and regenerating NAD+. Do not confuse it with pyruvate dehydrogenase, which acts only in aerobic respiration to make acetyl CoA.
Net 2 ATP per glucose. These 2 ATP come only from glycolysis (substrate-level phosphorylation). The lactic-acid-forming step itself makes no ATP; it only recycles NAD+. This is why fermentation releases less than 7% of the energy in glucose.
Yes. Skeletal muscle cells switch to lactic acid fermentation during vigorous exercise when oxygen supply cannot keep up. The lactic acid that builds up is linked to muscle cramps and fatigue. In the food world, Lactobacillus bacteria use it to turn milk into curd.
No. Lactic acid fermentation does not release CO2 because pyruvate (3 carbons) is directly converted to lactic acid (3 carbons) with no carbon lost. CO2 IS released in alcoholic fermentation, where pyruvate becomes ethanol. This is a favourite NEET distinguishing point.
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.
It is anaerobic respiration in which glycolysis is followed by the direct reduction of pyruvic acid to lactic acid by lactate dehydrogenase, which regenerates NAD+. It yields a net of 2 ATP per glucose.
Entirely in the cytoplasm (cytosol). No mitochondria are involved. It occurs in some bacteria such as Lactobacillus and in human/animal muscle cells during oxygen shortage.
NADH supplies the hydrogen (reducing power) that converts pyruvate to lactic acid. In giving up this hydrogen, NADH is oxidised back to NAD+, which is essential to keep glycolysis running.
Because glucose is only partially broken down. Without oxygen, the Krebs cycle and ETS cannot operate, so most of glucose's energy stays locked inside lactic acid. Only the 2 net ATP from glycolysis are gained (less than 7% of glucose energy).
Both start with glycolysis and both regenerate NAD+. But lactic acid fermentation makes lactic acid with NO CO2 (in muscles and Lactobacillus), while alcoholic fermentation makes ethanol PLUS CO2 (in yeast) and needs two enzymes, pyruvate decarboxylase and alcohol dehydrogenase.
