Respiration in Plants: Cellular Respiration, Glycolysis, Fermentation and Fate of Pyruvic Acid in Aerobic Respiration

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Cellular respiration is the enzymatic breakdown of glucose (C6H12O6) in the presence of oxygen (like glucose, amino acids and fatty acids) to produce CO2, water and cellular energy (ATP). It occurs within the cells and tissues.

Image showing Cell Respiration Pathways.

Image Showing Cell Respiration Pathways

Glycolysis (Also Known As-Embden-Meyerhof-Pranas Pathway)

  • Glycolysis is a ten-step process that occurs in the cytoplasm. Glucose is a six-carbon sugar undergoes a series of chemical transformations. In the end, it gets converted into two molecules of pyruvate, a three-carbon organic molecule. In these reactions, net yield of 2 ATP per glucose molecule, and net yield of 2 NADH per glucose (NADH is nicotine adenine dinucleotide, a co-enzyme that serves as a carrier for H+ ions liberated as glucose is oxidized.)

  • .

  • Two molecules of ATP were used up in the initial steps of glycolysis. Thus, the net gain of ATP during glycolysis is. Also, are produced. Only a small amount of energy is released at the end of glycolysis.


In aerobic respiration, oxidation of pyruvic acid requires O2 and enters in mitochondria. Under anaerobic conditions or insufficient supply of O2 microbes, plants and animals carry out fermentation. In fermentation (in yeast), pyruvic acid is converted to CO2 and ethanol and in muscle cells of animals, pyruvic acid is reduced to lactic acid by lactate dehydrogenase. The reducing agent is NADH + H+ which are deoxidized to NAD+ in both the processes, production of 2 ATPs can continue under anaerobic conditions and there is no further release of ATP during fermentation.

Image showing Pathways of anaerobic respiration.

Image Showing Pathways of Anaerobic Respiration

Significance of Fermentation

  • Fermentation has a number of commercial applications beyond those described thus far. Many occur in the food preparation and processing industry.

  • In bakeries for preparing bread, cakes and biscuits.

  • In breweries for preparing wine and other alcoholic drinks

  • In producing vinegar and in the tanning and curing of leather.

  • Ethanol is used to make gasohol, a fuel that is used for cars in Brazil.

  • In everyday life, fermentation is used while making idli, dosa, bhatura and dhokla.

Fate of Pyruvic Acid in Aerobic Respiration

Each pyruvate from glycolysis goes into the mitochondrial matrix—the innermost compartment of mitochondria. There, it’s converted into a two-carbon molecule bound to Coenzyme A, known as acetyl CoA. Carbon dioxide is released and NADH is generated. Acetyl CoA is thus the connecting link between glycolysis and the next series of reactions that yield more energy in the form of ATP. Acetyl CoA can also be generated from fats and proteins.

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