Anaerobic Respiration in Plants:

It is the process of release of energy in enzymatically controlled step-wise incomplete degradation of organic food without oxygen being used as oxidant.

Therefore, end products are never completely inorganic. The term anaerobic respiration is often used in connection with higher organisms where it occurs in the roots of some water-logged plants, muscles of animals and as supplementary mode of respiration in massive tissues.

Anaerobic respira­tion is the exclusive mode of respiration in some parasitic worms, many prokaryotes, several unicellular eukaryotes and moulds. In micro-organisms the term fermentation is more commonly used where anaerobic respiration is known after the name of product like alco­holic fermentation, lactic acid fermentation.

Carbon dioxide is evolved in some cases. It gives a frothy appearance (L. fermentum — to boil) to the medium.

Buchner (1897) was the first to find that fermentation could be caused without the living yeast cells by grinding them under pressure and mixing the extract with sug­ary solution. The enzyme complex present in the extract was named as zymase.

Be­cause of the latter, fermentation is also called zymosis. Commonly, fermentation is defined as the anaerobic breakdown of carbohydrates and other organic compounds into alcohols, organic acids, gases, etc. with the help of micro-organisms or their enzymes.

The mechanism of anaerobic respira­tion or fermentation is similar to common pathway of aerobic respiration up to gly­colysis. Glycolysis breaks down glucose enzymatically in several steps to form two molecules of pyruvate, 2 ATP and 2 NADH (+H+). Pyruvate is anaerobically broken down to yield various products depending upon the organism and the type of tissue.

The two common products are ethyl alco­hol and lactic acid.

1. Ethyl Alcohol Fermentation (Fig. 14.9):

It is quite common in fungi (e.g., Rhizopus, Yeast) and bacteria. Yeast can respire both aerobically and anaerobically.

Anaerobic respiration occurs in sugary solution if the fungus is not in contact with atmo­sphere. It causes fermentation. In the presence of pyruvate decarboxylase and TPP (thiamine pyrophosphate), pyruvate is broken down to form acetaldehyde. Carbon dioxide is released.

Acetaldehyde is reduced to ethyl alcohol or ethanol by alcohol dehydrogenase. Hydrogen is obtained from NADH produced during oxidation of glyceraldehyde 3-phosphate to 1: 3 diphosphoglycerate in glycolysis.

Ethyl alcohol does not stay inside microorganisms but is excreted. Accumulation of alcohol beyond a certain limit can, however, kill the microorganism (e.g., 13% in yeast). A higher concentration of alcohol in a beverage is achieved through distillation.

2. Lactic Acid Fermentation:

The fermentation occurs in lactic acid bacteria (e.g., Lactobacillus), some fungi and muscles. Lactic acid produced in muscles is sent to liver to regenerate glucose. In lactic acid fermentation pyruvic acid produced in glycolysis is directly reduced by NADH to form lactic acid. No C02 is produced. The enzyme is lactic dehydro­genase which requires FMN (Flavin Mono-nucleotide) and Zn2+.

Anaerobic respiration produces very little energy (about 5%) as compared to aerobic respiration.

The reasons are:

(a) There is incomplete breakdown of respiratory substrate,

(b) At least one of the products of anaerobic respiration is organic. It can be further oxidised to release energy,

(c) NADH produced during glycolysis is often used up.

(d) ATP formation does not occur during regeneration of NAD+.

(e) Electron transport chain is absent,

(f) Oxygen is not used for receiving electrons and protons.

Importance of Anaerobic Respiration:

(i) Anaerobic respiration is important during periods of oxygen deficiency,

(ii) Alcoholic fermentation is used in brewing industry for the production of various types of beers, whisky and other wines,

(iii) Carbon dioxide of alcoholic fermentation is used in baking industry for making the bread spongy,

(iv) Vinegar is obtained by the fermentation activity of acetic acid bacteria,

(v) Dairy industry depends upon the action of lactic acid bacteria which convert milk sugar to lactic acid. Lactic acid coagulates the milk protein casein and the droplets of milk fat fuse,

(vi) Production of industrial alcohols and organic acids,

(vii) Tea and tobacco leaves are cured (or removed of their bitterness) and provided with a fine flavour,

(viii) Retting or separation of stem fibres is carried out with the help of bacterial fermentation of softer tissues,

(ix) Ensilage or preserved fodder is prepared by keeping green chopped fodder in silo where bacterial action causes softening and release of preserving acids,

(x) Cleaning of raw hides is a fermentative bacterial activity,

(xi) Decom­position of organic remains is carried out by fermentation. However, the fermenting organ­isms also spoil out food and may cause food poisoning by releasing toxins or ptomaine.