Respiration in Plants Test - 10

• According to the mode of respiration, bacteria can be aerobic or anaerobic. Each of them is further of two types, obligate and facultative.
• Obligate anaerobic bacteria respire only anaerobically. They generally get killed under aerobic conditions, e.g., Clostridium tetani, C. botulinum etc.
• Facultative anaerobes are bacteria which generally respire aerobically but switch over to anaerobic mode of respiration, if oxygen becomes deficient, e.g., Saccharomyces cerevisiae.

Fermentation is the incomplete oxidation of glucose under anaerobic conditions by sets of reactions where pyruvic acid is converted to CO₂​ and ethanol (alcoholic fermentation) or lactic acid (lactic acid fermentation). In fermentation, there is a net gain of only 2 ATP molecules for each molecule of glucose degraded to pyruvic acid.

The given reaction represents alcoholic fermentation. In alcoholic fermentation, pyruvate is converted into ethanol in a two step pathway. Ethyl alcohol fermentation is quite common in fungi (e.g., Rhizopus, yeast) and bacteria. In yeast, pyruvate is decarboxylated to form acetaldehyde which is then reduced by NADH to form ethanol.

In glycolysis, NAD⁺ oxidises glyceraldehyde-3-phosphate by acting as a reducing agent and convert it to 3-phosphoglyceric acid.
(NAD⁺ + H⁺ → NADH)
In the absence of oxygen Pyruvic acid to either Lactic acid or ethanol depending on the organism.
1. In Lactic acid fermentation - pyruvic acid converts into Lactate while regenerating the NAD⁺.
(NADH → NAD⁺ + H⁺)
2. In Alcoholic fermentation - pyruvic acid converts into ethanol while regenerating the NAD⁺.
(NADH → NAD⁺ + H⁺)

Lactic acid fermentation occurs in lactic acid bacteria (e.g., Lactobacillus), some fungi and muscles. In lactic acid fermentation, pyruvic acid produced in glycolysis is directly reduced by NADH to form lactic acid. NoCO₂ is produced. The enzyme is lactate dehydrogenase which requires FMN and Zn²⁺.

In the test tube full of mercury (figure A) there is no air and, therefore, the introduced soaked seeds do not get air for their respiration. But they are capable of respiration in the absence of oxygen as is indicated by the evolution of carbon dioxide (figure B). Therefore, anaerobic respiration takes place in the seeds in the absence of free oxygen. The experiment also shows that CO₂ is evolved in anaerobic respiration of seeds.

Mercury is very reactive towards oxygen. It reacts with the oxygen in the air and also reacts with dilute acids containing oxygen. However, mercury is comparatively non-reactive in water. This is why it is used in anaerobic respiration experiments.

(i) Alcoholic fermentation is used is brewing industry for the productin of various types of beer, whisky and other wines.
(ii) Carbon dioxide of alcoholic fermentation is used in baking indutry for making the bread spongy.
(iii) Tea and tobacco leaves are cured (or removed of their bitterness) and provided with a fine falvour through fermentation Vinegar is obtained by the fermentation activity if acetic acid bacteria.

The given process is an example of alcoholic fermentation, thus the gas produced is CO₂.

• Fermentation can be described as an enzyme-catalyzed process in which one substrate is usually broken down by the enzymes produced by bacteria, yeast, and other microorganisms.
• Fermentation of glucose results in the formation of ethanol and carbon monoxide. This is an exothermic reaction and results in the release of energy. The changes observed in dough after keeping it overnight in warm weather is an example of fermentation.