Question
In the following flow chart, replace the symbols a,b,c and d with appropriate terms. Briefly explain the process and give any two application of it.
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Answer
a = Glyceraldehyde 3-phosphate b = Phosphoenol pyruvic acid c = Ethanol d = Lactic acid The given metabolic pathway in figure is anaerobic respiration or fermentation. Anaerobic respiration is an enzyme mediated energy liberating catabolic process of step-wise but incomplete breakdown of organic substrate without using oxygen as an oxidant. Energy is liberated during breaking of bonds between various types of atoms. The common products of anaerobic respiration are C02, ethyl alcohol and lactic acid. $\text{C}_6\text{H}_{12}\text{O}_6\xrightarrow{\text{Enzymes}}2\text{CO}_2+2\text{C}_2\text{H}_5\text{OH}+59\text{Kcal}\\\text{Ethyl alcohol}$ $\text{C}_6\text{H}_{12}\text{O}_6\xrightarrow{\text{Enzymes}}2\text{C}_3\text{H}_6\text{O}_3+36\text{Kcal}\\\text{Lactic acid}$
- Alcoholic fermentation: It takes place in two steps:
$\text{pyruvate}\xrightarrow[\text{decarboxylase},\text{Tpp},\text{Mg}^{2+}]{\text{pyruvate}}\\\text{Acetaldehyde}+\text{CO}_2(\uparrow)$
It produces alcohol from pyruvic acid and releases carbon dioxide.
- Lactic acid fermentation: It takes place in a single step:
Pyruvic acid + NADH
$\text{pyruvic acid}+\text{NADH}\\\xrightarrow[\text{FMN,Zn}^{2+}]{\text{Lactic acid}+\text{NAD}^+}$
It produces lactic acid from pyruvate and does not release carbon dioxide. Application of anaerobic respiration are as follows:- It has important role in brewing (Saccharomyces cerevisiae), baking, vinegar and milk industries (Streptococcus lactis). Vinegar is produced by fermentation of sugars in the presence of acetic acid bacteria (Acetobacter aceti).
- Production of industrial alcohols and organic acids like citric acid and malic acid.
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