Fermentation (anaerobic), aerobic respiration

The term anaerobic respiration is replaced by fermentation in microorganisms by Cruickshank in 1897.

Fermentation is defined as the anaerobic breakdown of carbohydrates and other organic compounds to form alcohol organic acids with the presence of microorganisms or their enzymes.

The two types of fermentation are alcoholic fermentation and lactic acid fermentation.

Pyruvate undergoes decarboxylation to produce acetaldehyde and carbon-di-oxide with the help of pyruvate decarboxylase, Mg²⁺, TPP [Thiamine Pyrophosphate] as cofactors.

Acetaldehyde then accepts hydrogen from NADH for the production of Ethanol.
Pyruvate \xrightarrow[Mg^{2+},TPP]{{Pyr.decarboxylase}}Acetaldehyde+CO_{2}\uparrow
Acetaldehyde+NADH\rightarrow NAD^{+}+C_{2}H_{5}OH

Accumulation of alcohol may also kill the micro organisms.

Alcoholic fermentation is involved in the brewing industry to produce beverages done by Saccharomyces Cerevisiae.

In lactic acid fermentation, Pyruvic acid function as H-acceptor and gets 2 hydrogen atoms from NADH₂ and reacts with lactate dehydrogenase to form lactic acid, where no CO₂ is evolved.
Pyruvate+NHDH_{2} \xrightarrow[FMN/Zn^{2+}]{{Lactate \ dehydrogenase}}Lactic \ acid+NAD^{+}

The family Enterobacteriaceae performs mixed acid fermentation, where it forms lactic acid, Ethanol, formic acid and succinic acid.

Aerobic Respiration:

Location: mitochondrial matrix

Under aerobic condition, glucose metabolism takes place in 2 stages known as Kreb’s cycle and Terminal oxidation, after glycolysis.

Pyruvate must be oxidised to enter Kreb’s cycle.

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The above reaction is called link reaction (or) gateway step as the acetyl CoA acts as the connecting link between glycolysis and Kreb’s cycle.

The complex Pyruvate dehydrogenase is made up of Pyruvate decarboxylase, TPP, FAD, Mg²⁺, Lipoic acid.