Anaerobic respiration ( Fermentation ) And its Types

Описание: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen Because most of the ATP generated by cellular respiration is due to the work of oxidative phosphorylation, our estimate
of ATP yield from aerobic respiration is contingent on an adequate supply of oxygen to the cell.
Fermentation is a way of harvesting chemical energy without using either oxygen or any electron transport chain—in other words, without cellular respiration.
Glycolysis oxidizes glucose to two molecules of pyruvate. The oxidizing agent of glycolysis is NAD+, and neither oxygen nor any electron transfer chain is involved. Overall, glycolysis is exergonic, and some of the energy made available is used to produce 2 ATP (net) by substrate-level phosphorylation.
If oxygen is present, then additional ATP is made by oxidative phosphorylation when NADH passes electrons removed from glucose to the electron transport chain. But glycolysis generates 2 ATP whether oxygen is present or not—that is, whether conditions are aerobic or anaerobic.
As an alternative to respiratory oxidation of organic nutrients, fermentation is an extension of glycolysis that allows continuous generation of ATP by the substrate-level phosphorylation of glycolysis. For this to occur, there must be a sufficient supply of NAD+ to accept electrons during the oxidation step of glycolysis. Without some mechanism to recycle NAD+ from NADH, glycolysis would soon deplete
the cell’s pool of NAD+ by reducing it all to NADH and would shut itself down for lack of an oxidizing agent.
Under aerobic conditions, NAD+ is recycled from NADH by the transfer of electrons to the electron transport chain. An anaerobic alternative is to transfer electrons from NADH to pyruvate, the end product of glycolysis.
Types of Fermentation.
In alcohol fermentation, pyruvate is converted to ethanol (ethyl alcohol) in two steps. The first step releases carbon dioxide from the pyruvate, which is converted to the two-carbon compound acetaldehyde. In the second step, acetaldehyde is reduced by NADH to ethanol. This regenerates the supply of NAD+ needed for the continuation of glycolysis.
During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate as an end product, with no release of CO2. (Lactate is the ionized form of lactic acid.) Lactic acid fermentation by certain fungi and bacteria is used in the dairy industry to make cheese and yogurt.
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