Question

Explain the process of fermentation that takes place in eukaryotic cells, such as muscle cells.

Why are the products of fermentation necessary for anaerobic respiration to continue?

Answer 1

The process of energy generation from glucose involves three key pathways: Glycolysis, TCA cycle and the Electron Transport chain where Oxygen serves as the final electron acceptor. Although glycolysis alone can produce 2 moles of ATP per round ( a process called substrate level phosphorylation), a substantial amount of ATP is produced ( 38/36 ATPs) when metabolites of glucose run through all of these pathways. But in events such as a strenous physical activity, the supply of  O2 from circulation is not fast enough to meet the demands of the muscle cells and therefore muscle cells go in an anerobic state. Due to unavailability of O2 , the proton gradient which drives the ATP synthesis machinery does not form. So, to continue producing ATP the cells depend heavily now on substrate level phosphorylation in glycolysis which occurs in the phosphoglycerokinase mediated synthesis of 3-phosphoglycerate from 1,3-bisphosphoglycerate.

But the problem is that substrate level phosphorylation depends on NAD+ supply as NAD+ is continually reduced to NADH in glycolysis. In oxygenated environment, NADH would have donated its electron to the Electron Transport Chain and reoxidized to NAD+. The Fermentation process provides a way out. In O2- starved eukaryotic cells, the glycolysis end product is reduced to lactate, regenerating NAD+ in the process. The reaction is carried out by Lactate dehydrogenase.

The overall reaction stands: 2Pyruvate + 2NADH +2H+   2Lactate + 2NAD+

So, now the anerobic process of ATP production can continue with NAD+ produced from fermentation of pyruvate. Not only in skeletal muscle cells, Fermentation also takes place in the erythrocytes ( they lack mitochondria, so no electron transport system) ,submerged parts of plants, and in solid tumors. The lactic acid produced in muscles is transported to liver where it is converted back to glucose, a process called Cori cycle. Also, the lactate accumulation in muscle and blood are responsible for the sense of fatigue.

So, in brief, the ferementation product NAD+ acts as the final electron acceptor in the anerobic glycolyis process so that glycolysis continues to produce ATP required by the cell.