Explain the ways in which glucose is broken down in absence of oxygen.

In the absence of oxygen, when aerobic respiration is not possible, glucose can be broken down through a process called anaerobic respiration. There are two main types of anaerobic respiration: lactic acid fermentation and alcoholic fermentation. Both processes serve to regenerate NAD+ (nicotinamide adenine dinucleotide), which is necessary for glycolysis to continue. Here’s an explanation of both processes:

Lactic Acid Fermentation:

Process: In lactic acid fermentation, glucose is partially broken down through glycolysis to produce two molecules of pyruvate. Since there is no oxygen available to complete the process through the Krebs cycle and electron transport chain, the cell resorts to lactic acid fermentation.

Conversion: In lactic acid fermentation, pyruvate is converted into lactic acid by accepting electrons from NADH. This regeneration of NAD+ allows glycolysis to continue, albeit at a reduced efficiency.
Example: This process is commonly observed in muscle cells during strenuous exercise when oxygen is temporarily depleted. The accumulation of lactic acid can lead to muscle fatigue and soreness.

Alcoholic Fermentation:

Process: In alcoholic fermentation, yeast and some bacteria can metabolize glucose in the absence of oxygen. Like lactic acid fermentation, the process begins with glycolysis, resulting in the formation of two molecules of pyruvate.

Conversion: The pyruvate is then converted into ethanol (alcohol) and carbon dioxide. This conversion also involves the regeneration of NAD+ to sustain glycolysis.
Example: Alcoholic fermentation is used in the production of alcoholic beverages, such as beer and wine, where yeast ferments sugars to produce ethanol and carbon dioxide.

Both lactic acid fermentation and alcoholic fermentation are less efficient in terms of ATP (adenosine triphosphate) production compared to aerobic respiration. However, they are crucial pathways for cells to generate energy when oxygen is limited or unavailable.