🏃 Anaerobic Respiration

In the absence of sufficient oxygen, the body resorts to anaerobic respiration as a survival mechanism. This process provides just enough ATP to sustain basic functions like muscle contraction and active transport. Anaerobic respiration breaks down complex molecules into simpler ones without oxygen or an electron transfer chain. There are two primary types: lactic fermentation in animals and alcoholic fermentation in plants. Fermentation is just another name for anaerobic respiration.

🥛 Lactate Fermentation

Lactate fermentation primarily occurs in animals. Lactate is essentially lactic acid, produced during intense exercise when oxygen supply is limited.

The process begins with glycolysis in the cytoplasm, which occurs regardless of the type of respiration. In low-oxygen conditions, pyruvate, the end product of glycolysis, is converted to lactate. This conversion utilizes reduced NADH to produce NAD++.

This process requires the enzyme lactate dehydrogenase.

Lactic acid accumulation can cause muscle fatigue by denaturing enzymes and proteins. Oxygen debt refers to the amount of oxygen needed to break down the accumulated lactic acid.

Lactate enters the bloodstream and is transported to the liver, where it reacts with oxygen to regenerate glucose. This glucose can then undergo glycolysis again to produce pyruvate. If sufficient oxygen is available, the pyruvate can proceed through normal aerobic respiration.

The regeneration of glucose from lactate, known as the Cori cycle, consumes about six ATP molecules. Given that glycolysis yields a net gain of two ATP, the Cori cycle results in a net loss of four ATP molecules, making it an unsustainable long-term solution.

🍺 Alcoholic Fermentation

Alcoholic fermentation takes place in plants and yeast, starting with pyruvate. This process produces ethanol (alcohol).

First, carbon dioxide is removed from pyruvate to form ethanal, a decarboxylation reaction catalyzed by the enzyme pyruvate decarboxylase. Then, ethanal gains a hydrogen atom from reduced NADH to produce ethanol. The enzyme involved in the second step is called ethanol dehydrogenase.

Unlike lactic fermentation, alcoholic fermentation is irreversible; once ethanol is formed, it cannot be converted back.

Alcoholic fermentation has practical applications for humans, notably in the production of food and beverages. For example, yeast and plants are used to make beer and wine. When brewing beer, the carbon dioxide produced during fermentation is retained, giving beer its fizz. In wine production, the carbon dioxide is allowed to escape, resulting in a flat beverage.

Yeast is also used in bread making, where the carbon dioxide produced causes the dough to rise. The ethanol produced during this process evaporates during baking.

High concentrations of ethanol are toxic to yeast, typically around 15%, which effectively kills the yeast and eliminates the need for extraction.