Fermentation

FERMENTATION

Some organisms undergo the process of fermentation in order to generate ATP in the absence of oxygen, using other substrates to generate this type of energy. In fact, the scientific definition of fermentation is a metabolic or enzymatic process that creates energy out of an organic molecule in the absence of oxygen or an electron transport system. In such cases, the final electron receptor is an organic molecule rather than oxygen.

Fermentation is believed to be the oldest metabolic pathway—something that prokaryotes and eukaryotes have in common. From an evolutionary standpoint, this was the pathway that developed before there was sufficient oxygen on earth. What it means is that, even though higher order animals get the vast majority of their ATP through aerobic mechanisms, the capacity to undergo fermentation still exists as part of an older evolutionary pathway.

In fact, mammals undergo fermentation in muscle cells when intense exercise is done and there is insufficient oxygen. The end result is the creation of lactic acid, a common metabolite in muscle during exercise. Invertebrates have fermentation capabilities, producing things like alanine and succinate. Bacteria undergo fermentation in low oxygen environments, creating acetate and formate. These bacteria interact with methanogens, which are Archaea species; these convert the acetate to methane gas. Figure 23 describes lactate fermentation in mammals:

Glycolysis is also an evolutionarily old pathway; its end product, pyruvate, gets bypassed out of going into the Krebs cycle, going on to fermentation instead. The main end products of fermentation include lactic acid, ethanol, carbon dioxide, and hydrogen gas, although a few organisms can produce acetone and butyric acid. They are not fully oxidized and become waste products as they cannot be metabolized any further.

Fermentation generally takes place in an anaerobic environment (which means without oxygen). Because more ATP is made with oxidative phosphorylation, this is the preferred method by most species. Certain yeast species, like Saccharomyces cerevisiaex , actually prefer fermentation when sugar is abundant. Other species do not tolerate oxygen at all and are considered obligate anaerobes.

Some yeast products make ethanol in the fermentation process. Glucose goes to 2 pyruvate molecules in glycolysis and then gets diverted into acetaldehyde and then

ethanol. Carbon dioxide is given off in the process, which is how dough rises when yeast is added. This is, of course, the same process that happens when sugar and yeast mixes together to make alcoholic beverages. Ethanol can be generated in large quantities using corn, sugar beets, and sugarcane—ultimately added to gasoline and used for fuel. Figure 24 illustrates ethanol fermentation:

The ethanol pathway takes glucose and makes 2 pyruvate molecules as in glycolysis. This is an exothermic reaction that gives off heat. It binds phosphate to ADP to make ATP and converts NAD into NADH. There is then a reaction that causes the pyruvate to break into acetaldehyde and carbon dioxide. The hydrogen ion from NADH and the energy of the previous reactions goes on to reduce acetaldehyde into ethanol, regenerating NAD again so that it can be reused. The two enzymes necessary for these last two reactions are pyruvate decarboxylase and alcohol dehydrogenase.