Fermentation and Anaerobic Respiration.

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Fermentation: If oxygen is absent, many cells are still able to use glycolysis to produce ATP. Two ways this can be done are through fermentation and anaerobic respiration. Fermentation is the process by which the electrons and hydrogen ions from the NADH produced by glycolysis are donated to another organic molecule.

The point of fermentation

The reason this is done is to produce NAD+ which in tern is needed to keep glycolysis going. Remember, that unless the cell has some sort of electron transport system, the NADH is not usable. At the same time NAD+ is needed for glycolysis and its much less expensive in terms of energy for the cell to simply take the NADH that would normally go to the mitochondrion and use it to regenerate the NAD+. This is shown in the figure for ethanol fermentation in yeast.

Notice that the NADH produced by glycolysis donates it's hydrogen ions and electrons that in aerobic respiration would have ended up powering electron transport phosphorylation.

Other fermentation pathways

There are a number of fermentation pathways that different cells use. Yeast cells produce ethyl alcohol by fermentation. Certain cells of our body, namely muscle cells, use lactic acid fermentation, while depending on the organism some of the other products of fermentation include acetic acid, formic acid, acetone and isopropyl alcohol.

Fermentation and running

In our bodies certain muscle cells, called fast twitch muscles, have less capability for storing and using oxygen than other muscles. When you run and these muscles run short of oxygen, the fast twitch muscles begin using lactic acid fermentation. This allows the muscle to continue to function by producing ATP by glycolysis.

 

The muscles get enough ATP for quick spurts or shall we say sprints, but quickly become fatigued as their stores of glycogen are used up. Eventually you cramp. This is in part because the muscles lack sufficient ATP to continue contracting. Also, lactic acid builds up and must be metabolized by the liver. Runners who sprint actually have more muscle cells specialized for lactic acid fermentation than do long distance runners.

White meat and dark meat

If you want to see what these muscles are like, when you eat chicken or turkey the white meat is fast twitch muscle. The dark meat is what is called slow twitch muscle. This meat is dark because it contains an oxygen holding protein called myoglobin. Note that the slow twitch muscles tend to be wing and leg muscles where long term endurance is required. The fast twitch muscles tend to be more common in the breast where quick response but not necessarily endurance is needed. Also, wild animals tend to have more slow twitch muscle than their domestic counterparts.

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pgd revised 02/20/00