Fermentation Primer: Lesson IV

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                 Fermentation Primer: Lesson IV

In last month’s lesson we discussed how yeast cells process pyruvate under aerobic conditions. This month’s issue explores the anaerobic processing.

Q: So, what happens to pyruvate under anaerobic conditions within the yeast?

In the absence of oxygen, pyruvate can follow two different routes, depending on the type of cell that is metabolizing it:

It can be converted into ethanol and carbon dioxide through the Alcoholic Fermentation Pathway, or

It can be converted into lactate through the Lactic Acid Fermentation Pathway.

Let’s start with the Alcoholic Fermentation Pathway:

In alcohol fermentation, NADH donates its electrons to a derivative of pyruvate, producing ethanol.

Going from pyruvate to ethanol is a two-step process:

In the first step, a carboxyl group is removed from pyruvate and released in as carbon dioxide, producing a two-carbon molecule called acetaldehyde.

In the second step, NADH passes its electrons to acetaldehyde, regenerating NAD+ and forming ethanol.

This process is irreversible as carbon dioxide diffuses away.

Let’s look now at the Lactic Acid Fermentation Pathway:

In lactic acid fermentation, NADH transfers its electrons directly to pyruvate, generating lactate as a byproduct. lactate, which is just the deprotonated form of lactic acid, gives the process its name. The bacteria that make yogurt carry out lactic acid fermentation, as do the red blood cells in your body, which don’t have mitochondria and thus can’t perform cellular respiration.

This process is reversible. when oxygen is available to the cell again the lactate can be converted back to pyruvate.

Interesting facts about Alcoholic and Lactic Acid Fermentations

Alcoholic fermentation is the process that causes bread dough to rise. When yeast cells in the dough run out of oxygen, the dough begins to ferment, giving off tiny bubbles of carbon dioxide. These bubbles are the air spaces you see in a slice of bread. The small amount of ethyl alcohol that is produced in the dough evaporates when the bread is baked.

Humans undergo lactic acid fermentation when the body needs a lot of energy in a hurry. When you are sprinting full speed, for example, your cells will only have enough ATP stored in them to last a few seconds. Once the stored ATP is used, your muscles will start producing ATP through lactic acid fermentation. Fermentation makes it possible for cells to continue generating ATP through glycolysis.

Join us again next month, as we continue our deep-dive into this fascinating world!

Source: Creative Commons (creativecommons.org) and Khan Academy (khanacademy.org)