Loading [MathJax]/extensions/TeX/boldsymbol.js
Skip to main content
Library homepage
 

Text Color

Text Size

 

Margin Size

 

Font Type

Enable Dyslexic Font
Chemistry LibreTexts

Fermentation

( \newcommand{\kernel}{\mathrm{null}\,}\)

Fermentation is the process by which living organisms recycle NADH \rightarrow NAD^+. NAD^+ is a required molecule necessary for the oxidation of Glyceraldehyde-3-phosphate to produce the high energy molecule 1,3-bisphosphoglycerate (Step 6 of Glycolysis). Fermentation occurs in the cytosol of cells.

Introduction

Because NAD^+ is used in Glycolysis it is important that living cells have a way of recycling NAD^+ from NADH. One way that a cell recycles NAD^+ is through the process of respiration, a set of sequential electron transfers involving an electron transport chain to a terminal electron acceptor. In aerobic organisms, the terminal electron acceptor is oxygen. In anaerobic organisms, the terminal electron acceptor can vary from species to species and include but are not limited to various metals like Fe(III), Mn(IV) and Co(III), CO2, nitrate, sulfur This process reduces NADH back to NAD^+ which can then be used again in step 6 of Glycolysis or other red/ox reactions in the cell. Another way that NAD^+ is recycled from NADH is by a process called fermentation.

Example: Lactic acid fermentation in contracting muscle

Lactic acid fermentation occurs by converting pyruvate into lactate using the enzyme Lactate dehydrogenase and producing NAD^+ in the process. This process takes place in oxygen depleted muscle and some bacteria. It is responsible for the sour taste of sauerkraut and yogurt. NAD^+ is required for the oxidation of glyceraldehyde-3-P to produce 1,3-Bisphosphoglycerate (Step 6 of Gycolysis). If the supply of NAD^+ is not replenished by the ETC or fermentation, glycolysis is unable to proceed. Fermentation is a necessary process for anaerobic organisms to produce energy. The yield of energy is much less than if the organism were to continue on through the TCA cycle and ETC, but energy is produce nonetheless.

ferm_02.jpg

Example: Alcoholic fermentation in yeast

The purpose of fermentation in yeast is the same as that in muscle and bacteria, to replenish the supply of NAD+ for glycolysis, but this process occurs in two steps:

  1. Alcoholic fermentation consists of pyruvate being first converted into acetaldehyde by the enzyme pyruvate decarboxylase and releasing CO_2.
  2. In the second step acetaldehyde is converted into ethanol using alcohol dehydrogenase and producing NAD^+ in the process. It is this recycled NAD^+ that can be used to continue on with glycolysis.

ferm_01.jpg

References

  • Garrett, H., Reginald and Charles Grisham. Biochemistry. Boston: Twayne Publishers, 2008.
  • Raven, Peter. Biology. Boston: Twayne Publishers, 2005.

Problems

  1. Draw the chemical structures of pyruvate, ethanol and lactate (the reactant and products of fermentation)
  2. Why is fermentation necessary? (Hint: see step 6 of Glycolysis)
  3. What type of environment is necessary for fermentation to occur?
  4. Where does fermentation occur? What part of the cell?
  5. Explain the alternative to fermentation and why it is able to proceed. (Hint: Final electron acceptor)

Contributors and Attributions


Fermentation is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

  • Was this article helpful?

Support Center

How can we help?