FERMENTATION

Learning Objectives for this Section

Fermentation (def) is an anaerobic (def) breakdown of carbohydrates in which an organic molecule (def) is the final electron acceptor. It does not involve an electron transport system. Furthermore,:

• it is a partial breakdown of glucose giving a little energy, 2 net ATP's per glucose by way of substrate-level phosphorylation (def);
• it involves only glycolysis; and
• is found in anaerobic (def) and facultative anaerobic (def) bacteria.

  A. Glycolysis (def) during Fermentation (def)

  Function: As during aerobic respiration, glycolysis is a partial breakdown of a six-carbon glucose molecule into two, three-carbon molecules of pyruvate, 2NADH +2H⁺, and 2 net ATP as a result of substrate-level phosphorylation (def), as shown in (see Fig. 1 and 2). Glycolysis occurs in the cytoplasm of the cell. As mentioned above, the overall reaction is:

  glucose (6C) + 2 NAD⁺ +2 ADP +2 inorganic phosphates (P_i)

  yields 2 pyruvate (3C) + 2 NADH + 2 H⁺ + 2 net ATP

  Glycolysis also produces a number of key precursor metabolites (def), as shown in Fig. 3.

  Since there is no electron transport system, the protons and electrons donated by certain intermediate precursor molecules during glycolysis generate no additional molecules of ATP. Instead, they combine with the coenzyme (def) NAD⁺, the organic molecule which serves as the final electron and proton acceptor, reducing it to NADH + H⁺ (see Fig. 1 and 2).

  Animation illustrating glycolysis

  McGraw-Hill Flash animation illustrating glycolysis

  he 2 pyruvic acids are then converted into one of many different fermentation end products in several non-energy-producing steps.

B. Fermentation end products

Some fermentation end products produced by microorganisms are very beneficial to humans and are the basis of a number of industries (brewing industry, dairy industry, etc.). Examples of fermentation end products include:

• Saccharomyces: ethyl alcohol and CO₂
• Streptococcus and Lactobacillus: lactic acid
• Propionibacterium: proprionic acid, acetic acid, and CO₂
• Escherichia coli: acetic acid, lactic acid, succinic acid, ethyl alcohol, CO₂, and H₂
• Enterobacter: formic acid, ethyl alcohol, 2,3-butanediol, lactic acid, CO₂, and H₂
• Clostridium: butyric acid, butyl alcohol, acetone, isopropyl alcohol, CO₂, and H₂
• QUIZ YOURSELF ON THIS SECTION
• RETURN TO CELLULAR RESPIRATION TABLE OF CONTENTS
• GO TO PRECURSOR METABOLITES

Copyright © Gary E. Kaiser
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Updated: August, 2009