Fermentation

II. BACTERIAL GROWTH AND MICROBIAL METABOLISM

D. Cellular Respiration

3. Fermentation

Learning Objectives for this Section

Cellular respiration (def) is the process cells use to convert the energy in the chemical bonds of nutrients to ATP energy. Depending on the organism, cellular respiration can be aerobic, anaerobic, or both. Aerobic (def) respiration is an exergonic pathway that requires molecular oxygen (O₂). Anaerobic (def) exergonic pathways do not require oxygen and include anaerobic respiration and fermentation. We will now look at these three pathways.

Fermentation

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 Fig. 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 Fig. 2)

GIF animation illustrating glycolysis

McGraw-Hill Flash animation illustrating glycolysis

The 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

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