Chapter 7 Section 3: Fermentation

Overview and Biological Role of Fermentation

• Complete biological breakdown of glucose under optimal environmental conditions requires molecular oxygen.

• Under anaerobic conditions—where cells in the environment or inside the human body lack access to oxygen—cells utilize an alternate metabolic pathway to extract energy from glucose without accumulating intolerable levels of toxic waste products.

• Fermentation is defined as the anaerobic breakdown of glucose.

• The total yield of adenosine triphosphate ($\text{ATP}$) from glucose breakdown during anaerobic conditions is $2\,\text{ATP}$.

• All $2\,\text{ATP}$ molecules are produced exclusively during the pathway of glycolysis.

• Fermentation itself does not generate any $\text{ATP}$; its net yield of $\text{ATP}$ is exactly $0\,\text{ATP}$.

Mechanisms of NAD+ Regeneration

• Glycolysis reduces nicotinamide adenine dinucleotide ($\text{NAD}^+$) into $\text{NADH}$ by harvesting high-energy electrons from glucose molecules.

• To continue extracting electrons from subsequent glucose molecules, the cell must continuously regenerate its supply of oxidized $\text{NAD}^+$.

• During fermentation, $\text{NADH}$ transfers its acquired electrons and proton ($\text{H}^+$) onto pyruvic acid (pyruvate).

• Once $\text{NADH}$ unloads its electrons and proton, the restored $\text{NAD}^+$ returns to the enzymes involved in glycolysis to collect additional electrons from new glucose molecules, allowing glycolysis and its associated $2\,\text{ATP}$ yield to continue.

Fermentation Pathways in Yeast and Human Muscle

• The specific end products generated from pyruvic acid depend entirely on the organism involved and the specific metabolic enzymes it possesses.

• Ethanol Fermentation in Yeast (Saccharomyces cerevisiae):

  • In the absence of oxygen, Saccharomyces cerevisiae utilizes pyruvic acid to oxidize $\text{NADH}$ back to $\text{NAD}^+$.

  • Each molecule of pyruvic acid ($3$ carbons) is converted into a $2$-carbon alcohol (ethanol).

  • The remaining third carbon from each pyruvic acid is released as a molecule of carbon dioxide ($\text{CO}_2$).

  • Because one glucose molecule yields $2$ pyruvic acid molecules, the net products per glucose are $2$ ethanol molecules and $2\,\text{CO}_2$ molecules.

  • This specific yeast pathway is responsible for the commercial production of beer and wine.

• Lactic Acid Fermentation in Human Muscle Cells:

  • During intense exercise (such as running long distances), muscle cells can reach their maximal oxygen intake capacity, resulting in localized oxygen deprivation.

  • Under these anaerobic conditions, muscle cells utilize electrons and protons ($\text{H}^+$) from $\text{NADH}$ to reduce pyruvic acid directly into lactate (lactic acid).

  • Each pyruvic acid molecule is converted into one lactate molecule, yielding a total of $2$ lactate molecules per glucose molecule.

  • Lactate is transported away from muscle tissues via the bloodstream.

  • In the liver, lactate can be processed and synthesized back into glucose.

  • In other muscle cells, lactate is processed or released based on the overall metabolic demands of the body.

Physiological Impacts of Lactic Acid Accumulation

• Overexertion during intense physical training (such as initiating new workout regimens in early January) causes an accumulation of lactic acid within muscle tissue.

• The localized burning sensation experienced during high-intensity exercise is directly caused by this accumulation of lactic acid.

• It typically requires a duration of $1$ to $2$ days for the bloodstream to completely carry away and clear the accumulated lactic acid following significant exertion.

Microbial Fermentation and Practical Applications

• Microorganisms across the domains Bacteria and Archaea similarly perform anaerobic breakdown by converting glucose into pyruvic acid.

• The specific fermentation pathway chosen by a bacterium or archaeon in the absence of oxygen is determined by its genetic profile—specifically, which genes and corresponding enzymes it possesses.

• Bacterial Fermentation Diversity:

  • Certain bacteria reduce pyruvic acid into lactic acid.

  • Other bacteria convert pyruvic acid into alcohol and carbon dioxide ($\text{CO}_2$).

• Applications of Saccharomyces cerevisiae:

  • Used in winemaking and brewing due to its ability to convert sugars into ethanol alongside substantial amounts of $\text{CO}_2$.

  • Used as baker's yeast in bread production, where fermentation within raw dough generates microscopic bubbles of $\text{CO}_2$ gas that cause the dough to expand and rise prior to baking.

Comparison and Next Steps in Aerobic Cellular Respiration

• Summary of Glycolysis vs. Fermentation Inputs/Outputs:

  • Glycolysis takes in glucose along with initial investments of $\text{ATP}$ and produces $\text{NADH}$ and a net yield of $2\,\text{ATP}$.

  • Fermentation functions strictly to recycle $\text{NADH}$ back to $\text{NAD}^+$ and produces zero additional $\text{ATP}$.

• Aerobic Respiration in Eukaryotes:

  • When oxygen is available, eukaryotic cells process glucose through mitochondrial metabolic pathways instead of ending in fermentation.

  • Steps of Mitochondrial Respiration:

    1. An intermediate (preparatory) step that processes pyruvate.

    2. The citric acid cycle (also referred to as the cyclic acid cycle).

    3. The electron transport chain.

  • All three steps of aerobic respiration take place inside the mitochondrion.