Chapter 30: Fermentation Notes

In aerobic respiration, the final electron acceptor is an oxygen molecule $(O_2)$ . ATP production relies on high-energy electrons carried by NADH or FADH to the electron transport chain (ETC).
If aerobic respiration does not occur, NADH must be reoxidized to $NAD^+$ for reuse as an electron carrier for the glycolytic pathway to continue.
Fermentation: Processes that use an organic molecule as the final electron acceptor to regenerate $NAD^+$ from NADH.
Anaerobic cellular respiration: The electron transport chain (ETC) uses an inorganic molecule as a final electron acceptor.
Both processes (fermentation and anaerobic cellular respiration) allow organisms to convert energy for their use in the absence of oxygen.

What is the main purpose of fermentation in anaerobic respiration?
- C. To regenerate NAD from NADH

Certain prokaryotes, including some species in the domains Bacteria and Archaea, use anaerobic respiration.
Methanogens: A group of archaeans that reduces carbon dioxide to methane to oxidize NADH.
- Found in soil and in the digestive tracts of ruminants, such as cows and sheep.
Sulfate-reducing bacteria: Reduce sulfate to hydrogen sulfide to regenerate $NAD^+$ from NADH.

Lactic acid fermentation is used by animals and certain bacteria, such as those in yogurt.
Occurs routinely in mammalian red blood cells (which do not have mitochondria) and in skeletal muscle that has an insufficient oxygen supply to allow aerobic respiration to continue (i.e., muscles used to the point of fatigue).
In muscles, lactic acid accumulation must be removed by the blood circulation, and when the lactic acid loses a hydrogen, the resulting lactate is brought to the liver for further metabolism.
Chemical reactions of lactic acid fermentation:
$Pyruvic \text{ acid} + NADH \leftrightarrow \text{lactic acid} + NAD^+$
The enzyme used in this reaction is lactate dehydrogenase.
The reaction can proceed in either direction, but the reaction from left to right is inhibited by acidic conditions.
Lactic acid accumulation was once believed to cause muscle stiffness, fatigue, and soreness, although more recent research disputes this hypothesis.
Once the lactic acid has been removed from the muscle and circulated to the liver, it can be reconverted into pyruvic acid and further catabolized for energy.
Tremetol, a metabolic poison found in the white snakeroot plant, prevents the metabolism of lactate. When cows eat this plant, tremetol is concentrated in the milk they produce. Humans who consume the milk can become seriously ill.
- Symptoms include vomiting, abdominal pain, and tremors, become worse after exercise.

Which of the following is the end product of lactic acid fermentation?
- C. Lactic acid

Another familiar fermentation process is alcohol fermentation, which produces ethanol.
The first chemical reaction of alcohol fermentation ( $CO<em>2$ does not participate in the second reaction): $Pyruvic \text{ acid} + H^+ \rightarrow CO</em>2 + \text{acetaldehyde} + NADH + H^+ \rightarrow \text{ethanol} + NAD^+$
The first reaction is catalyzed by pyruvate decarboxylase, a cytoplasmic enzyme, with a coenzyme of thiamine pyrophosphate (TPP, derived from vitamin B and also called thiamine).
- A carboxyl group is removed from pyruvic acid, releasing carbon dioxide as a gas.
- The loss of carbon dioxide reduces the size of the molecule by one carbon, producing acetaldehyde.
The second reaction is catalyzed by alcohol dehydrogenase to oxidize NADH to $NAD^+$ and reduce acetaldehyde to ethanol.
The fermentation of pyruvic acid by yeast produces the ethanol found in alcoholic beverages.
Ethanol tolerance of yeast is variable, ranging from about 5% to 21%, depending on the yeast strain and environmental conditions.

What type of fermentation process is used by yeast to produce ethanol in alcoholic beverages?
- B. Alcohol fermentation

Other fermentation methods take place in bacteria.
Many prokaryotes are facultatively anaerobic, meaning they can switch between aerobic respiration and fermentation, depending on the availability of free oxygen.
Certain prokaryotes, such as Clostridia, are obligate anaerobes. Obligate anaerobes live and grow in the absence of molecular oxygen.
- Oxygen is a poison to these microorganisms and kills them on exposure.
All forms of fermentation, except lactic acid fermentation, produce gas.
The production of particular types of gas is used as an indicator of the fermentation of specific carbohydrates, which plays a role in the laboratory identification of bacteria.
Various methods of fermentation are used by assorted organisms to ensure an adequate supply of $NAD^+$ for the sixth step in glycolysis.
- Without these pathways, this step would not occur, and ATP could not be harvested from the breakdown of glucose.

Which of the following is NOT a purpose of fermentation?
- D. Generating ATP efficiently in the presence of oxygen

What is the main function of pyruvate decarboxylase in alcohol fermentation?
- B. To release carbon dioxide from pyruvic acid