Anaerobic Fermentation in Animals and Yeasts

Overview of Anaerobic Fermentation

• Definition: A metabolic pathway that occurs in the absence of oxygen.
• Purpose: To regenerate NAD+ so glycolysis can continue, allowing for a small amount of ATP production.
• ATP Yield: Net gain of 2 ATP molecules per glucose molecule (from glycolysis).
• Location: Cytoplasm

KEY TAKEAWAY: Anaerobic fermentation is an oxygen-independent pathway that allows cells to produce a small amount of ATP when oxygen is limited, by regenerating NAD+ for glycolysis.

Anaerobic Fermentation Inputs and Outputs

• Input: Pyruvate (product of glycolysis)
• Output: Varies depending on the organism (animals or yeasts)

Anaerobic Fermentation in Animals (e.g., Human Muscle Cells)

• Location: Cytoplasm of muscle cells (and other cells under anaerobic conditions).
• Inputs:
  • Pyruvate
  • NADH
• Outputs:
  • Lactate (lactic acid)
  • NAD+

• Process: Pyruvate is reduced by NADH to form lactate, regenerating NAD+ which is essential to continue glycolysis.

  Pyruvate + NADH $\rightarrow$ Lactate + NAD+

• When it occurs: During strenuous exercise when oxygen supply to muscles is insufficient for aerobic respiration.

COMMON MISTAKE: Confusing lactic acid fermentation with aerobic respiration. Lactic acid fermentation does NOT produce CO2 or H2O.

Anaerobic Fermentation in Yeasts

• Location: Cytoplasm of yeast cells.
• Inputs:
  • Pyruvate
  • NADH
• Outputs:
  • Ethanol (alcohol)
  • Carbon dioxide (CO2)
  • NAD+

• Process:

  1. Pyruvate is decarboxylated to acetaldehyde, releasing CO2.
  2. Acetaldehyde is reduced by NADH to ethanol, regenerating NAD+.

  Pyruvate $\rightarrow$ Acetaldehyde + CO2

  Acetaldehyde + NADH $\rightarrow$ Ethanol + NAD+

• Application: Used in brewing (ethanol production) and baking (CO2 causes bread to rise).

APPLICATION: The fermentation process carried out by yeast is crucial in the production of alcoholic beverages like beer and wine, as well as in the baking industry where carbon dioxide production leads to the rising of dough.

Comparison of Anaerobic Fermentation in Animals and Yeasts

EXAM TIP: Be able to compare and contrast anaerobic fermentation in animals and yeasts. Pay attention to the specific inputs and outputs.

Why Different Outputs?

• Different organisms have different enzymes that catalyze different reactions.
• The specific enzymes available in animal cells vs. yeast cells determine the final products of fermentation.

VCAA FOCUS: VCAA frequently asks questions comparing the inputs, outputs, and purposes of anaerobic fermentation in different organisms.

Facultative Anaerobes

• Definition: Organisms that can switch between aerobic respiration and anaerobic fermentation depending on the availability of oxygen.
• Examples: Escherichia coli (E. coli), Salmonella enterica, some yeast strains.
• Advantage: Allows survival in environments with fluctuating oxygen levels.

STUDY HINT: Create a table summarizing the key steps, inputs, outputs, and locations of glycolysis, aerobic respiration, and anaerobic fermentation to help you compare and contrast these processes.