Alcoholic and Lactic Acid Fermentation — Revision Notes

⚡ 30-Second Revision

Glycolysis: — Common initial pathway, 1 Glucose $\rightarrow$ 2 Pyruvate + 2 ATP (net) + 2 $\text{NADH}$. Occurs in cytoplasm.
Purpose of Fermentation: — Regenerate $\text{NAD}^+$ from $\text{NADH}$ to allow glycolysis to continue.
Alcoholic Fermentation:

- Organisms: Yeasts, some bacteria. - Steps: Pyruvate $\xrightarrow{\text{Pyruvate decarboxylase}}$ Acetaldehyde + $\text{CO}_2$; Acetaldehyde + $\text{NADH} \xrightarrow{\text{Alcohol dehydrogenase}}$ Ethanol + $\text{NAD}^+$. - Products: Ethanol, $\text{CO}_2$. - ATP Yield: 2 net ATP (from glycolysis).

Lactic Acid Fermentation:

- Organisms: *Lactobacillus*, animal muscle cells. - Steps: Pyruvate + $\text{NADH} \xrightarrow{\text{Lactate dehydrogenase}}$ Lactate + $\text{NAD}^+$. - Products: Lactate (lactic acid). - ATP Yield: 2 net ATP (from glycolysis).

**No $\text{O}_2$ required for either.**
Partial oxidation of glucose.

2-Minute Revision

Fermentation is an anaerobic metabolic process crucial for regenerating $\text{NAD}^+$ from $\text{NADH}$ produced during glycolysis, allowing glycolysis to continue and yield a small amount of ATP (2 net ATP per glucose).

It occurs in the cytoplasm. There are two main types: alcoholic and lactic acid fermentation. Alcoholic fermentation, common in yeasts, converts pyruvate into acetaldehyde (releasing $\text{CO}_2$ via pyruvate decarboxylase) and then into ethanol (via alcohol dehydrogenase), regenerating $\text{NAD}^+$.

This process is vital for baking and brewing. Lactic acid fermentation, found in certain bacteria (like *Lactobacillus*) and animal muscle cells, directly converts pyruvate into lactate (via lactate dehydrogenase), also regenerating $\text{NAD}^+$.

No $\text{CO}_2$ is released in lactic acid fermentation. This type is responsible for the souring of milk and contributes to muscle fatigue. Both pathways represent partial oxidation of glucose and are significantly less efficient in ATP production compared to aerobic respiration.

5-Minute Revision

Fermentation is a vital anaerobic pathway that enables cells to produce a limited amount of ATP (2 net ATP per glucose, solely from glycolysis) when oxygen is unavailable. Its primary function is to reoxidize $\text{NADH}$ to $\text{NAD}^+$, ensuring that glycolysis, the initial breakdown of glucose, can continue.

Glycolysis occurs in the cytoplasm, producing two molecules of pyruvate, two net ATP, and two $\text{NADH}$ from one glucose molecule. Without $\text{NAD}^+$ regeneration, glycolysis would halt.

Alcoholic Fermentation:

This pathway is characteristic of yeasts (*Saccharomyces cerevisiae*) and some bacteria. It proceeds in two steps after glycolysis:

1
Decarboxylation: — Each pyruvate molecule (3 carbons) is converted into acetaldehyde (2 carbons) with the release of one molecule of carbon dioxide ($\text{CO}_2$). This reaction is catalyzed by pyruvate decarboxylase. For example, if you start with one glucose, you get two pyruvates, leading to two acetaldehydes and two $\text{CO}_2$ molecules.
2
Reduction: — Acetaldehyde is then reduced by $\text{NADH}$ to form ethanol (2 carbons). This step is catalyzed by alcohol dehydrogenase, and in the process, $\text{NADH}$ is oxidized back to $\text{NAD}^+$. The ethanol and $\text{CO}_2$ are the end products. This process is crucial in industries like baking ($\text{CO}_2$ makes dough rise) and brewing (ethanol is the alcohol).

Lactic Acid Fermentation:

This pathway is performed by certain bacteria (e.g., *Lactobacillus* species, used in dairy production) and by animal muscle cells during intense exercise when oxygen supply is insufficient. It is a single-step process after glycolysis:

1
Reduction: — Each pyruvate molecule (3 carbons) is directly reduced by $\text{NADH}$ to form lactate (3 carbons). This reaction is catalyzed by lactate dehydrogenase, which simultaneously reoxidizes $\text{NADH}$ to $\text{NAD}^+$. No $\text{CO}_2$ is released in this process. Lactate is the sole organic end product. In muscles, accumulated lactate can contribute to fatigue, but it can also be transported to the liver and converted back to glucose (Cori cycle). The sour taste of yogurt and cheese is due to lactic acid.

Key Differences to Remember:

Products: — Alcoholic produces ethanol and $\text{CO}_2$; Lactic acid produces lactate.
Enzymes: — Alcoholic uses pyruvate decarboxylase and alcohol dehydrogenase; Lactic acid uses lactate dehydrogenase.
$\text{CO}_2$ Release: — Yes in alcoholic; No in lactic acid.
ATP Yield: — Both yield 2 net ATP from glycolysis only. No additional ATP is generated in the fermentation steps.

Prelims Revision Notes

Alcoholic and Lactic Acid Fermentation: NEET Revision Notes

I. General Principles of Fermentation:

Definition: — Anaerobic process (occurs without oxygen) involving partial degradation of glucose.
Location: — Cytoplasm of the cell.
Primary Purpose: — Regeneration of $\text{NAD}^+$ from $\text{NADH}$ to sustain glycolysis.
ATP Yield: — Net 2 ATP per glucose molecule, exclusively from glycolysis (substrate-level phosphorylation). Fermentation steps themselves yield no ATP.
Efficiency: — Much less efficient than aerobic respiration (which yields $\approx$ 30-32 ATP).
Common Precursor: — Glycolysis, which converts 1 Glucose $\rightarrow$ 2 Pyruvate + 2 ATP (net) + 2 $\text{NADH}$.

II. Alcoholic Fermentation:

Organisms: — Yeasts (*Saccharomyces cerevisiae*), some bacteria.
Pathway (Post-Glycolysis):

1. Decarboxylation: Pyruvate (3C) $\xrightarrow{\text{Pyruvate decarboxylase}}$ Acetaldehyde (2C) + $\text{CO}_2$. 2. Reduction: Acetaldehyde (2C) + $\text{NADH} + \text{H}^+ \xrightarrow{\text{Alcohol dehydrogenase}}$ Ethanol (2C) + $\text{NAD}^+$.

Key Enzymes: — Pyruvate decarboxylase, Alcohol dehydrogenase.
End Products: — Ethanol and Carbon Dioxide ($\text{CO}_2$).
Significance: — Used in baking ($\text{CO}_2$ for rising), brewing (ethanol production), winemaking, biofuel production.

III. Lactic Acid Fermentation:

Organisms: — Certain bacteria (*Lactobacillus* species), animal muscle cells (during strenuous exercise), some fungi.
Pathway (Post-Glycolysis):

1. Reduction: Pyruvate (3C) + $\text{NADH} + \text{H}^+ \xrightarrow{\text{Lactate dehydrogenase}}$ Lactate (3C) + $\text{NAD}^+$.

Key Enzyme: — Lactate dehydrogenase.
End Product: — Lactate (lactic acid).
$\text{CO}_2$ Release: — None.
Significance: — Used in dairy industry (yogurt, cheese production), silage preservation. In muscles, temporary energy source, lactate transported to liver for Cori cycle (conversion back to glucose).

IV. Comparison (Key Differences):

Aspect	Alcoholic Fermentation	Lactic Acid Fermentation
End Products	Ethanol, $\text{CO}_2$	Lactate
$\text{CO}_2$ Release**	Yes	No
Intermediate	Acetaldehyde	None
Key Enzymes	Pyruvate decarboxylase, Alcohol dehydrogenase	Lactate dehydrogenase
Organisms	Yeast, some bacteria	*Lactobacillus*, muscle cells

V. Common Misconceptions to Avoid:

Fermentation produces a lot of ATP (Incorrect: only 2 net ATP from glycolysis).
Fermentation is the same as anaerobic respiration (Incorrect: anaerobic respiration uses ETC, fermentation does not).
Lactic acid is always a waste product (Incorrect: can be recycled via Cori cycle).

Vyyuha Quick Recall

To remember the products and key features:

Alcoholic Fermentation Emits Carbon Dioxide, Ethanol, and uses Acetyl-CoA (Acetaldehyde intermediate).

Lactic Acid Fermentation Lacks Carbon Dioxide, Leads to Lactate, and uses Lactate Dehydrogenase.