Fermentation Notes

Conditions and Definition

• Fermentation is an anaerobic pathway: it operates when no O$_2$ is available to serve as a terminal electron acceptor.
• It allows organisms to keep extracting (limited) energy from glucose ($\text{C}<em>6\text{H}</em>{12}\text{O}_6$) after glycolysis, even though the normal electron-transport chain is inoperative.
• Central purposes of fermentation:
  • Regenerate the oxidized cofactor $\text{NAD}^+$ from $\text{NADH}$ so glycolysis can continue.
  • Remove/convert pyruvate ($2\;\text{C}<em>3\text{H}</em>4\text{O}_3$) that would otherwise accumulate and stall metabolism.
• No additional ATP is produced directly by fermentation; all ATP comes from glycolysis in this context.

Glycolysis: First Phase Shared with Cellular Respiration

• Occurs in cytoplasm; does not require oxygen.
• Overall glycolytic equation (one glucose):
  • Input: $1\;\text{Glucose} + 2\;\text{ATP} + 2\;\text{NAD}^+$
  • Output: $2\;\text{Pyruvate} + 4\;\text{ATP} + 2\;\text{NADH} + 2\;\text{H}^+$
• ATP accounting:
  • $4$ ATP generated – $2$ ATP invested = net $2$ ATP.
• After glycolysis in the absence of O$_2$:
  • $\text{NADH}$ holds high-energy electrons that have nowhere to go.
  • Pyruvate remains unreduced to CO$_2$ via the Krebs cycle, so it must be disposed of another way.

The Fermentation Step: General Mechanism

• Fermentation takes the 2 pyruvate + 2 NADH yielded from glycolysis and converts them into:
  • 2 $\text{NAD}^+$ (recycled back to glycolysis)
  • End-products that leave the cell or accumulate harmlessly (lactate or ethanol + CO$_2$).
• No extra ATP beyond the glycolytic net of $2\;\text{ATP}$ is harvested.

Two Major Types of Fermentation

1. Lactic Acid Fermentation

• Carried out by certain bacteria (e.g.
  Lactobacillus) and by animal muscle cells under O$_2$ debt.
• Reaction per glucose (after glycolysis):
  $2\;\text{Pyruvate} + 2\;\text{NADH} \;\longrightarrow\; 2\;\text{Lactic Acid} (\text{C}<em>3\text{H}</em>6\text{O}_3) + 2\;\text{NAD}^+$
• Consequences:
  • Lactic acid accumulation in muscles → soreness/burning sensation during intense exercise.
  • Used industrially to acidify and preserve dairy (yogurt, cheese).

2. Ethanol (Alcohol) Fermentation

• Typical of yeast (a facultative anaerobe) and some plant tissues.
• Two-step decarboxylation + reduction sequence:
  1. $2\;\text{Pyruvate} \;\xrightarrow{\text{decarboxylase}}\; 2\;\text{Acetaldehyde} + 2\;\text{CO}_2$
  2. $2\;\text{Acetaldehyde} + 2\;\text{NADH} \;\longrightarrow\; 2\;\text{Ethanol} (\text{C}<em>2\text{H}</em>6\text{O}) + 2\;\text{NAD}^+$
• Industrial & culinary relevance:
  • Alcoholic beverages (wine, beer, spirits) derive ethanol from this pathway.
  • Bread rising: CO$_2$ bubbles from fermenting yeast expand the dough; ethanol evaporates during baking.

Energy Yield Summary (Glycolysis + Fermentation)

• Input (per glucose): $1\;\text{Glucose} + 2\;\text{ATP (investment)}$
• Output: $4\;\text{ATP (gross)} \Rightarrow 2\;\text{ATP (net gain)}$
• Additional outputs depend on pathway:
  • Lactic acid fermentation → 2 lactate molecules.
  • Ethanol fermentation → 2 ethanol + 2 CO$_2$ molecules.

Comparison with Aerobic Cellular Respiration

• Aerobic respiration: $\approx 36–38\;\text{ATP}$ / glucose.
• Fermentation (with glycolysis): 2 ATP / glucose.
• Ratio: fermentation captures ~1⁄18 the ATP of full aerobic respiration.

Thermodynamic Efficiency & Entropy

• Cellular respiration efficiency:
  • $\approx 55\%$ of glucose’s energy conserved in ATP; $\approx 45\%$ lost as heat.
• Fermentation efficiency:
  • $\approx 3\%$ energy captured; $\approx 97\%$ lost as heat.
• Both pathways increase entropy; higher efficiency corresponds to smaller entropy increase and more low-entropy (usable) energy retained.
• No biochemical reaction is 100 % efficient due to unavoidable heat loss and entropy production.

Representative Examples & Practical Implications

• Obligate anaerobes (e.g.
  certain lactic-acid bacteria) rely solely on fermentation for ATP.
• Facultative anaerobes (yeast): switch between aerobic respiration and fermentation depending on O$_2$ availability.
• Food & beverage industry:
  • Cheese, yogurt: flavor & texture from lactic acid fermentation by Lactobacillus.
  • Wine/beer/distilled spirits: ethanol fermentation by yeast accumulates alcohol.
  • Baking: CO$_2$ bubbles from yeast fermentation create dough rise.
• Human physiology:
  • During strenuous exercise, skeletal muscle temporarily shifts to lactic acid fermentation, leading to post-exercise muscle soreness until O$_2$ repays and lactate is cleared.

Key Equations at a Glance

• Glycolysis (net):
  $\text{C}<em>6\text{H}</em>{12}\text{O}<em>6 + 2\;\text{ADP} + 2\;\text{P}</em>i + 2\;\text{NAD}^+ \;\rightarrow\; 2\;\text{C}<em>3\text{H}</em>4\text{O}_3 + 2\;\text{ATP} + 2\;\text{NADH} + 2\;\text{H}^+$
• Lactic Acid Fermentation:
  $2\;\text{C}<em>3\text{H}</em>4\text{O}<em>3 + 2\;\text{NADH} \rightarrow 2\;\text{C}</em>3\text{H}<em>6\text{O}</em>3 + 2\;\text{NAD}^+$
• Ethanol Fermentation:
  $2\;\text{C}<em>3\text{H}</em>4\text{O}<em>3 \rightarrow 2\;\text{CO}</em>2 + 2\;\text{C}<em>2\text{H}</em>4\text{O}$
  $2\;\text{C}<em>2\text{H}</em>4\text{O} + 2\;\text{NADH} \rightarrow 2\;\text{C}<em>2\text{H}</em>6\text{O} + 2\;\text{NAD}^+$

Flow-Chart Style Summary

1. Glucose
   ↓ Glycolysis (invest 2 ATP, produce 4)
   ➜ 2 Pyruvate + 2 ATP (net) + 2 NADH
2. No O$_2$ available ⇒ ETC stalls, NADH accumulates.
3. Fermentation step regenerates NAD$^+$:
   • Option A: Lactate produced (animal cells, some bacteria).
   • Option B: Ethanol + CO$_2$ produced (yeast, some plants).
4. NAD$^+$ feeds back to step 1, enabling continued (though inefficient) ATP generation.

Bottom line: Fermentation is a metabolic "pressure-release valve" that sacrifices efficiency for survival when oxygen is scarce.