U2L2 - Fermentation

Fermentation

Key Concept

• a metabolic process that occurs in the absence of oxygen

  • known as ANAEROBIC RESPIRATION.

  • an important pathway for organisms to generate ATP when oxygen is unavailable

    • breaks down organic molecules. In many organisms, it serves as the sole means of ATP production.

    • This process regenerates NAD$^+$ from NADH, crucial for glycolysis to continue.

    • Common types include lactic acid fermentation + alcoholic fermentation, each yielding different byproducts.

Stages of Fermentation

1. Lactic Acid Fermentation

   • Occurs in animal muscles during strenuous exercise when oxygen is scarce, in some bacteria and fungi.

   • Step 1: Glycolysis - Glucose ($C6H{12}O_6$ ) is broken down into two molecules of pyruvate, producing a net of 2 ATP and 2 NADH.

   • Step 2: Pyruvate Conversion to Lactic Acid - two pyruvate molecules are directly converted into lactic acid (lactate).

   • Step 3: NAD$^+$ Regeneration - During conversion, NADH donates its electrons to pyruvate, oxidizing NADH back to NAD$^+$. This regeneration of NAD$^+$ is essential for glycolysis to continue producing a small amount of ATP.

   • The net products are lactic acid and NAD$^+$. No $CO_2$ is released.

2. Alcoholic Fermentation

   • Occurs in yeast and some bacteria, commonly used in brewing and baking.

   • Step 1: Glycolysis - Glucose is broken down into two pyruvate molecules, producing a net of 2 ATP and 2 NADH.

   • Step 2: Pyruvate Decarboxylation - Each pyruvate molecule is decarboxylated (loses a molecule of $CO_2$) to form acetaldehyde.

   • Step 3: Acetaldehyde Reduction to Ethanol - Acetaldehyde then accepts electrons from NADH, reducing it to ethanol and regenerating NAD$^+$. This allows glycolysis to continue.

   • The net products are ethanol, $CO_2$, and NAD$^+$.

Importance for Athletes

• Understanding fermentation is essential for athletes, as it plays a key role in energy production under anaerobic conditions, particularly during high-intensity, short-duration activities like sprinting or weightlifting. When oxygen supply to muscle cells becomes insufficient, muscles rely on fermentation (lactic acid fermentation) to rapidly produce ATP. This leads to the buildup of lactic acid, which can contribute to muscle fatigue and soreness. Athletes often train to improve their lactic acid tolerance and clearance.

Glycolysis

Overview

• Glycolysis is the initial and simplest stage of cellular respiration, occurring in the cytoplasm of the cell.

  • It is a universal metabolic pathway, considered the first process to have evolved over 3.8 billion years ago.

  • This evolutionary development predates the accumulation of significant oxygen in the Earth's atmosphere, occurring when prokaryotes existed in an atmosphere that was devoid of oxygen (anaerobic).

  • The process involves the breakdown of a single six-carbon glucose molecule into two three-carbon pyruvate molecules.

Energy Production

• These early organisms relied exclusively on glycolysis to produce limited energy. Although glycolysis directly produces four ATP molecules through substrate-level phosphorylation, it consumes two ATP molecules in its initial energy-investment phase. Therefore, it yields a net gain of only 2 ATP molecules per glucose molecule.

• Despite the low net yield of ATP, these 2 ATP are sufficient for the basic biological processes of many simple bacteria and single-celled organisms, enabling essential functions such as eating, growing, and dividing.

Potential Energy

• There exists excess potential energy in the form of the two Pyruvate molecules and two NADH molecules (reduced nicotinamide adenine dinucleotide) after glycolysis.

• In anaerobic conditions (like those where fermentation occurs), pyruvate can be converted into byproducts (like lactic acid or ethanol) to regenerate NAD$^+$, allowing glycolysis to continue.

• In aerobic conditions (with oxygen present), pyruvate would typically enter the mitochondria for further oxidation in the Krebs cycle, and NADH would donate its electrons to the electron transport chain to produce significantly more ATP.