Fermentation is important because, like cellular respiration, it provides a way for cells to release energy stored in organic molecules for cellular work.
Understanding the connection between fermentation and cellular respiration reveals their complementary roles in energy extraction. Through cellular respiration and fermentation, those bonds are broken releasing the potential energy of organic molecules into kinetic energy that cells use to do work. While complete cellular respiration (aerobic respiration) yields a large amount of energy, fermentation plays a crucial role when the conditions for full respiration are not met.
Fermentation: Keeping Energy Production Going
Cellular respiration typically involves glycolysis, the Krebs cycle, and oxidative phosphorylation, requiring oxygen for the latter stages to be most efficient. Fermentation is an anaerobic process, meaning it occurs in the absence of oxygen. Its primary importance lies in its relationship with glycolysis, the initial stage of cellular respiration.
- Glycolysis: This process breaks down glucose into pyruvate, producing a small amount of ATP (usable energy) and NADH. Glycolysis can occur with or without oxygen.
- The Need for NAD+: For glycolysis to continue, it requires a molecule called NAD+. As glycolysis proceeds, NAD+ is converted to NADH. If NAD+ is not regenerated, glycolysis stops, and ATP production ceases.
Regenerating NAD+
This is where fermentation becomes essential. When oxygen is scarce or absent, the later stages of aerobic respiration cannot effectively regenerate NAD+ from NADH. Fermentation pathways take over, converting pyruvate (the end product of glycolysis) into other molecules, such as lactic acid or ethanol. This conversion step regenerates NAD+ from NADH, allowing glycolysis to continue producing ATP.
Common Types of Fermentation
- Lactic Acid Fermentation: Occurs in animal muscle cells during intense exercise when oxygen supply is low, and also in some bacteria (used in making yogurt and cheese). Pyruvate is converted to lactic acid.
- Alcoholic Fermentation: Occurs in yeasts and some plant cells. Pyruvate is converted to ethanol and carbon dioxide. This process is used in baking (CO2 makes bread rise) and brewing alcoholic beverages.
Why This Matters for Energy Production
Fermentation itself does not produce ATP directly. Its importance lies in enabling glycolysis, which does produce a net gain of 2 ATP molecules per glucose molecule, to continue.
While the 2 ATP from fermentation-coupled glycolysis is far less than the significant amount produced by full aerobic respiration (around 30-32 ATP), it is a vital emergency energy source. It allows cells to generate ATP rapidly for short bursts of activity or survival in environments lacking oxygen, preventing a complete shutdown of energy production.
In summary, while not a part of aerobic respiration itself, fermentation is critically important in the context of cellular energy release because it serves as an anaerobic alternative or supplement that ensures the initial, universal energy-producing pathway (glycolysis) can continue when oxygen is unavailable, thereby allowing cells to still harness some potential energy from organic molecules for work, as stated in the provided reference.
