Here's what happens instead:
1. Fermentation:
* Anaerobic Respiration: Some organisms can use alternative electron acceptors like sulfate, nitrate, or iron instead of oxygen. This process, known as anaerobic respiration, generates less ATP than aerobic respiration but still allows for energy production.
* Fermentation: Most organisms, including humans, resort to fermentation when oxygen is scarce. This process involves using pyruvate, a product of glycolysis, as an electron acceptor. This regenerates NAD+ from NADH, allowing glycolysis to continue producing a small amount of ATP.
2. Different Fermentation Pathways:
There are different fermentation pathways depending on the organism. Common examples include:
* Lactic Acid Fermentation: Occurs in muscle cells during intense exercise and some bacteria. Pyruvate is converted to lactic acid.
* Ethanol Fermentation: Occurs in yeast and some bacteria. Pyruvate is converted to ethanol and carbon dioxide.
Consequences of No Oxygen:
* Reduced ATP Production: Fermentation produces far less ATP than aerobic respiration.
* Build-up of Byproducts: Fermentation pathways generate byproducts, such as lactic acid or ethanol, which can accumulate and have negative effects.
* Limited Growth: Organisms relying on fermentation have limited growth potential due to low ATP production.
In Summary:
In the absence of oxygen, NADH cannot be reoxidized through the ETC. Cells resort to fermentation to regenerate NAD+ and maintain glycolysis. This leads to a significantly reduced ATP production and accumulation of byproducts.
