|Ethanol Fermentation; Credit: Davidcarmack|
Oxygen plays a vital role in aerobic respiration, but isn’t always readily available in every environment, or situation – conditions that lack the presence of oxygen can be described as ‘anaerobic’. In these conditions only a few types of organism can thrive:
Facultative anaerobes – are organisms that can respire aerobically if oxygen is present, but also have the ability to switch to ‘fermentation’ to meet their energy requirements.
Obligate anaerobes – can only survive in the absence of oxygen. The organisms die if oxygen is introduced into their environment.
To fully explain ‘anaerobic respiration’ and ‘fermentation’ it’s first important to understand what oxygen actually does in aerobic respiration.
The Role of OxygenWhen a typical eukaryotic cell respires, it does so aerobically. This process involves ‘changing’ the energy stored in glucose, or similar molecules, into a more useable form – adenosine triphosphate (ATP). There are several processes in this:
- Oxidative decarboxylation of pyruvate
- Citric acid cycle (Krebs cycle)
- Oxidative phosphorylation
The final process in that list, ‘oxidative phosphorylation’, is where oxygen plays such an important role. The electron transport chain in the mitochondrial cristae creates a proton gradient, which drives the phosphorylation of adenosine diphosphate (ADP) to ATP – oxygen acting as the final acceptor for the electron transport chain.
Anaerobic RespirationAnaerobic respiration isn’t wildly different from aerobic; it still involves an electron transport chain, but oxygen isn’t used as the final electron acceptor. In place of oxygen a range of other electronegative compounds can be used, such as sulfate.
Oxygen is a perfect final electron acceptor because it is extremely electronegative, more so than sulfate, and when it’s reduced can form water, whereas sulfate forms hydrogen sulfide. A sulfate electron acceptor still works however, it just produces less ATP.
FermentationFermentation takes anaerobic respiration one step further, it removes the need for an electron transport chain entirely. It starts out the same with the glycolysis step, which creates two molecules of pyruvate for every one of glucose, and in the process creates two molecules of ATP (by substrate level phosphorylation).
At this stage in aerobic respiration NAD+ is regenerated from the electron transport chain, but for fermentation, another process needs to occur if glycolysis is to continue. This new process involves transferring electrons from NADH to pyruvate, or a form of pyruvate. This can happen many different ways, but two of the most common are described below:
1. Alcohol fermentation
This involves removing carbon dioxide from pyruvate, to form acetaldehyde. Acetaldehyde can be reduced by NADH to form ethanol and NAD+, so glycolysis can continue.
2. Lactic acid fermentation
In this method pyruvate is directly reduced by NADH to form lactate – lactate is the ion of lactic acid.
Both of these produce ATP through the conversion of glucose to pyruvate, but are able to continue this process of glycolysis by regenerating NAD+ from the reduction of pyruvate, or its derivatives.