Anaerobic Respiration – Concept, types, examples and glycolysis

We explain what is anaerobic or anaerobic respiration in biology, we talk about the existing types, and we offer examples of regions where it occurs.

What is anaerobic respiration?

In biology, the metabolic process of oxidoreduction of sugars is called anaerobic respiration. In other words, energy is obtained by oxidation of glucose without the presence of oxygen, but using oxidizing agents (electron acceptors), a process of cellular respiration in which oxygen molecules are not involved, nor needed.

If anaerobic respiration does not require oxygen, the aerobic respiration processes sugar molecules using oxygen molecules. The anaerobic respiration uses other types of chemical elements or even more complex organic structures, through an electron transport chain.

It should not be confused with fermentation, since it the latter does not involve the electron transport chain. However, both anaerobic an fermentation occur in the absence of oxygen.

This type of cellular respiration is unique to certain prokaryotic organisms. (bacteria or archaea), especially those that live in conditions with little or no oxygen presence. In many cases, it can also constitute a backup process, let’s say an emergency shown when oxygen is missing accidentally from the environment.

Types of anaerobic respiration

Anaerobic respiration can be classified according to the type of chemical element used to replace oxygen, the electron acceptors during the metabolic process. Naturally, there are several types of processing without oxygen via electron accetors, but the most common are:

• Anaerobic respiration through nitrates. In this case, the microorganisms consume nitrates (NO₃^–) to reduce them to nitrites (NO₂^–) by incorporating electrons. However, since nitrites are usually toxic to most life forms, it is much more common for the end product of this process to go further, to biatomic nitrogen (N₂) which is an inert gas. This process is known as denitrification.
• Anaerobic respiration through sulfates. Similar to the previous case, but with sulfur derivatives (SO₄^2-), is a much rarer case, belonging exclusively to anaerobic bacteria, while the previous case can occur as an alternative to the momentary shortage of oxygen. The process of sulfate reduction produces sulfur radicals (S^2-).
• Anaerobic respiration by carbon dioxide. Some groups of archaea producing methane gas (CH₄) consume carbon dioxide (CO₂) to use it as an electron acceptors. These are the microorganisms that inhabit the digestive tract of ruminants, for example, where other microorganisms supply them with the hydrogen they require for the process.
• Anaerobic respiration through iron ions. It is common among certain bacteria, capable of consuming ferric ions (Fe³⁺), reducing them to ferrous ions (Fe²⁺). Since this type of iron molecules are very common in the earth’s crust, it usually happens on the bottom of swamps, where iron sediments show up due to bacterial action.

Examples of anaerobic respiration

Organisms that live in hot springs perform anaerobic respiration.

Examples of this type of process are common in the prokaryotic world, especially in the most inhospitable regions of the planet, but where life is still possible. Such regions are:

• The bowels of bigger animals.
• The seabed and the deep fissures on the bottom of the sea.
• The geothermal lakes or marine geothermal springs, where the magma gushes at the bottom.
• Geysers, hot springs and other forms of geothermal bursts.
• Swamps and waters rich in clay, full of organic matter, but low in oxygen.

Glycolysis

Glycolysis is a metabolic pathway that allows animals to obtain energy from glucose. In other words, it is a successive series of biochemical reactions, applied by most living beings, to break the glucose molecule (C₆H₁₂OR₆) and obtain the necessary chemical energy (in the form of adenosine triphosphate or ATP) to keep the cellular metabolism going.

Glycolysis consists of 10 enzymatic reactions that occur consecutively, either in the presence (aerobic) or in the absence (anaerobic) of oxygen, five during the preparatory phase, and five during the pay-off phase. The result is the formation of two molecules of pyruvate or pyruvic acid (C₃H₄OR₃) which, in turn, feed other metabolic routes to continue obtaining energy for the body (the so-called Krebs Cycle).