This unit is about Anaerobic respiration and its processes.




In animals and some bacteria, anaerobic respiration produces lactic acid, while in plants, some fungi and bacteria form alcohol through fermentation process. This process has been economically utilised in industries to produce food items, such as making yoghurt, cheese, alcohol and bread. However, various communities have basic ways of food production by fermentation process. Suggest improvement techniques suitable for the methods.

Anaerobic respiration involves the breakdown of organic compounds without using of oxygen. Less energy is produced than in aerobic respiration. Other products depend on the organism, for example, yeast, plants and some bacteria produce alcohol in the form of ethanol and carbon dioxide while animals and some bacteria produce lactic acid.  i.e. Glucose   Enzymes    Ethanol + Carbon dioxide+ energy (in plants) This fermentation process in organisms is utilised by man in the industrial process of brewing beer, bread making, and producing vinegar and producing biogas.

In humans anaerobic respiration occurs for a few seconds during strenuous exercises when the muscles require extra energy. This is because we cannot breathe fast enough to obtain the extra oxygen required to obtain the extra energy. So the muscles respire anaerobically to produce energy and lactic acid, thus acquiring oxygen debt. Lactic acid produced by bacteria through anaerobic respiration can be put to maximum use by humans, e.g. in the making of yoghurt, butter, etc.  It is also called lactic-fermentation.

human body

Microorganisms are the only things that use anaerobic respiration, as far as we know. Some bacteria that live in vents near the ocean floor are called sulfate-reducers, since they use sulfate instead of oxygen as their electron receptor. Microbes called methanogens also use anaerobic respiration—they use carbon dioxide as an electron receptor and make methane in the process.

If a cell has a short supply of oxygen because happens to be in a body running at full speed away from a lion, however, it will not use anaerobic respiration but will start fermentation. Fermentation extends glycolysis with extra reactions that replenish NAD+, so that glycolysis can keep running and keep producing small amounts of ATP. Fermentation comes in two types:

• Lactic acid fermentation
• Alcohol fermentation

anaerobic respiration


Lactic acid fermentation happens in bacteria, fungi, and animal cells. It is a pretty simple follow-up to glycolysis: the pyruvate molecules are reduced to lactate, while NADH is oxidized to NAD+. In this way, NAD+ is replenished for glycolysis. Lactic acid fermentation happens in some fungi and bacteria, but also in animal muscle cells. The dairy industry uses bacteria and fungi for their fermentation to make cheese and yogurt.

BIO2: ANAEROBIC RESPIRATION 1Alcohol fermentation is pretty similar to lactic acid fermentation. Instead of the pyruvate being reduced to lactate, it is reduced to ethanol, and lets off two molecules of CO2 along the way.


Two kinds of organisms use alcohol fermentation: bacteria and yeast (yeast are fungi). Humans “use” alcohol fermentation in another way, by using it to make bread, beer and wine. We’ll talk more about that in another section.

Anaerobic respiration as compared to fermentation



Cellular respiration (both aerobic and anaerobic) utilizes highly reduced species such as NADH and FADH2 (for example produced during glycolysis and the citric acid cycle) to establish an electrochemical gradient (often a proton gradient) across a membrane, resulting in an electrical potential or ion concentration difference across the membrane.

The reduced species are oxidized by a series of respiratory integral membrane proteins with sequentially increasing reduction potentials with the final electron acceptor being oxygen (in aerobic respiration) or another species (in anaerobic respiration). The membrane in question is the inner mitochondrial membrane in eukaryotes and the cell membrane in prokaryotes. A proton motive force or pmf drives protons down the gradient (across the membrane) through the proton channel of ATP synthase. The resulting current drives ATP synthesis from ADP and inorganic phosphate.

Fermentation in contrast, does not utilize an electrochemical gradient. Fermentation instead only uses substrate-level phosphorylation to produce ATP. The electron acceptor NAD+ is regenerated from NADH formed in oxidative steps of the fermentation pathway by the reduction of oxidized compounds. These oxidized compounds are often formed during the fermentation pathway itself, but may also be external. For example, in homofermentative lactic acid bacteria, NADH formed during the oxidation of glyceraldehyde-3-phosphate is oxidized back to NAD+ by the reduction of pyruvate to lactic acid at a later stage in the pathway. In yeast, acetaldehyde is reduced


Anaerobic respiration plays a major role in the global nitrogen, sulfur, and carbon cycles through the reduction of the oxyanions of nitrogen, sulfur, and carbon to more-reduced compounds. Dissimilatory denitrification is the main route by which biologically fixed nitrogen is returned to the atmosphere as molecular nitrogen gas. Hydrogen sulfide, a product of sulfate respiration, is a potent neurotoxin and responsible for the characteristic ‘rotten egg’ smell of brackish swamps.


Along with volcanic hydrogen sulfide, biogenic sulfide has the capacity to precipitiate heavy metal ions from solution, leading to the deposition of sulfidic metal ores. Many terrestrial environments become temporarily flooded, and the resulting decrease in oxygen availability results in transient anoxia.

Sequential changes in redox conditions and associated adapted microorganisms will follow a flooding event (such as initially aerobic conditions becoming nitrate-reducing followed by iron-reducing, sulfate reducing and eventually methanogenic). Redox gradients such as these may occur in either time (called sequential reduction) or space (the redox regime becomes increasingly negative with distance from an oxygen source). Environmental redox cycling often has strong effects on natural biogeochemical cycling as well as biodegradation of anthropogenic organic pollutants.

Anaerobic Byproducts

As you can see, both of these anaerobic conditions leads to glycolytic products other than pyruvate. These different products are necessary because the NADH molecule must be reoxidized so that it can function in the next round of glycolysis of newly introduced glucose. If oxygen is not present to help oxidize it, other reactions, such as those of homolactic and alcoholic fermentation, must occur.




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