Anaerobic respiration is the type of respiration that allows cells to break down sugars to produce energy in the absence of oxygen. This is in contrast to the highly efficient process of aerobic respiration, which relies on oxygen for energy.
Molecular oxygen is the most efficient electron acceptor for respiration due to its high affinity for electrons. However, some organisms have evolved to use other terminal electron acceptors and as such can breathe without oxygen.
Respiration is the process by which the energy stored in fuel is converted into a form that a cell can use. Typically, energy stored in the molecular bonds of a sugar or fat molecule is used to make ATP by taking electrons from the fuel molecule and using it to drive an electron transport chain.
Respiration is vital to a cell’s survival because if it cannot release energy from fuels, it does not have enough energy to carry out its normal functions. This is why air-breathing organisms die so quickly without a constant supply of oxygen: our cells cannot generate enough energy to stay alive without it.
Instead of oxygen, anaerobic cells use substances like sulfate, nitrate, sulfur, and fumarate to fuel their cellular respiration. Many cells can breathe either aerobically or anaerobically, depending on whether oxygen is available.
Anaerobic vs. aerobic respiration
Both aerobic and anaerobic respiration are methods of extracting energy from a food source such as fat or sugar. Both processes begin with the splitting of a six-carbon sugar molecule into two three-carbon pyruvate molecules in a process called glycolysis. This process uses two ATP molecules and creates four ATP, for a net gain of two ATP per sugar molecule split.
In both aerobic and anaerobic respiration, the two pyruvate molecules undergo another series of reactions that use electron transport chains to create more ATP.
It’s these reactions that need an electron acceptor – be it oxygen, sulfate, nitrate, etc. – to power them.
Many bacteria and archaea can only breathe anaerobically. Many other organisms can breathe either aerobically or anaerobically, depending on whether oxygen is present.
Humans and other animals rely on aerobic respiration to stay alive, but can extend the lifespan or performance of their cells in the absence of oxygen through anaerobic respiration.
After glycolysis, both the aerobic and anaerobic cells send the two pyruvate molecules through a series of chemical reactions to create more ATP and extract electrons for use in their electron transport chain.
However, what these reactions are and where they take place varies between aerobic and anaerobic respiration
During aerobic respiration, the electron transport chain and most chemical reactions of respiration take place in the mitochondria. The membrane system of the mitochondria makes the process much more efficient by concentrating the chemical reactants of respiration in a small space.
In contrast, anaerobic respiration typically occurs in the cytoplasm. This is because most cells that exclusively carry out anaerobic respiration do not have any specialized organelles. The series of reactions is typically shorter in anaerobic respiration and uses a final electron acceptor such as sulfate, nitrate, sulfur, or fumarate instead of oxygen.
Anaerobic respiration also produces less ATP for each sugar molecule digested than aerobic respiration, making it a less efficient method of generating cellular energy. It also produces various waste products – including alcohol in some cases!
Cellular respiration in different organisms
- Organisms can be classified based on the type of cellular respiration they perform.
- Obligatory Aerobes – Organisms that cannot survive without oxygen. For example, humans are obligate aerobes.
- Obligatory Anaerobes – Organisms that cannot survive in the presence of oxygen. Certain types of bacteria are obligate anaerobes, such as B. Clostridium tetani, which causes tetanus.
- Aerotolerant Organisms – Organisms that can live in the presence of oxygen but do not use it to grow. For example, the bacterium Streptococcus, which causes throat infections.
- Facultative Aerobes – Organisms that can use oxygen to grow, but can also perform anaerobic respiration. For example Saccharomyces cerevisiae, the yeast used in brewing.
With a simple experimental setup using thioglycolate broth, scientists can use this method to classify microbes. This medium contains a range of oxygen concentrations that create a gradient. This is due to the presence of sodium thioglycolate, which consumes oxygen, and the continuous supply of oxygen from the air; Oxygen is present at the top of the tube and no oxygen is present at the bottom.
Types of anaerobic respiration
The types of anaerobic respiration are as diverse as their electron acceptors. Important types of anaerobic respiration are:
- Lactic Acid Fermentation – In this type of anaerobic respiration, glucose is broken down into two molecules of lactic acid to produce two ATPs. It is found in certain types of bacteria and some animal tissues such as muscle cells
- Alcoholic fermentation – In this type of anaerobic respiration, glucose is broken down into ethanol or ethyl alcohol. This process also produces two ATP per sugar molecule. This occurs in yeast and even in some species of fish such as goldfish.
- Other Types of Fermentation – Other types of fermentation are performed by some bacteria and archaea. These include propionic acid fermentation, butyric acid fermentation, solvent fermentation, mixed acid fermentation, butanediol fermentation, stickland fermentation, acetogenesis, and methanogenesis.
Anaerobic Respiration Equations
The equations for the two most common types of anaerobic respiration are:
- Lactic acid fermentation:
C6H12O6 (glucose) + 2ADP + 2Pi → 2lactic acid + 2ATP
- Alcoholic fermentation:
C6H12O6 (glucose) + 2ADP + 2pi → 2C2H5OH (ethanol) + 2CO2 + 2 ATP
Examples of anaerobic respiration
Sore muscles and lactic acid
During intense exercise, our muscles use oxygen to produce ATP faster than we can supply it.
In this case, muscle cells can perform glycolysis faster than they can oxygenate the mitochondrial electron transport chain.
The result is that anaerobic respiration and lactic acid fermentation take place in our cells – and after prolonged exercise, the accumulated lactic acid can make our muscles sore!
yeast and alcoholic beverages
Alcoholic beverages such as wine and whiskey are typically made by bottling yeasts — which perform alcoholic fermentation — with a solution of sugar and other flavorings.
Yeasts can use complex carbohydrates, including those found in potatoes, grapes, corn, and many other grains, as sugar sources for cellular respiration.
Putting the yeast and its fuel source in an airtight bottle ensures that there is not enough oxygen and thus the yeast switches to anaerobic respiration. This creates alcohol.
Alcohol is actually toxic to the yeasts that produce it – if the concentration of alcohol gets high enough, the yeast will begin to die.
For this reason it is not possible to brew wine or beer with more than 30% alcohol content. However, the distillation process, which separates alcohol from other components of the brew, can be used to concentrate the alcohol and make spirits like vodka.
Methanogenesis and dangerous homebrews
Unfortunately, alcoholic fermentation is not the only type of fermentation that can take place in plant matter. Another alcohol, called methanol, can be made from the fermentation of cellulose. This can lead to methanol poisoning.
The dangers of “moonshine”—cheap, home-brewed alcohol that often contains high levels of methanol due to poor brewing and distillation techniques—was promoted during Prohibition in the 20th century.
Fatalities and nerve damage from methanol poisoning are still a problem in areas where people try to cheaply brew alcohol. So if you want to be a brewer, do your homework!
Swiss cheese and propionic acid
The propionic acid fermentation gives the Swiss cheese its unmistakable taste. The holes in Swiss cheese are actually caused by carbon dioxide gas bubbles released as a waste product from a bacterium that uses propionic acid fermentation.
With the introduction of stricter hygiene standards in the 20th century, many Swiss cheese makers were amazed to find their cheese losing its holes – and its flavor.
The cause was discovered to be the absence of a specific bacterium that produces propionic acid. Over the centuries, this bacterium was introduced as a contaminant from the hay that cows ate. But after the introduction of stricter hygiene standards, this no longer happened!
These bacteria are now intentionally added during production to ensure Swiss cheese retains its flavor and retains its instantly recognizable pitted appearance.
Vinegar and Acetogenesis
Bacteria that engage in acetogenesis are responsible for the production of vinegar, which is primarily composed of acetic acid.
Vinegar actually requires two fermentation processes because the bacteria that make acetic acid need alcohol for fuel!
Vinegar is first fermented into an alcoholic preparation like wine. The alcoholic mixture is then fermented again with the acetogenic bacteria.