Cellular respiration is a process which takes place in the cells of organisms; during this process, the stored food material is converted into chemical energy, which is known as adenosine triphosphate. Anaerobic respiration is a type of respiration which occurs in the absence of oxygen or mitochondria. In this process, ATP is generated in very different ways, which are as follows:

1. Anaerobic cellular respiration: During glycolysis, highly reduced compounds such as NADH are formed, further oxidised by a series of respiratory integral membrane proteins known as the electron transport chain (ETC). In organisms which lack mitochondria, ETC is present in the plasma membrane; instead of oxygen is the terminal electron acceptor, organic (Dimethyl sulfoxide) or inorganic compounds (sulfate, nitrate and ferric ion) act as terminal electron acceptors; these electron acceptors have lower reduction potential than oxygen thus ATP generated per molecule of glucose is lower when compared to aerobic respiration. This process occurs in organisms such as sulphur bacteria.
2. Fermentation: In this process, ETC is not involved. Instead, substrate-level phosphorylation occurs to produce ATP. During this process, highly reduced compounds such as NADH, formed during glycolysis, are oxidised by enzymatic reduction of pyruvate to lactic acid or ethyl alcohol. This process occurs in organisms such as yeast.

What is the anaerobic respiration equation?

During lactic acid and alcoholic fermentation, two ATPS are produced, which is comparatively lower than the 36 ATPs generated during typical aerobic respiration. Yeasts and some bacteria carry out alcoholic fermentation. Lactic acid fermentation is carried out by organisms such as Lactobacillus and Streptococcus.

Diagram of anaerobic respiration

During alcoholic fermentation, an enzyme, alcohol dehydrogenase reduces pyruvate to form Ethyl alcohol and carbon dioxide, and NADH (Nicotinamide adenine dinucleotide hydrogen) is oxidised to form NAD⁺.

During lactic acid fermentation, an enzyme, lactic acid dehydrogenase, reduces pyruvate to form lactate or lactic acid, and NADH is oxidised to form NAD⁺.

What are the end products of anaerobic respiration?

In yeast, the end products of anaerobic respiration are ethyl alcohol; this fermentation process is used in making alcoholic beverages. 

The end product of lactic acid is the lactic acid in bacteria such as Lactobacillus, Streptococcus or mammalian muscle cells. The process of lactic acid fermentation is employed in making fermented dairy products such as cheese, yoghurt, fermented soy products, pickled foods etc.

In acetic acid bacteria, fermentation converts sugars or ethanol to acetic acid. This process of fermentation is utilised in producing vinegar.

In nitrogen-fixing bacteria, where nitrate is used as a terminal electron acceptor, the end product of anaerobic cellular respiration is nitrogen gas.

In methanogenic bacteria, where carbon monoxide is used as a terminal electron acceptor, the end product of anaerobic cellular respiration is methane gas.

In sulphate-reducing bacteria, where sulphate is used as a terminal electron acceptor, the end product of anaerobic cellular respiration is hydrogen sulphide gas.

Explain anaerobic respiration in yeast?

Yeast is a typical example of a facultative anaerobe. In the presence of oxygen, pyruvate undergoes a series of chemical reactions. Further, it proceeds to the electron transport chain; as a result, 36 molecules of ATPs are generated per glucose molecule, but in the absence of oxygen, they can still survive by changing their mode of respiration to anaerobic. During anaerobic respiration in yeast, glucose, a six-carbon compound, is broken down into pyruvate, a three-carbon compound, by a metabolic pathway called glycolysis. Later two pyruvate molecules are partially broken down into two molecules of ethyl alcohol, carbon dioxide and adenosine triphosphate, an enzyme alcohol dehydrogenase, catalyses this reaction.

During the bread-baking process, alcohol produced in the reaction gets evaporated, whereas, during the wine-making process, the alcohol produced is collected and further processed into alcoholic beverages.

Examples of Anaerobic Respiration

• Fermentation of milk by Lactobacillus and streptococcus to produce cheese and yoghurt.
• Soyabean is fermented by microbes such as Lactobacillus, Streptococcus, and Aspergillus to produce various fermented foods such as tofu, natto, soy sauce, tempeh etc.
• Various fruits such as grapes, apples, pomegranates, strawberries etc., are fermented by yeast to make alcoholic beverages such as wine.
• Toddy is an alcoholic beverage produced from the fermented sap of different species of palms.
• Vinegar, i.e., acetic acid, is made by fermentation of fruits and grains.
• Vodka, also called Russian whisky, is made from fermented grains or potatoes, sugar beet molasses.
• Butyric acid is produced when anaerobic bacteria such as Clostridium are involved in the fermentation of substances such as lactic acid or butter.
• Nitrogenous compounds are added to the soil by nitrogen-fixing bacteria, this process is essentially anaerobic respiration.
• By anaerobic respiration, methane gas produced by archaebacteria 
• Through anaerobic respiration, sulphate-reducing bacteria produce hydrogen sulfide gas.

Experimental Setup of Anaerobic Respiration

Experiment Title – Experimental Setup of Anaerobic Respiration

Experiment Description – Sometimes, animals and plants cannot get enough oxygen to respire aerobically, but they still need to respire to survive. So, how do they respire then?  Learn in this Virtual lab.

Aim of Experiment – Observation and comment on the setup. (Anaerobic Respiration)

Material Required – Germinating seeds (gram, moong, urad), Test tube, Petri dish, Mercury, Forceps, Potassium hydroxide pellets, Burette stand with clamp.

Procedure – 

• Take a test tube and fill it with mercury. 
• Invert it over a Petri dish which is also filled with mercury. There must be a continuous column of mercury in the test tube. 
• Tilt the test tube slightly and, with the help of forceps, introduce 3 – 4 healthy germinating gram seeds into the test tube.
• Gently tap the test tube with your fingernail/forceps so the seeds move upwards in the mercury column. 
• Fix the test tube to a stand with a clamp and keep the setup undisturbed for two hours.
• Observe the setup. 
• Introduce 3-4 KOH pellets in the same way the seeds were introduced. Observe the changes.

Precautions – 

• Ensure that there should be a continuous mercury column in the test tube.
• The seed coats should be removed before introducing the seeds into the test tube.
• Be careful while handling KOH pellets. Always use forceps to transfer the pellets into the test tube.

FAQs on Experimental Setup of Anaerobic Respiration