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Aerobic and Anaerobic Respiration

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IVSAT 2024

Organisms such as prokaryotes and eukaryotes use respiration mechanisms for the breakdown of food that may require environmental oxygen. The process by which mitochondria use to transfer the energy in foods to ATP is known as cellular respiration. In this process, food molecule breaks down in mitochondria, may consume oxygen, and transfer energy to cells (in which, it is stored as ATP molecule) and the environment (in the form of heat).

There are two types of cellular respiration- they are aerobic respiration and anaerobic respiration. The cells of animals, plants, and many bacteria need oxygen (O2) to facilitate energy transfer during cellular respiration. In these organisms, the type of cellular respiration that takes place is called aerobic respiration. The meaning of the word aerobic is with air. On the other hand, in the case of anaerobic respiration, the organisms do not require oxygen (O2) for cellular respiration. Alcohol fermentation, lactic acid fermentation, etc. are examples of anaerobic respiration.

Cellular respiration is different from normal respiration. Respiration is more commonly referred to as breathing and it is a physical act of inhaling and exhaling process. While cellular respiration is the process that occurs inside cells and involves the use of oxygen to transfer energy from food to ATP. Some organisms are capable of respiration anaerobically (in the absence of oxygen). In the absence of oxygen, these organisms manage to accept the electrons by using inorganic molecules. For example, in place of oxygen, many bacteria use sulfur, nitrate, or other inorganic compounds as electron acceptors. Based on the inorganic molecule used by bacteria as an electron acceptor, they are classified into methanogens and sulfur bacteria. The details on these classes are as follows-

  1. Methanogens Bacteria performing anaerobic respiration are primitive archaebacteria such as thermophiles. Some of these bacteria are called as methanogens. These methanogens use carbon dioxide (CO2) as the electron acceptor. In this reaction, CO2 is reduced to methane (CH4) by using the hydrogens derived from organic molecules produced by other organisms.

  2. Sulfur Bacteria: Among primitive bacteria, evidence of a second anaerobic respiratory process is seen in a group of rocks about 2.7 billion years old. It is known as the Woman River iron formation. A high amount of sulfur (32S) is present as organic material in these rocks. The isotope of sulfur 32S is a comparatively light isotope as compared with heavy isotope 34S. Hence, due to the high abundance of sulfur, several organisms use them for cellular respiration. These types of sulfur bacteria are capable of deriving energy from the reduction of inorganic sulfates (SO4) to hydrogen sulfide (H2S). The hydrogen atoms used in this reaction are obtained from organic molecules that are produced by other organisms. These bacteria function in the same manner as methanogens function, except they use sulfates as the oxidizing agent (i.e. electron acceptor agent) in place of carbon dioxide.During the first form of photosynthesis reaction, the hydrogen molecules were obtained from H2S by using the energy of sunlight. These sulfate reducers set the platform for the evolution of photosynthesis and that is responsible for creating an environment that is rich in H2S.

Steps Involved in Anaerobic Reaction

The  cellular respiration that takes place within the anaerobic bacteria consist of two different reactions. These reactions are made up of several complex reactions. These reactions are named Glycolysis and fermentation. The glycolysis reaction is followed by a fermentation reaction, ultimately yielding different types of an organic molecules based on the organism. The fate of pyruvate (it is a final product of glycolysis) is dependent on the availability of oxygen molecules. If it is present, then it passes through the Krebs cycle which is followed by the electron transport chain. However, in anaerobic conditions, pyruvic acid undergoes a fermentation reaction.

  1. Stage One Glycolysis: Glycolysis occurs in the cytoplasm of the cell. During this process, cells break glucose into pyruvate. Pyruvate is a three-carbon-containing compound. After this step, pyruvate is broken down into a two-carbon molecule which is known as acetyl-coenzyme A (acetyl-coA) and carbon dioxide (CO2). Glycolysis is the first step of extracting energy from glucose. Glycolysis reaction is a 10-reaction biochemical pathway. The location of glycolysis is the cytoplasm of the cell because all enzymes required to carry out glycolysis are present in the cytoplasm. They are not bound to any membrane or organelle. In this reaction, two ATP molecules are used up during the initial steps. However, at the end of the cycle, four ATP molecules are formed by substrate-level phosphorylation. Hence, there is a net yield of 2 ATP while catalyzing one glucose molecule by glycolysis. Additionally, four electrons are captured during the formation of NADH and that can be used in the production of ATP by aerobic respiration. Also, by this reaction, two molecules of pyruvate are formed that still contain most of the energy the original glucose molecule held. This step occurs in both, aerobic as well as anaerobic respiration.

  2. Fermentation Reaction: Aerobic metabolism cannot take place in the absence of oxygen. Hence, cells are entirely dependent on the glycolysis process to produce energy molecule ATP. The hydrogen atoms generated by glycolysis are transferred to organic molecules under these conditions. This process is called fermentation. Bacteria are capable of carrying out a dozen kinds of fermentation reactions. However, they all use some type of organic molecule as a hydrogen acceptor from reduced NAD (NADH) and by this way, NAD+ is reproduced. This reaction can be summarized as Organic molecule + Reduced NAD (NADH)→ Reduced organic molecule + NAD+As a result of this reaction, the reduced organic molecule is produced such as acetic acid, butyric acid, propionic acid, or lactic acid or alcohol. In this way, this reaction has a high commercial value in production by fermentation.

Several Fermentation Reactions are Discussed as Follow- 

  1. Ethanol Fermentation: Only a few types of fermentation reactions are carried out by eukaryotic cells. Yeast is capable of carrying out fermentation reactions. In yeast, the molecule that accepts hydrogen from reduced NAD (NADH) is pyruvate which is an end product of glycolysis. A terminal carbon dioxide group from pyruvate is removed by the yeast enzyme and it carries out a decarboxylation reaction. As a result of this decarboxylation reaction, a two-carbon molecule, acetaldehyde, is produced. The CO2 released by this reaction causes bread made with yeast to rise, while the bread that is made without using yeast does not have that texture. The acetaldehyde molecule produced by the above-mentioned reaction then accepts a hydrogen atom from the NADH molecule, and it ultimately produces ethyl alcohol (ethanol) and NAD+. This type of fermentation process is called ethanol fermentation as the final product of this reaction is ethanol. It involves the fermentation of ethanol by anaerobic organisms in the absence of oxygen. Since this reaction is the source of ethanol in wine and beer, this particular type of fermentation reaction is of great interest. In a sense, ethanol is a byproduct of this reaction as it is a toxic product to yeast. The ethanol concentration of about 12% is lethal and it could begin to kill the yeast. This is the reason why naturally fermented wine contains only about 12% of ethanol. The chemical reaction that takes place during ethanol fermentation can be summarized as follow:

Glucose → Pyruvic acid → Acetaldehyde + Carbon dioxide

Acetaldehyde → Ethanol

C6H12O6→ 2 C2H5OH + 2 CO22.

Lactic Acid Fermentation

Unlike yeast cells, most animal cells do not carry out a decarboxylation reaction. They regenerate NAD+ without decarboxylation which means during this reaction, CO2 is not liberated. As the name suggests, this reaction involves the fermentation of lactic acid (the final product is lactic acid). Muscle cells are capable of carrying out lactic acid fermentation. For this reaction, they use an enzyme called lactate dehydrogenase. This enzyme is responsible for the transfer of hydrogen atoms from NADH back to the end product of glycolysis i.e. pyruvate. In lactic acid fermentation, lactic acid is generated from pyruvate, and during this reaction, NAD+ is regenerated from its reduced form NADH. Hence, this completes a cycle that allows the glycolysis reaction to continue as long as glucose is available. In the body, there is a counter mechanism for the removal of lactic acid from muscle. Circulating blood is capable of removing excess lactate (it is the ionized form of lactic acid.) from muscles. 

However, when the removal of lactate does not take place with full efficiency, the accumulation of lactic acid takes place, and this tends to interfere with normal muscle function and also contributes to muscle fatigue. This reaction is possible only due to the availability of lactate dehydrogenase. The final product of this reaction, lactic acid, is less toxic than compared to ethanol. Lactic acid may produce some toxic symptoms such as they produce a painful sensation in muscle in case of oxygen depletion. During heavy exercise, there are high chances of hampered oxygen supply. Hence, this is the reason for painful sensation after heavy exercise.

The overall reaction of lactic acid fermentation can be summarized as

Glucose → Pyruvic acid →Lactate

Cellular respiration is a process that takes place inside the cells where energy is released by the breakdown of glucose molecules. The process can be easily divided into two categories based on the usage of oxygen, namely aerobic and anaerobic respiration.

What is the Difference Between Aerobic and Anaerobic Respiration? 

The basic difference between aerobic and anaerobic respiration is the presence or absence of oxygen during the processes. More detailed differences are between the two are as follows:

Difference Between Aerobic and Anaerobic Respiration

Aerobic Respiration   

      Anaerobic Respiration 

Oxygen is used when this form of respiration takes place.

Oxygen is not used when this form of respiration takes place

It can be found in the cytoplasm and the mitochondria.

It can only be found in Cytoplasm

Gases are exchanged in this form of respiration.

Gases are never exchanged in this form of respiration.

Mammals have this type of respiration.

Bacteria and yeast use this type. In other organisms, it occurs during heavy activities

Glucose breaks down into carbon dioxide and water in this type of respiration

Glucose breaks down into ethyl alcohol, carbon dioxide, and energy

However, it is a wrong conception that humans and other multicellular organisms use only aerobic respiration. This is disproven by the fact that our muscles, during vigorous exercises, undergo anaerobic respiration, where lactic acid is produced as the waste-byproduct instead of carbon dioxide.

Definition of Aerobic Respiration 

As stated earlier, cellular respiration is mainly of two types: aerobic and anaerobic. Aerobic means “with air”. Hence, aerobic respiration can be defined as the process of cellular respiration that uses oxygen to produce energy from food. This type of respiration is common in most plants and animals including humans, birds, and other mammals.

While breathing, we inhale air that contains oxygen and we exhale air rich in carbon dioxide. As we breathe in, the oxygen-rich air is circulated to all the parts of our body and ultimately to each cell. Inside the cell, the food, which contains glucose, is broken down into carbon dioxide and water with the help of oxygen. The process of breaking down the food particles releases energy, which is then utilized by our body. The energy released via aerobic respiration leads to the growth of plants, animals, and human beings.

The process can be simply explained with the help of the following equation:

Glucose + Oxygen → Carbon dioxide + Water + Energy

Aerobic respiration is a continuous process and it happens all the time inside the cells of animals and plants.

Anaerobic means “without air”. Therefore, this type of cellular respiration does not use oxygen to produce energy. Sometimes there is not enough oxygen around for some organisms to respire, but they still need the energy to survive. Due to lack of oxygen, they carry out respiration in the absence of oxygen to produce the energy they require, which is referred to as anaerobic respiration. Anaerobic respiration usually occurs in lower plants and microorganisms. In the absence of oxygen, the glucose derived from food is broken down into alcohol and carbon dioxide along with the production of energy. 

Glucose → Alcohol + Carbon dioxide + Energy

Definition of Anaerobic Respiration 

Anaerobic respiration is also utilized by multiple cell organisms, like us, as a temporary response to oxygen-less conditions. During heavy or intensive exercise such as running, sprinting, cycling, or weight lifting, our body demands high energy. As the rotation of oxygen is limited, the muscle cells inside our body change to anaerobic respiration to complete the energy demand.

After exercising much, how do you feel? Have you ever wondered why you get those muscle cramps when you run very fast? Anaerobic respiration is the culprit to be blamed. Cramps occur when muscle cells respire anaerobically. The partial breakdown of glucose, due to lack of oxygen, produces lactic acid and the accumulation of lactic acid causes muscle cramps. That is why a hot shower after heavy sports relieves the cramps as it improves blood circulation in the body which in turn enhances the supply of oxygen to the cells.

Glucose → Lactic acid + Energy

Anaerobic respiration produces a relatively lesser amount of energy as compared to aerobic respiration as glucose is not completely broken down in the absence of oxygen.


The primary difference between aerobic and anaerobic respiration is the usage of oxygen in the process of cellular respiration. Aerobic respiration, as the name suggests, is the process of producing the energy required by cells using oxygen. The by-product of this process produces carbon dioxide along with ATP – the energy currency of the cells. Anaerobic respiration is the same as aerobic respiration, except, the process takes place without the presence of oxygen. And consequently, the by-products of this process are lactic acid and ATP.

Contrary to popular belief, multicellular organisms, including humans use anaerobic respiration to produce energy, though this only happens when the muscles do not get adequate oxygen due to extremely vigorous activities.

Aerobic and anaerobic respiration is the basic foundation in the study of biology. This topic is explained very briefly in class seven of the  NCERT book of science in chapter 10 called Respiration In Organisms. This chapter forms the basis of biology as it deals with the many critical terms that are important to understand in order to study further, for example, terms like cellular respiration, anaerobic, aerobic respiration, nasal cavity, diaphragm, chest cavity, spiracles, trachea.

This study notes provided by Vedantu on aerobic and anaerobic respiration mainly talk about the definition of aerobic and anaerobic respiration in-depth and The notes talk about methanol and sulphur bacteria both of which are important in anaerobic respiration. The steps that are involved in an anaerobic reaction are stage one glycolysis, stage two, fermentation reaction, the notes also deal with fermentation reactions such as ethanol fermentation, lactic acid fermentation, they give a detailed description of the difference between aerobic and anaerobic respiration with examples and important formulas.

Important Points Discussed in the Chapter That Are Related to Aerobic and Anaerobic Respiration Are-

  • Oxygen which we inhale breaks down glucose, carbon dioxide, and water, and in this process energy is released

  • The concept of aerobic and anaerobic respiration is extremely simple as the food which breaks down in the presence of oxygen is called aerobic respiration while the food that breaks down without oxygen is called anaerobic respiration

  • anaerobic respiration usually takes place during heavy exercise, as the oxygen supply to our muscle cells is insufficient therefore food breakdowns without oxygen

  • Breathing falls under respiration as it is a process during which an organism takes in oxygen and gives out air rich in carbon dioxide

  • During inhalation, the lungs expand and then revert back to the original state after the air is moved out during exhalation.

  • Increased rate of physical activity increases the rate of breathing

  • Animals like cows, buffalo, dogs, and cats have the same process of breathing which is similar to humans

  • There are different ways in which the exchange of gases takes place for example in fishes it takes place through gills and in insects it takes place through the trachea.

  • The roots of the plants take in oxygen through the soil and the leaves of the plants have stomata which are responsible for the exchange of gases, the glucose breakdown in plant cells is similar to humans.

Key Concepts Needed to Understand the Importance of Aerobic and Anaerobic Respiration-

  • 10.1 Why do we respire?

  • 10.2 Breathing

  • 10.3 How do we breathe?

  • 10.4 What do we breathe out?

  • 10.5 Breathing in other animals

  • 10.6 Breathing underwater

  • 10.7 Do plants also respire?

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FAQs on Aerobic and Anaerobic Respiration

1. What role do mitochondria play in cell death?

The scientific term for cell death is apoptosis. As cells age, they are cleared and mitochondria help in deciding, which cells should be destroyed. The cell organelle releases cytochrome activating caspase which is a key enzyme involved in the process of apoptosis. Cytochrome C activates the process of cell death.

2. What is the purpose of mitochondrial membranes?

The mitochondrial structure has two membranes, the inner and the outer. The outer membrane has protein pores which allow ions and large molecules to pass. These can also be simple proteins. However, the inner membrane has restricted permeability. It also has a wide range of proteins that are used in ATP synthesis and the electron transport system.

During the electron transport chain, the participating proteins push protons from the matrix to the intermembrane space. This leads to the creation of a concentration gradient of protons that ATP synthase uses to produce ATP. So the membranes of mitochondria have a direct role in the production of these energy molecules.

3. What is aerobic respiration?

The meaning of aerobic is -with air. Therefore, aerobic respiration takes place in the presence of air is oxygen, It is a type of cellular respiration that uses oxygen to produce carbon dioxide, water, and energy. This type of respiration takes place in many living organisms like plants, animals, humans, birds, and other mammals. Breathing is a type of respiration and while breathing oxygen is inhaled and air rich in carbon dioxide is exhaled. In the process of exhaling oxygen, the oxygen is distributed to all the parts of the body and reaches the cell, inside the cell the breakdown of glucose takes place which gives carbon dioxide, water, and energy, this energy is then utilized by our body. This type of respiration is called aerobic respiration.

4. What is anaerobic respiration?

Anaerobic means- without air. anaerobic respiration is a temporary response to conditions where there is no oxygen, this type of respiration takes place mainly when there has been an increase in physical activity such as cycling, running, jumping or weightlifting. Therefore, the oxygen in the body is not enough to perform the process of respiration, and hence the muscles then turn to anaerobic respiration to complete the energy demand.

5. Where can I find the study material based on aerobic and anaerobic respiration?

The study material is available on Vedantu’s website, which can be easily accessed by downloading the material available for free.

6. What is glycolysis?

Glycolysis is the process in which cells break glucose into pyruvate. Glycolysis happens in the cytoplasm of the cell. Pyruvate is a compound that contains three carbons. This step leads to the breakage of pyruvate into two-carbon molecules called acetyl coenzyme and carbon dioxide.

7. What is a fermentation reaction?

Fermentation reaction only takes place in the presence of aerobic respiration and as there is the use of oxygen in the breakage of glucose, other cells in aerobic respiration are entirely dependent on glucose to produce energy molecule ATP.  In this process, hydrogen atoms are generated by glycolysis which is then transferred to organic molecules. This whole process is called a fermentation reaction.

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