Aerobic and Anaerobic Respiration

Aerobic and Anaerobic Respiration - Definition and Steps

Organisms such as prokaryotes and eukaryotes use respiration mechanism 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 the energy-transfer during cellular respiration. In these organisms, the type of cellular respiration takes place is called as aerobic respiration. The meaning of word aerobic is with air. On the other hand, in the case of anaerobic respiration, the organisms do not require oxygen (O2) for the 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 occurs inside cells and that 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 acceptor. 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 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 takes place within the anaerobic bacteria consist of two different reaction. These reactions are made up of several complex reactions. These reactions are named as- Glycolysis, and fermentation. The glycolysis reaction is followed by fermentation reaction, ultimately yield different type of organic molecule based on the organism. The fate of pyruvate (it is a final product of glycolysis) is dependent on the availability of oxygen molecule. If it is present, then it passes through Krebs cycle which is followed by the electron transport chain. However, in anaerobic condition, pyruvic acid undergoes fermentation reaction.

  • A. Stage One: Glycolysis

  • Glycolysis occurs in the cytoplasm of the cell. During this process, cells break glucose in to 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. Location of glycolysis is 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 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 contains most of the energy the original glucose molecule held. This step occurs in both, aerobic as well as anaerobic respiration.

  • B. Fermentation Reaction

  • Aerobic metabolism cannot take place in the absence of oxygen. Hence, cells are entirely dependent on 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 as fermentation. 

    Bacteria are capable of carrying out 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 reaction. In yeast, the molecule that accepts hydrogen from reduced NAD (NADH) is pyruvate that is an end product of glycolysis. A terminal carbon dioxide group from pyruvate is removed by yeast enzyme and it carries out decarboxylation reaction. As a result of this decarboxylation reaction, two-carbon molecule, acetaldehyde, is produced. The CO2 released by this reaction causes bread made with yeast to rise, while the bread that are made without using yeast does not have that texture. Acetaldehyde molecule produced by the above-mentioned reaction then accepts a hydrogen atom from NADH molecule, and it ultimately produces ethyl alcohol (ethanol) and NAD+

    This type of fermentation process is called as ethanol fermentation as the final product of this reaction is ethanol. It involves fermentation of ethanol by anaerobic organisms under 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 a naturally fermented wine contains only about 12% of ethanol.

    The chemical reaction takes place during ethanol fermentation can be summarized as follow:

    Glucose → Pyruvic acid → Acetaldehyde + Carbon dioxide
    Acetaldehyde → Ethanol
    C6H12O6→ 2 C2H5OH + 2 CO2


  • 2. Lactic acid fermentation

  • Unlike yeast cells, most animal cells do not carry out a decarboxylation reaction. They regenerate NAD+ without decarboxylation that 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 atom 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 complete 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 the 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