Respiration : Cellular respiration, Aerobic and Anaerobic Respiration

Published on 01-Oct-2022


In all the living plant cells and animal cells, there are various chemical reactions constantly taking place. A lot of energy is required for these reactions. And the source of this energy is cellular carbohydrates, proteins, lipids and other chemicals. Carbohydrates are the primary source of energy. The fixed energy stored in starch, sucrose or glucose is not released all at once but gradually through several sequential reactions controlled by different enzymes. The periodic oxidation-reduction reactions in the cells to release the static energy of these chemical substances as working kinetic energy are collectively known as respiration. So, respiration is a combination of several oxidation-reduction reactions that release energy. Complex food products are converted into simple products during energy production.

The biochemical process in which complex biomass (food) in living cells is oxidized. As a result, static energy stored in biomass is converted into kinetic energy or chemical energetic is, called respiration. The energy released as a result of respiration is used in various energy-absorbing activities of the organism. Taking glucose as the primary respiratory substance, the chemical signals for respiration are as follows.

Static energy is the static and kinetic energy that is locked up in a stored state (e.g., in food). The functional and dynamic energy (e.g., ATP) is kinetic energy.

Respiratory Organs

In every living cell of a plant, respiration continues 24 hours a day and night. Cellular cytoplasm and mitochondria are the prominent organelles of respiration. Compound substances oxidized and become simple substances in the process of respiration are called respiratory substances. Carbohydrates (sugars), proteins (meats), fats and organic acids are used as respiratory substances. The light energy of sunlight is stored in these materials as chemical static energy, and as a result of respiration, the static energy is released as kinetic energy. So, sunlight is the primary source of all energy.

ATP-source of energy in cells

Making ATP ADP + P ATP Making ATP also requires energy. The energy required for this reaction: ATP, comes from the breakdown of organic compounds. ATP can never be transferred from one cell to another, but all cells need a continuous supply of ATP. For this reason, respiration is required in every living cell so that all the necessary activities of life can be carried out inside each cell. Hence, this type of respiration is called cellular respiration.

Cellular respiration

The cells need energy for three reasons, and they are:

  1. Synthesize large biomolecules, such as DNA, RNA, proteins, etc. 
  2. Active transport involves the exchange of molecules or ions across a membrane.
  3. Moving objects around inside the cell (e.g., chromosomes, protein fibres in muscle cells).

When ATP is used inside the cell, all of it is converted to heat. Thermal energy is needed to keep the cell warm but cannot be reused in any of the cell's functions. So, it eventually gets lost in the environment.

Types of Respiration

 Respiration can be divided into two depending on the oxygen requirement, and they are:

(a) Aerobic respiration 

(b) Anaerobic respiration

Anaerobic respiration

Respiration that requires free oxygen is called aerobic respiration, and respiration that takes place in the absence of free oxygen is called aerobic respiration.

During respiration, oxygen completely oxidizes the respiratory material and produces more energy. During respiration, some cellular enzymes partially oxidize the respiratory material and produce less energy.

 Aerobic Respiration

The process of respiration that requires free oxygen and completely oxidizes the respirant to produce CO2, H2O and a large amount of energy is called aerobic respiration. Because the presence of oxygen means the presence of air, this type of respiration is aerobic respiration. Respiration in most organisms (many bacteria, most fungi, all protists, plants and animals) is aerobic respiration.

Steps or stages of the respiration process

Although aerobic respiration is a continuous process, it is divided into several successive steps or stages according to the place of reaction and the course of action. The steps are as follows:

The first step: Glycolysis (Glycolysis): 

The process in which one molecule of glucose is oxidized in different chemical reactions into two molecules of pyruvic acid is called glycolysis. No oxygen is required for this process. Glycolysis is the first or most common step in both aerobic and aerobic respiration. Glycolysis is also called the EMP pathway, the general pathway of respiration or cytoplasmic respiration. The starch stored in plants is first oxidized to glucose by various enzymes, and glucose is used as the first product of glycolysis. All enzymes in this step are soluble. 


When glucose is taken up as a respiratory substance, the process of glycolysis is sequential as follows:

  • (i) Glucose accepts a phosphate from ATP to form glucose-6-phosphate. The hexokinase enzyme is activated in this reaction, and an ADP is formed. The reaction is unidirectional. Glucose-6-phosphate is converted to fructose-6-phosphate. The phosphoglucoisomerase enzyme is activated in this reaction. The reaction is bidirectional.
  • (iii) Fructose-6-phosphate accepts a phosphate from ATP to form fructose-1,6-bisphosphate. The enzyme phosphofructokinase is activated in this reaction, and an ADP is formed. The reaction is unidirectional. 
  • (iv) Fructose-1-6-B phosphate (6 carbons) is broken down to form one molecule of phosphoglyceraldehyde (3 carbons) and one molecule of dihydroxyacetone phosphate (3 carbons). Aldolase enzyme is active in this reaction. They can be converted into one another by the action of the isomerase enzyme. Both reactions are bidirectional.
  • (v) 3-Phosphoglyceraldehyde accepts a molecule of inorganic phosphate to form 1,3-bisphosphoglyceric acid. In this reaction, the enzyme phosphoglyceraldehyde dehydrogenase is activated, inorganic phosphate and NAD participate, and NADH + H+ is formed. The reaction is bidirectional. Sucrose is the major translocated sugar in plants, so sucrose should be considered as the respiratory substance in plants, not glucose.
  • (vi) 1,3-Bisphosphoglyceric acid loses phosphate to 3-phosphoglyceric acid. In this reaction, the enzyme phosphoglyceric acid kinase is activated, and ATP is produced from ADP. The reaction is bidirectional.
  • (vii) 3-phosphoglyceric acid, under the action of phosphoglyceromutase enzyme, is converted to 2-phosphoglyceric acid. The reaction is bidirectional.
  • (viii) 2-Phosphoglyceric acid is converted to phosphonyl pyruvic acid under the action of the enzyme. The reaction is bidirectional. Here a molecule of water comes out.
  • (ix) Phosphonyl pyruvic acid, under the action of a pyruvic acid kinase enzyme, is converted to pyruvic acid. An ATP is formed from ADP in this reaction. Glycolysis ends with the formation of pyruvic acid from glucose. The reaction is unidirectional.

 Among the nine reactions of glycolysis, the 1st, 3rd, and last three reactions are unidirectional, while the others are bidirectional. Glycolysis produces ATP (two molecules), NADH + H+ (two molecules), and pyruvic acid (two molecules). ATP and NADH + H' are energy molecules. 

Two molecules of ATP are consumed from glucose to fructose-1,6-bisphosphate. In the next step, each triose (3-carbon glyceraldehyde and dihydroxyacetone) to pyruvic acid produces two molecules of ATP, and one molecule of NADH + H+, i.e., two molecules. A total of four ATP and two NADH + H+ are produced from triose. So, it can be seen that if the two molecules of ATP used first are removed from the four molecules of ATP produced, a net of two ATP and two NADH + H+ are accumulated in the glycolysis process. All its enzymes are soluble. 

Regulation of glycolysis

  1. Glycolysis is accelerated when ATP consumption is high; while ATP consumption decreases, the rate of the process slows down.  
  2. The availability and supply of glucose regulate this process. 
  3. The process of glycolysis is dependent mainly on the activity of the allosteric enzyme 'phosphomujokinase,' which catalyzes the formation of fructose 6-phosphate to fructose 1.6, bisphosphate. Its action is inhibited by ATP and stimulated by ADP. 

Second step: Oxidation of pyruvic acid: 

Location- matrix of the mitochondrion 

Entry of pyruvic acid into the matrix of mitochondria: Pyruvic acid is produced in the cytoplasm of the cell and directly crosses the outer membrane of the mitochondria via pores from the cytoplasm. Later, through transporters, OH ions cross the inner membrane of mitochondria and enter the matrix. 

The 3-carbon pyruvic acid in the matrix of the mitochondrion, under the action of the enzyme pyruvate dehydrogenase (a complex of several enzymes) (i) produces one molecule of CO (decarboxylation), (ii) produces one molecule of NADH + H' (oxidation) and (iii)) produces one molecule of two-carbon acetic acid, which is linked to coenzyme-A by a thioester bond to form two-carbon acetyl Co-A (Co-A conjugation). It is a three-step reaction that produces one molecule of CO2, one molecule of NADH + H+, and one molecule of acetyl Co-A. Acetyl Co-A is the chemical link between glycolysis and the Krebs cycle. So, the reaction producing acetyl Co-A from pyruvic acid is called Link Reaction. 

The Third step: Citric acid cycle or Krebs cycle Place- matrix of the mitochondrion in which acetyl CoA combines with oxaloacetic acid to produce CO2, energy molecules (ATP, FADH NADH + H') and oxaloacetic acid is regenerated in case cycle.

1. Acetyl Co-A. The 4-carbon oxaloacetic acid located in the matrix combines with the 6-carbon citric acid, and Co-A dissociates. Citrate Synthera aids in enzyme reactions. The reaction is unidirectional. Oxaloacetic acid is called a resident molecule due to its permanent location in the matrix. The first substance produced in this cycle is called citric acid. This cycle is called the citric acid cycle. 

2. Citric acid undergoes isomeric transformation to isocitric acid. Aconitase (aconitase) enzyme helps in this reaction. The reaction is bidirectional. 

3. Isocitric acid loses CO and 2H to form alpha-ketotoric acid. One molecule of NAD produces one molecule of NADH+H,' and one molecule of CO is formed. Isocitrate dehydrogenase (isocitrate dehydrogenase) enzyme helps in the reaction. The reaction is unidirectional. 

4. Alpha ketobutyric acid combines with Co-A to form succinyl Co-A. Here one molecule of NAD produces one molecule of NADH + H and one molecule of CO. Alpha-ketoglutarate dehydrogenase enzyme helps in this reaction. The reaction is unidirectional. 

5. Succinyl Co-A. Co-A is lost to succinic acid. Substrate level phosphorylation produces one molecule of ATP (ADP + Pi = ATP). Co-A separates. Succinyl Co-A synthetase (Succinyl Co-A synthetase) enzyme helps in the reaction. The reaction is unidirectional.  

6. Succinic acid loses 2H to form fumaric acid. Here one molecule of FAD is formed from one molecule of FADH. Succinate dehydrogenase (Succinate dehydrogenase) enzyme helps in the reaction. The reaction is bidirectional. 

7. Fumaric acid accepts a molecule of water to form malic acid. Fumarase (fumarase) enzyme helps in this reaction. The reaction is bidirectional. 

The main regulator of the Krebs cycle is the isocitrate dehydrogenase enzyme. ADP, NAD is its stimulant; ATP and NADH + H are inhibitors. This cycle is stopped when ATP or NADH + H' accumulates too much. In the Krebs cycle, four molecules of CO2, six molecules of NADH + H', two molecules of FADH, and two molecules of ATP are produced per glucose molecule. At the end of the Krebs cycle, the glucose molecule is completely dissociated. Carbon and oxygen are released as CO2 and released as waste. Hydrogens are accepted by NADH + H' and FADH2. Electrons in hydrogen remain as chemical potential energy. 

Difference between plant and animal Krebs cycle

1. Succinyl Co-A synthetase produces ATP in plants but GTP in animals. GTP is then converted to ATP by an enzyme reaction.

2. NAD-malic enzyme has been found in all plant mitochondria tested to date. This enzyme converts malic acid (malate) to pyruvic acid, which enters the Krebs cycle to form acetyl Co-A. No such reaction occurs in animals.


 Importance of Glycolysis Process

The glycolysis process is an important step in metabolism.  

- (1) Different substances from glucose to pyruvic acid generate a number of cellular components in different subtopic pathways. 

- (2) The ATP or NADH + H+ available to go from glucose to pyruvic acid is only 17% of the total latent energy. Only 3% of the energy is released as thermal energy, and about 80% of the energy is still stored in pyruvic acid remains  

- (3) Formation of pyruvic acid is key to this process. If pyruvic acid is not produced, respiration will stop. If respiration stops, the biosphere will be destroyed.



Glucose is produced in the reverse process of glycolysis. Gluconeogenesis. It is less in plants than in animals. Still, oil stored in castor seeds, sunflower seeds, etc., is converted into sucrose or glucose in the process of gluconeogenesis, which subsequently aids in the growth of seedlings that germinate from the seed. 

Difference between glycolysis and photolysis

Matter of difference



1. the process of occurrence

It occurs during respiration

The process occurs during photosynthesis.

2. the Venue

It is done in the cytoplasm of the cell.

Done in the granum region of the chloroplast.

3. The light

It does not require sunlight.

It requires sunlight.

4. the product

In this process, pyruvic acid is produced from glucose.

In this process, electrons, protons and oxygen are produced from water.

5. name of the process

This process is called the EMP pathway.

This process is similar to the Hill reaction.

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