What is Respiration? Definition of its Process & Equation

What is respiration in biology? Explain with Simple

The activities of living things require energy. This energy inside the cell is provided by breathing. Breathing is a common process. The decomposition of complex carbon compounds and the release of the maximum energy available in cells is called respiration.

There are two ways to use the term breathing in biology:

  • The exchange of respiratory gases (CO2 and O2) between the organism and its environment is called external respiration. This is the most common term.
  • The gradual degradation of C chain molecules and the release of energy within the cell is called cellular respiration.
  • Respiration is a series of oxidation-reduction reactions that are controlled by enzymes, during which carbohydrates (respiratory substrates) are oxidized which are produced during photosynthesis to carbon dioxide, and oxygen is reduced to water. With the respiratory substrate fracture, the energy is released as a result of the association.
respiration of plants

Most of this energy is stored in adenosine triphosphate (ATP) molecules. Since this process takes place inside the cell, it is also called tissue respiration. The process of obtaining oxygen from the environment or discharging carbon dioxide into the environment is called gas exchange or external respiration. The process can be summarized as follows:

respiration formula

Types of Respiration

Glucose is the most common fuel in cells. Provides energy through cellular respiration. Glucose metabolism depends on oxygen availability. The glucose molecule forms two pyruvate molecules before they divide into mitochondria. This reaction is called glycolysis. Ethylene glycol means glucose, and cleavage means cleavage. So, glycolysis means literally dividing sugar. It occurs in the cytoplasm.

respiration formula 2

This reaction occurs in all cells. Biologists believe that the same reaction may have occurred in the first cell that formed on Earth.

In different cell types, the next step differs in cellular respiration. There are three ways to treat pyruvate:

  • Alcoholic fermentation
  • Fermentation of lactic acid
  • Air-breathing

The first two reactions occur in anaerobic conditions. Therefore, these reactions are called anaerobic (anaerobic) reactions. The complete decomposition of glucose molecules occurs only in the presence of oxygen, that is, during air respiration. During breathing, glucose is oxidized in carbon dioxide and water, and energy is released.

Breathing that requires oxygen is called aerobic respiration, and it occurs without oxygen, and this is anaerobic breathing, but it can also happen with oxygen. Some organisms (such as Clostridia) can only breathe without oxygen. All animals and plants need oxygen and aerobic respiration, so they are called aerobic bacteria. The final products of air-breathing are carbon dioxide and water.

1. Anaerobic Respiration

Anaerobic respiration occurs in the absence of oxygen. It has two types:

  1. Alcoholic fermentation: It occurs in some primitive cells and some eukaryotic cells, such as yeast. In this case, pyruvic acid is dissolved into alcohol and carbon dioxide by fermenting alcohol.
Anaerobic Respiration

2. Lactic acid fermentation: In this case, each pyruvate molecule is converted to C3H6O3 lactic acid.

Lactic acid fermentation

This anaerobic respiration occurs in the muscle cells of humans and other animals. It occurs in extreme sports like running. In such extreme activities, oxygen cannot pass into cells. Therefore, pyruvate becomes lactic acid.

Fermentation of alcohol and lactic acid produces a small amount of energy from glucose molecules. Only 2% of glucose energy is converted to ATP (adenosine triphosphate).

2. Aerobic Respiration

It occurs in the presence of oxygen. (

Significance of Respiration

The important importance of the breathing process is: It is basically a power supply mechanism. The stored energy of the compound is converted to the energy available to ATP.

Although the most important aspect of the respiratory tract is energy saving, many important metabolic intermediates that are produced during the glycolysis pathway and the TCA cycle are converted into important metabolic products for cells. Respiratory metabolites include amino acids. Pentose for cell wall and synthesis of nucleotides, porphyrin precursors, and Acetyl CoA (CoA) to synthesize fatty acids, carotenoid, gibberellin, and acetic acid.


The quotient of the respiratory system is the ratio of the number of moles of carbon dioxide emitted from tissue over a period of time to the number of moles of O2 absorbed. Therefore, the reaction

respiration formula 3

The nature of the respiratory matrix in plant tissues can be determined by calculating the respiratory quotient of tissues.

  1. If glucose (carbohydrates) is a respiratory substrate, then the respiratory quotient will be:
respiration formula 4

And therefore,

Glucose (carbohydrates) RQ = 1

2. The oxygen content of fats is very low, as these fats will not directly oxidize. First, dissolve it in fatty acids and glycerin. In this process, part of the oxygen is consumed, so the fat needs more oxygen to be fully oxidized, and the RQ is less than 1. e. , 1

respiration formula 5

And therefore,

Fat RO = 0.7

  • Like fat, protein is not directly involved in breathing. Turn them first into an amide or ammonia. Therefore, these require more oxygen completely for oxidation. The range of RO values ​​is 079 (for amide) and 0.99 (for ammonia).
  • In succulents with malic acid as the only respiratory substrate, the respiratory quotient is:
respiration formula 6

Respiratory Substrates

Any organic vegetable component that is partially or completely oxidized to carbon dioxide and water during breathing is called the respiratory substrate. Carbohydrates are the main respiratory pillars of high plant cells.

The main ones are sucrose and starch. Sucrose, fructose, and glucose are the main soluble sugars in plant cells. Likewise, sucrose is the main carbohydrate transported in plants. Starch is the main reserve for plants.

In addition to carbohydrates, other substances are sometimes used as respiratory substrates, for example. For example, castor oil seeds are rich in fat reserves stored in the endosperm tissues. During the germination of these seeds, the fat is converted to sucrose, which the growing fetus breathes.

In some tissues, organic acids can be used as respiratory substrates. For example, at night, 4-C malic acid accumulates in the leaves of succulent plants (plants with flesh, sediments belonging to Crassulaceae) and is oxidized in CO2 and water. Likewise, glycolic acid (2-C organic acid produced under higher plant lighting) is also used as a respiratory substrate.

Protein rarely breathes in special cases. In isolated leaves, protein degradation is observed. In seeds with high protein reserves, the protein acts as a substrate for respiration in the early stages of germination. The protein is first broken down into amino acids, then converted into an intermediate, and then oxidized to carbon dioxide and water.

Role of Mitochondria in Respiration

Mitochondria are large granular or filamentous organelles. It is distributed throughout the cytoplasm of animal and plant cells. Mitochondria consist of two membranes. The film is wrapped around the outside. The inner membrane contains folds or precise markings. These cracks extend to the mitochondria.

Mitochondria are involved in cellular respiration. They convert the energy of organic molecules into chemical energy in the form of ATP. Mitochondria contain many enzymes and coenzymes. These enzymes and enzymes slowly release energy from glucose molecules. This energy is used in many cell functions. Therefore, mitochondria are the “energy source” of cells.

Adenosine Triphosphate (ATP) and its Importance

Adenosine triphosphate is usually abbreviated as ATP. They are found in every living cell. ATP is an essential chemical in life. It plays a major role in converting most of the vital energy.

By convention, “P” represents the entire phosphate group. The second and third phosphate molecules have high energy bonds. Upon hydrolysis, these bonds release more energy than other ATP molecules. The split of the final ATP phosphate molecule releases about 7.3 kcal of energy. Therefore, the unit can store a large amount of energy in a small space. ATP power is always available.

Cells use the energy of ATP molecules to achieve different functions. Some of these functions are a synthesis of complex compounds, active transport across cell membranes, muscle contraction, and nerve conduction.

Biological Oxidation

Living organisms need to provide free energy constantly. This energy comes from various redox reactions. The photosynthesis and chemical synthesis of some bacteria is in itself an oxidation reaction. Therefore, they do not need free energy. All other interactions require free energy.

This energy comes from oxidation while breathing. In some cases, biological oxidation involves removing hydrogen. The reaction is stimulated by dehydrogenase. Dehydrogenase is associated with some other specific enzymes. Cellular respiration is an oxidation process.


Respiration indicates that breathing is a multi-step process in which glucose is oxidized in a series of reactions. These reactions can be divided into three stages:

I. Glycolysis

It is made by a group of soluble enzymes present in the cytoplasm (the liquid part of the cytoplasm). Chemically, glucose undergoes a limited amount of oxidation to produce two molecules of pyruvate (3C compounds), ATP, and reduced NADH nucleotide.

2. Carboxylic acid cycle (TCA) or Krebs cycle

This cycle completely oxidizes the pyruvate to carbon dioxide and water. In this cycle, about 10 NADH are produced. The TCA cycle works in the mitochondrial matrix where the soluble enzyme is present. This stage is also called oxidizing carboxyl removal because during this stage carbon atoms are oxidized to carbon dioxide.

3. The electronic transmission chain

It consists of electronic vectors bound to mitochondrial membrane proteins. The system converts the electrons in the NADH from glycolysis and the TCA cycle to oxygen. Wire transfer releases a lot of energy. Most of them are stored in ATP produced by ADP and Pi.

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