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Types of Respiration

Respiration is classified into two major types, depending on the availability of oxygen:
  1. Aerobic respiration
  2. Anaerobic respiration
Aerobic Respiration
Aerobic respiration is the release of energy from glucose or another organic substrate in the presence of oxygen. Strictly speaking, aerobic means in air, but it is the oxygen in the air, which is necessary for aerobic respiration. This type of respiration is most common in higher organisms.

Anaerobic Respiration
Some organisms can respire in the absence of air. This is called anaerobic respiration. This does not release so much energy and it produces much more toxic waste products. This type of respiration generally occurs in lower organisms, such as bacteria and fungi. It also occurs in higher plants and animals under certain conditions, particularly when O2 is limited. In anaerobic respiration, the carbohydrate is incompletely oxidized into some carbonic compounds, such as ethyl alcohol or acetic acid or lactic acid and CO2 and the amount of energy released is also much less when compared to aerobic respiration. This process can be shown by the following equation:
C6H12O6 → 2C2H5OH + 2CO2 + Energy (247 kJ)

This process of oxidation in microbes is known as fermentation, which is very much similar to that of anaerobic respiration in the case of higher plants.

Mechanism of Respiration

In respiration, the carbohydrates are converted into pyruvic acid through a series of reactions. These reactions, put together, are known as glycolysis and take place in the cytosol. The pyruvic acid thus formed enters the mitochondria, where O2 and the necessary enzymes are available, and pyruvic acid is finally converted into CO2 and H2O. This reaction series is known as Krebs cycle or tricarboxylic acid (TCA) or citric acid cycle.


The term glycolysis has originated from the Greek words, glycos for sugar, and lysis for splitting. The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof and J. Parnas, and is often referred to as the EMP pathway, after the abbreviation of their last names. Glycolysis is the first stage in the breakdown of glucose and is common to all organisms. In anaerobic organisms, it is the only process in respiration. Glycolysis occurs in cytoplasm of the cell. In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid. In plants, this glucose is derived from sucrose, which is the end product of photosynthetic carbon reactions, or from storage carbohydrates. Sucrose is converted into glucose and fructose by the enzyme invertase, and these two monosaccharides can readily enter the glycolytic pathway. 

Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate and fructose-6-phosphate, respectively, by the activity of the enzyme hexokinase. The phosphorylated form of glucose then isomerises to produce fructose-6-phosphate. Subsequent steps of metabolism of glucose and fructose are the same. Fructose-6-phosphate is phosphorylated and fructose-1, 6-bisphosphate produced by the action of the enzyme phosphofructokinase, is split into two molecules of triose phosphate, that is, 3-phosphoglyceraldehyde and dihydroxyacetone phosphate, which are interconvertible. Once 3-phosphoglyceraldehyde is formed, the glycolytic pathway enters the energy conserving phase and is oxidized to a carboxylic acid (1, 3-bisphosphoglycerate), and NAD+ is reduced to NADH. In the next step of glycolysis, phosphoglycerate kinase catalyses the formation of 3-phosphoglycerate from 1, 3-bisphosphoglycerate, generating ATP in the process. This type of ATP generation, whereby a phosphate group is directly transferred from substrate to ADP to form ATP, is distinctly different from the ATP produced by ATP synthase during oxidative phosphorylation in the mitochondria or photophosphorylation in chloroplasts. Subsequently, 3-phosphoglycerate is successively converted into 2-phosphoglycerate and phosphoenolpyruvate (PEP). PEP is a good donor for the formation of pyruvate and liberates ATP.

In the above pathway, the molecules of ATP are produced in two ways:
  1. Direct transfer of phosphate to ADP and
  2. Oxidation of NADH produced during glycolysis to NAD+.
Each molecule of NADH gives rise to three molecules of ATP. In the glycolysis scheme described, it is obvious that two triose phosphate molecules are formed from one glucose molecule and 4 ATP molecules are produced. Out of these 4 ATP molecules, 2 ATP molecules are consumed initially in converting glucose to fructose-1, 6-bisphosphate. In addition, three molecules of ATP are produced from the oxidation of each of the two molecules of NADH produced during catabolism of glucose. Therefore, a net gain of 8 ATP molecules occurs during glycolysis.

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