Mechanism of Respiration

 Respiratory Substrate

The substrates, which are broken down in respiration for the release of energy, maybe carbohydrates, fats, or proteins. Proteins are used up in respiration only when carbohydrates and fats are not available.

As regards carbohydrates, not only simple hexose sugars like glucose and fructose but complex disaccharides particularly sucrose and polysaccharides such as lignin, inulin, and hemicellulose are also used as respiratory substrates. Fats are used as respiratory substrates after their hydrolysis to fatty acids and glycerol by lipase and their subsequent conversion to hexose sugars. Proteins serve as respiratory substrates after their breakdown into aminoacids by proteolytic enzymes.

During respiration, the complex substrates are broken down into simpler ones and finally, CO2 is liberated and water is formed. During the oxidation of the respiratory substrate, some energy is released. Part of this energy is trapped in the form of energy-rich compounds such as ATP while the remaining part is lost in the form of heat. The energy trapped in ATP molecules can be used in various ways.

All complex carbohydrates are firstly converted into hexoses (glucose or fructose) before actually entering into the respiratory process. The oxidation of glucose to CO2 and water consists of three distinguishable

phases:

A. Glycolysis

B. Kreb’s cycle

C. Electron Transport System (ETS) or Terminal Oxidation

Details of reactions involved in these three processes are furnished below:

A. Glycolysis

(EMP=Embden Meyerhof Paranas Pathway; Common Respiratory Pathway; Cytoplasmic Respiration) The course of step-wise degradation from glucose to pyruvic acid is termed glycolysis. After the name of its tracers, the glycolytic pathway is also known as Embden Meyerhof Paranas Pathway (EMP pathway). The fate of pyruvic acid, however, depends on the presence or absence of O2. In the presence of O2, the final degradation products are CO2 and water in Kreb’s cycle; while in the absence of O2 ethyl alcohol and CO2 in fermentation.

Glycolysis, fermentation, anaerobic respiration, and lactic acid formation processes occur freely in the cytoplasm while Kreb’s cycle occurs in the matrix of mitochondria in the eukaryotic cells and on the surface of mesosomes in prokaryotic cells. Enzymes of glycolysis are found in the cytoplasm in the soluble form, called cytosol. These remain active throughout their lifetime and are required again and again. Such enzymes are called constitutive enzymes.

Therefore, it states that one molecule of glucose which is a 6-carbon compound is broken down into two molecules of pyruvic acid which is a 3-carbon compound through a large number of reactions. It occurs in the following three important phases.

Phase I

In the first phase of glycolysis, the glucose molecule is phosphorylated with the introduction of two phosphate groups. For this, two ATP molecules are needed.

Phase II

It involves the breaking up of 6-carbon compounds, Fructose 1,6-diphosphate into two molecules of 3-carbon compounds, 3- phosphoglyceraldehyde (3-PGAld), and Dihydroxyacetone phosphate. These two 3-carbon compounds are inter-convertible.

Phase III

During this phase, degradation of 3-PGAld into pyruvic acid takes place with the production of four molecules of ATP. As in the phosphorylation of glucose during the first phase, two molecules of ATP have already been used up, there is a net gain of only two molecules of ATP during glycolytic reactions.

Transphosphorylation (Phosphorylation in Glycolysis)

The kind of reaction in which a phosphate group is transferred from another already phosphorylated compound i.e., ADP to form ATP is called transphosphorylation. With the production of pyruvic acid, glycolysis comes to an end. During these reactions, two ATP molecules are used up, and four ATP molecules are produced. Thus, there is a net gain of two ATP molecules. However, during glycolysis, two NADH2 molecules are also produced, from each, three ATP molecules are produced. As a result, 6ATP molecules are formed from Electron Transport System (ETS) chain. Thus, there is a total gain of 8 ATP molecules in glycolysis.