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GENERAL SCIENCE | Respiration

GENERAL SCIENCE | Respiration

Light energy is converted into chemical energy and stored in complex organic molecules called carbohydrates – glucose and starch. 
The breaking of C – C bonds of such compounds through oxidation releases a considerable amount of energy. 
This energy is utilized for various metabolic activities at cellular level. 
This phenomenon of release of energy by oxidation of various organic molecules is known as respiration. 
The compounds that are oxidised during this process are known as respiratory substrates. 
Carbohydrate is the common respiratory substrate. 
During respiration, the whole energy contained in the respiratory substrate is not released all at once. 
In respiration, oxygen is utilized and carbondioxide, water and energy are released.
Respiration is an exothermic reaction and the oxidation of glucose is given in the following equation. 

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The energy released during this process is transformed into usable form of energy as adenosine triphosphate (ATP). 
ATP molecules act as carriers of free energy between energy yielding and energy requiring reactions of the cell. 
Thus, ATP is described as energy currency of the cell. 
It is a nucloetide consisting of adenine, ribose sugar and three phosphate groups.
It is an energy rich compound and contains two high energy terminal bonds. 
A large amount of free energy is liberated, when these bonds are broken by hydrolysis

Mechanism of respiration 
Oxidation of glucose involves following four distinct stages – glycolysis, oxidative decarboxylation of pyruvic acid, Krebs cycle and Electron transport chain. 
In the first three stages, the hydrogen acceptor Nicotinamide adenine dinucleotide – oxidized form (NAD+) and Flavin adenine dinucleotide – oxidized form (FAD+) are reduced to NADH2 and FADH2 respectively. 
Both the coenzymes, (NAD+) and (FAD+) act as hydrogen carriers from respiratory substrate to electron transport chain, where H+ and electrons are transferred to oxygen to form water. 
This electron transport results in the release of energy, which is used to phosphorylate ADP to ATP. 
Hence, the electron transport chain reactions are referred to as oxidative phosphorylation.

Glycolysis
The process by which the glucose (6C compound) is split into two molecules of pyruvic acid (3C compound) is called glycolysis. 
Three German Microbiologists – Embden, Meyerhof and Parnas, first demonstrated this process in yeast cell. 
Hence, it is otherwise known as EMP pathway. 
It occurs in cytoplasm. 
It is common in all organisms.
It is divided into two phases – hexose phase and triose phase. 
Glyceraldehyde 3-phosphate and DHAP are the products of hexose phase and two molecules of pyruvic acid are the products of triose phase.

The overall reaction of glycolysis is given in the following equation. 

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Reactions involved in glycolysis are as follows 
1. The glucose is phosphorylated with ATP to form glucose-6- phosphate. The reaction is catalyzed by the enzyme hexokinase. 
2. Glucose-6-phosphate is isomerized to form fructose-6-phosphate by phosphoglucoisomerase. 
3. Fructose-6-phosphate is then phosphorylated using ATP to form fructose 1,6-bisphosphate. 
     This reaction is catalyzed by phosphofructokinase. The ATP is dephosphorylated to ADP. 
4. Fructose 1,6-bisphosphate is cleaved by the enzyme aldolase to two molecules of 3C compounds – dihydroxy acetone phosphate (DHAP) and glyceraldehyde 3-phosphate. These two trioses are isomers.
5. DHAP and glyceraldehyde-3-phosphate are interconvertible by the action of triose phosphate isomerase. These five series of reaction constitute hexose phase and produce two molecules of 3-carbon compound called 3- phosphoglyceraldehyde. In hexose phase two ATP molecules are consumed. 
6. A molecule of glyceraldehyde-3-phosphate is phosphorylated and oxidized to 1,3-bisphosphoglyceric acid in the presence of glyceraldehyde- 3-phosphate dehydrogenase. During this reaction, one NADH2 is formed. 
7. 1,3-bisphosphoglyceric acid is dephosphorylated to a molecule of 3-phosphoglyceric acid by phosphoglyceric kinase. During this reaction one ATP is formed. This type of ATP synthesis is called direct phosphorylation or substrate level phosphorylation. 
8. A molecule of 3-phosphoglyceric acid is then converted into a molecule of 2-phosphoglyceric acid by phosphoglyceric mutase. In this reaction, phosphate molecule is shifted form third carbon to second carbon. 
9. A molecule of 2-phosphoglyceric acid is dehydrated to a molecule of 2-phosphoenol pyruvic acid by enolase. Removal of water molecule from the substrate is called enolation. 
10. A molecule of 2-phosphoenol pyruvic acid is dephosphorylated to pyruvic acid and ADP is phosphorylated to ATP. This reaction is catalyzed by pyruvic kinase. Thus, in the triose phase, two molecules of a molecule of 3-phospho glyceraldehyde produce 2 molecules of pyruvic acid. 

In glycolysis, 4ATP and 2NADH2 molecules are formed and 2ATP molecules are consumed in hexose phase. Hence, the net gain is 2ATP and 2NADH2 .

Energy yield
  • Complete oxidation of one glucose molecule yields a net gain of 38ATP. 
  • Out of 38ATP molecules, 4ATP are obtained by direct substrate level phosphorylation, 30ATP through oxidation of NADH2 and 4ATP through oxidation of FADH2 . 
  • Since, a large number of ATP molecules are produced in the mitochondria, they are called the 'power houses of the cell'.


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