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*Self-Practice*

Std.10th -Science Part 2

Topic 2: *Life Process in Living Organisms Part-1*

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श्रीशैल मठपती, कोल्हापूर

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*Today's exercise *:

 1) Draw a neet and well labeled diagram of Kreb's Cycle as given with this post. 

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 *Kreb's Cycle*

◆ The Krebs cycle is the second of three stages of cellular respiration. 

This stage is found only in aerobic organisms.

◆ All the reactions in the Kreb's cycle takes place in the matrix of mitochondria.

◆ Pyruvate formed in the process of glycolysis has to be converted to 'acetyl co-enzyme- A' by a chemical reaction before entering the Krebs cycle.  This reaction, which converts pyruvate to acetyl Co-A, is known as linking reaction in between glycolysis and the Krebs cycle.

◆ In an eight-step reaction sequence, the acetyl group of acetyl-CoA is oxidised into two molecules of CO2. These reactions are catalysed by eight different enzymes.

 ◆ Acetyl Co-A is first converted into six carbon citric acid by combining with four carbon oxalo-acetic acid.

◆ The Kreb's cycle is a cyclical reaction. It means first and last product of the reaction are same. 

◆ Citric acid is the first and last product in a series of 8 chemical reactions that take place in the Kreb's cycle.  Hence the Krebs cycle is known as the 'Citric acid cycle'.

◆ As the citric acid consists of three carboxylic groups, the Krebs cycle is also known as the 'Tri-carboxylic acid cycle'.

◆ Molecules like CO2, H2O, NADH, FADH2 and ATP are derived through Kreb's cycle.

◆ Instead of generating a large amount of ATP in the Kreb's cycle, 8 electrons are removed from the 'Acetil Co-A' and transported to co-enzyme NAD+ and FAD.  Therefore, NAD+ is reduced to NADH and FAD to FADH2. Through this co-enzymes the electrons are transported to the third stage i.e. the ETC reaction.

◆ Thease cyclical  reactions were discovered by Sir Hans Kreb. Hence this cyclical process is also called as  "Kreb's cycle".

◆ The Kreb's cycle is useful for energy production as well as for the bio-synthesis of glucose, fatty acids and amino acids.

◆ Research of Kreb's cycle has shown that even a single cell in an organism has an ability to produce glucose, fatty acids and amino acids.  Thus, the theory of evolution is confirmed by the evolution of life from unicellular organisms.

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*Shrishail Mathapati,* Kolhapur 

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