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Growth is the natural phenomenon in every living organisms. Through growth they maintain their ability to sustain in the environment and to reproduce. Living organisms take the food to produce energy through by which they work and perform such physical or chemical changes for their growth.

The sum of all physical and chemical changes for a living organism in its growth stage is called as metabolism. Here in Greek 'metabole' means 'to change'. Hence the word metabolism mainly deals with changes that occur in the living organism.

This change can be of two types.

  1. Anabolism or constructive metabolism: In this pathway many small molecules are combined together to give a complex molecule. For example the synthesis of proteins from amino acids is an anabolism activity. During this energy is consumed and new cells are generated. Insulin, testosterone and Estrogen are some anabolism hormones.
  2. Catabolism or destructive metabolism: In this pathway a large molecule is decomposed to give many small molecules along with energy. The breaking of a polysaccharide to simple glucose, breaking of lipid to simple saccharides and protein to amino acid are examples of catabolism. Cortisol and Adrenalin are some examples for catabolism hormones.


Catabolism Definition

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"Catabolism is the biological process through by which large molecules are broken into smaller molecules. Energy is released during this reaction."

In short we can say that anabolism process requires energy for its activity like producing enzymes, generating new cells etc for which energy is required and that energy is supplied by the catabolism reaction.

Examples of Catabolism

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There are many catabolism reactions in our body. Some of the reactions are listed below.
  1. Polysaccharides like cellulose and starch are complex molecules of glucose monomer unit. They are broken down to glucose units.
  2. Nucleic acids like DNA and RNA are made up of bases like purines and pyrimidines along with pentose sugar. They are broken down to their constituent bases and sugars which can be used as energy source.
  3. Proteins are complex polymer of many α-amino acids. They are broken down to simple amino acids which can in turn be again recycled to get another protein or even they may be decomposed to give glucose which is energy source.
  4. Glucagon is a hormone produced in our body. It interacts with glycogen stored in the pancreas of our body to break it to simple glucose. Glucose molecules are stored as glycogen in our body and they are broken when required by glucagon.

Carbohydrate Catabolism

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Carbohydrates are complex polymers of glucose and other sugar monomer units like fructose, arabinose etc. The number of monomer units will vary from 2 to 10 in the edible carbohydrates like starch, sugar etc. It may exceed upto many numbers in cellulose. They are degenerated to simple glucose units and other monossacharides by the way of carbohydrate catabolism.

Hence, carbohydrate catabolism includes all reaction for the breaking of complex carbohydrates to simple sugars and breaking of simple sugars to give waste products and energy.
  1. Polysaccharides are simply broken to mono-saccharides by hydrolysis with some enzymes.
  2. The simple monosaccharides are broken into carbon dioxide and water by the following redox reaction.
C6H12O6 + 6O2 6CO2 + 6H2O

Although the reaction is prescribed as a simple redox reaction it proceeds through many complex steps in which energy is transferred to ADP to give ATP.

The catabolism of glucose molecule is given in the following steps.


In this step glucose molecule is degraded to pyruvate molecule with two ADP to ATP conversions and two NAD+ to NADH conversions.

The energy is stored in ATP and NADH. The overall reaction is given below.

glucose + 2 NAD+ + 2 ADP + 2 Pi 2 pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O

The pyruvate produced may be decomposed to lactate with the consumption of one NADH. This is called as lactic acid fermentation. It is used to release energy in case of critical anaerobic conditions.

In the regular aerobic respiration, the pyruvate is first converted to acetyl coenzyme A by kreb's cycle. This is further decomposed to carbon dioxide and water. NADH are produced in the steps.

NADH produced will undergo oxidation and the potential developed is used to convert ADP to ATP. There are around 2.5 ATP molecules produced for each NADH oxidation.

Protein Catabolism

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Proteins are complex polymers of amino acids. The sequence of amino acids determine the nature and functional behavior of protein like in DNA and RNA. In protein catabolism the proteins are hydrolyzed back to amino acids.

The amino acids produced may be used in two ways.
  1. They may be used to synthesize again another set of protein. Amino acids being a small molecule can be transported easily to the desired location across membranes and used to synthesize the desired protein in the desired place.
  2. Amino acids can be completely broken to give energy and waste products. This is the anabolism way by which energy is released from proteins in the absence of carbohydrates.

Amino Acid Catabolism

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Amino acids are used as energy sources in some of the cases. In such cases they undergo disproportionation with enzymes to give NADH or pyruvate etc. Some of the examples for amino acid catabolism is given here.
  • Transaminase shifts the alkyl group between amino acid and pyruvate to get the desired amino acids.
The scheme of the reaction can be given as

Amino Acid Catabolism
  • Serine dehydratase catalyses the conversion of serine to pyruvate and ammonium ion.
Serine $\to $ Pyruvate + NH4+
  • Some of the amino acids are converted to the ketoacids with the conversion of NAD to NADH

Lipid Catabolism

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  1. The catabolism of lipid (otherwise long chain fatty acid) follows b-oxidation pathway because oxidation occurs at b-carbon or C3 carbon.
  2. Usually lipids will contain even number of carbon atoms. In the catabolism the fatty acids are first esterified to give coenzyme-A before oxidation.
  3. During the decomposition of fatty acid ATP molecules are produced from ADP which can be used as energy source.

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