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The human body is a device for taking in energy and converting it to other forms. Mast obviously this is in the form of physical work such as lifting heavy objects.

Energy is taken into the body in the form of food. The components of food may be classified as macronutrients and micronutrients. Micronutrients play vital roles in the metabolism than macronutrients.


What is Metabolism?

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If there is an activity going continuously in a body it requires two actions to be taken. One is storing energy and the other is spending the energy. The mechanism of storing the energy available from outside so that it can be spent in installments when the energy source is not there, and the mechanism of converting the stored form in to active energy to utilize for useful purpose for the activities to carry on is called metabolism.
The source of energy may be direct as in plants which can convert the solar energy and store it in the form of carbohydrates by synthesizing from carbon di oxide and water. In animals this mechanism is not there and they depend on those carbohydrates from plants to get energy. These carbohydrates are further transformed in to various other forms for storing (Fats) and for other activities Enzymes, Vitamins, hormones and Proteins etc.

Conversion of energy into useful purposes like growth and regeneration is called anabolism. Anabolism is an endothermic process since it absorbs energy. This continuous energy is supplied by various substances at the expense of their being and this process is called catabolism. Since catabolism releases energy it is an exothermic process. Together the Anabolism and Catabolism is called metabolism. When there is more anabolism and less catabolism, the growth occurs and when the catabolism over powers the anabolism, the death occurs.
One of the important category of organic compounds is the amino acids. These are the molecules having amino group as well as carboxylic group in their molecular structure. In general terms these are carriers of acidic as well as basic components with them separated by a neutral molecular chain. These amino acids combine in numbers and form large molecular units called proteins.

Anabolism is defined as a process in which simpler molecules combines to form newer and more complex compounds. Living cells combine amino acids to form structural proteins and functional proteins. Structural proteins are those which help repair and replace tissues. Enzymes, antibodies and hormones are functional proteins. The purpose of these is to catalyze chemical reactions, help fight disease and regulate body processes respectively.

Protein Anabolism

  1. Protein anabolism is a process of building up of the proteins. With the involvement of enzymes and Ribonucleic acid (RNA) amino acids combine with their amino group in peptide linkages and forms long chain proteins which is called protein anabolism.
  2. Protein anabolism is necessary for body muscle building, strength growth and recovery.
  3. Adenosinetriphosphate (ATP) is one of the essential conduits that help in bringing together the smaller amino acids in to proteins.
  4. These are assisted by growth hormones called anabolic steroids. Which will act as stimulants for the growth of proteins and in turn the muscles.
  5. Protein anabolism is essential to reduce fat and increase the muscular strength.
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Catabolism is the process of breaking down of sugars like glucose, fatty acids and amino acids. This process gives energy and heat. The food taken gets digested in the stomach with the help of various enzymes and digestive juices in to energy carriers. These energy carriers along with storage glycogen or body protein are the compounds involved in the process of breaking down in to smaller units.

The breaking down of fatty acids takes place into two steps.
In the first step, enzymes act on fatty acids and convert them into acetyl coenzyme A. Acetyl coenzymeA in turn undergoes a series of chemical reactions to release energy and break up in to smaller units which are again used as material for anabolism step. This cycle is known as Krebs cycle.

In the same way Glucose catabolism takes place in two stages.
  1. Glycolysis is the first step in which in the absence of oxygen glucose breaks down into pyruvic acid with the release of small amount of energy. The absence of oxygen prevents the conversion of pyruvic acid to Acetyl CoA.
  2. In the second stage the reactions of Krebs cycle takes place. In this stage firstly in presence of oxygen the puruvic acid gets converted in to Acetyl coenzyme A which in turn undergoes a series of chemical reactions to produce carbon dioxide, water and energy. About 40% of the energy produced during the process of catabolism is utilized in the formation of a compound commonly known as ATP, adenosine triphosphate. This ATP breaks down again when required releasing the energy. The remaining 60% of the energy is in the form of heat.
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Oxidative Metabolism

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Oxidative metabolism is often referred to as cellular respiration because the energy released can be utilized at cellular level. In this process the chemical bonds of molecules such as glucose are converted into energy. Since the sugar conversion in to energy happens only with oxygen as primary source It is referred to as "oxidative metabolism".
  • Oxidative metabolism happens during the process of respiration. For example in the oxidative metabolism of glucose, Oxidation of glucose with oxygen produce carbon dioxide, water, and energy.
  • Production of carbon di oxide and water with the release of energy as heat occurs even in the burning of hydrocarbons but the energy released is uncontrollable.
  • Oxidative metabolism occurs in gradual steps and the energy released in controlled fashion is converted in to chemical energy and this chemical energy is stored in adenosine triphosphate (ATP) in the form of bonds.
  • These ATP bonds carry energy throughout cells, releasing it when required by enzyme catalytic reactions.
  • The oxidative metabolism in presence of air is termed as aerobic respiration reaction.
  • In the absence of oxygen the oxidative metabolism occurs in very low efficiency through fermentation process. This is called anaerobic respiration and by this process only 1/18th of the ATP is produced.

Sugar Metabolism

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Sugar metabolism is the process of conversion of carbohydrates and fats in to energy. This type of metabolism requires enzymatic and hormonal action in order to ensure the distribution of energy to the cell level. The diagram given below gives a general idea of the chain of reactions that take place in the human body. Starches, sugars are first converted to glucose and fructose by the enzymatic action.

These on further action of the enzymes get converted to glucose 6 phosphate and fructose 1 phosphate. G-6-P gets converted to F-6-P which in turn becomes F-1,6-biP by a specific enzyme. The DHAP( di hydro adenine phosphate) with glyceraldehyde breaks in to pyruvate and ATP. ATP finally gives CO2 and releases energy.

Sugar Metabolism

Lipid Metabolism Pathway

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There are multiple steps involved in metabolism of lipid. As a first step is hydrolysis of lipid happens in cytoplasm which produces glycerol and fatty acids. Since glycerol belongs to three atoms of carbon alcohol, it very easily gets metabolized into an intermediate in glycolysis, dihydroxyacetone phosphate. The last reaction is readily reversible if glycerol is needed for the synthesis of a lipid.

Lipid Metabolism

Hydroxyacetone which is obtained from the glycerol is further metabolized into useful compounds. Through glycolysis, dihydroxyacetone is converted to pyruvic acid. It also finds application in gluconeogenesis and is used to produce glucose-6-phosphate which is actually glucose to blood or glycogen that depends on requirement. With the help of the fatty acid spiral, oxidation of fatty acids takes place CoA in mitochondria to convert it to acetyl. With the help of electron transport chain and citric acid cycle acetyl CoA is changed into CO2, ATP and H2O.

Most of the fatty acids are obtained from carbohydrates and sometimes from proteins. Proteins and Carbohydrates are first catabolized to obtain acetyl CoA. Acetyl CoA enters into citric acid cycle depending upon the requirement of energy; it is also used in the synthesis of fatty acids through the process of Lipogenesis.

More topics in Metabolism
Biochemical Catabolism
Anabolism Oxidative Phosphorylation
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