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The cell is too small to be seen with our own naked eye and it can be seen with the help of the microscope. It is the fundamental unit of life. They are commonly referred as essential packed of chemical essential for life. The most important characteristics of the cell are its ability to grow and divide to produce daughter cells. This daughter cell again further divide to produce new progeny of cells. They are combined to form tissues, and the tissue may be grouped into organs and organs may be combined into an organism.
A typical animal cell is described diagrammatically in fig below.
A living cell is composed of about 50 elements and the 11 elements are the most abundant element. They are O, C, H, N, Ca, P , K, S, Cl, Na, and Mg. The four first element mainly the oxygen, carbon, and nitrogen together accounts for 96% of the total mass. The most important substance in a living cell is water which amounts to about 70% of the weight. The water plays an important role in every aspect of life. In addition of water, the cell contains a large amount of carbon compound. These compounds can be divided into two classes. They are.
1. Small molecules
The chemical compound which contains molecular masses in the range of about 100 to 1000 and containing up to 30 carbon atoms are called small molecules. These small molecules are found free in solution in the cytoplasm of the cell. They form a number of the intermediate from which the molecules belonging to the second class are synthesis.
The compound of the second class is macromolecules. Carbohydrate, protein nucleic acids and lipids are the most important component of macromolecules.
The amino acid is the monomer of protein. They contain the carboxylic group (COOH), an amino group (NH2) and a side chain attached to the same carbon atom. Carboxylic acid which contains amino group in αcarbon is called amino acid. As the amino group is attached with () carbon, all amino acid are called as α amino acid.
There are about more than 3oo amino acid in nature. Among them, only 20 of them are found in protein. Such amino acid are called as standard amino acid or proteinogenic). Different types of the amino acid depend on nature of side chain.
In amino acid central carbon atom attached to four different groups. Therefore, it exhibits stereoisomerism. There are two stereoisomers of amino acid I,e L-amino and D-amino form.
An amino acid having NH2 group in right side belongs to D-form. And those heaving NH2 group in left side belong to L- form. L-form is most common and found I plant and animal and also in a microorganism. D- form is very rare found in a special bacterial structure such as peptidoglycan and polypeptide capsule of Bacillus anthracis.
At physiological P.H both amino acid group and carboxylic group of amino acid are ionised. Therefore, in the cell, all amino acids exist as an ionizable molecule.
Types of amino acid.
On the basis of body requirement, an amino acid is classified into two types.
Essential amino acid.
An amino acid which is not synthesis in inside cell of human beings is called essential amino acids. This amino acid must be essentially present in the diet. Among 20 amino acid 10 are essential and must be included in our diet. The essential amino acid is PVTTIMHALL.
Histidine and Arginine are considered as non-essential or semi-essential amino acid. This is not required for adults because adults can synthesise this two amino acid. However, infants and babies require this amino acid in the diet.
Non-essential amino acid.
An amino acid which is the synthesis in the cell of human beings is called non-essential amino acids. They did not essentially present in the diet.
Example. Glycine, Alanine, Serine, Cystein, Asparagin, Glutamine, Aspartate, Glutamate, Tyrosine, Proline.
Zwitter ion or dipole ion.
An amino acid contains both an acidic carboxylic (-COOH) group and a basic amino (-NH2) group in the same molecule. In aqueous solution, Carboxylic group (-COOH) of an amino acid loses its proton to give carboxylate ( –COO- ) and the amino group (-NH2) of same amino acid gains a proton to give –NH3+. This type of ion which contains both positive and negative charge in a single molecule is called twitter ion or dipole ion. In other words, the amino acid which develops both +Ve and -ve charge inside the molecule is called Zwitter ion or dipole ion.
Evidence in support of Zwitter ion.
The evidence to support the Zwitter ion is as follows.
Bahl, B S, Bahl, and Arun. Advanced Organic chemistry. S. Chand and company Ltd., n.d.
Sthapit, M K, R R Pradhananga, and K B Bajracharya. Foundations of chemistry. Taleju Prakashan, n.d.
Tewari, K S, S N Mehrotra, and N K Vishnoi. A textbook of organic chemistry. Vikash publishing House Pvt. ltd., n.d.
Verma, N K and S K Khanna. Compressive chemistry. 8th edition. Laxmi publications P. Ltd., 1999.