In this article we will discuss about the organic and inorganic compounds present in living body.
These are represented by A. Water and B. Inorganic salts.
This is most important for the living body. Major bulk of protoplasm is made up of water (66% in man; nearly 90% in jelly fish).
It does the following functions:
(i) acts as solvent for other inorganic and organic substances,
(ii) serves as a medium of every chemical reaction that occurs within the living body,
(iii) remains as a liquid for considerable range of temperature and is an excellent transporting medium and
(iv) plays a great role in regulating the effects of external temperature.
B. Inorganic Salts:
Though often present in insignificant quantity, the inorganic salts play following important roles:
(i) help in certain chemical reactions,
(ii) serve as precursor material for’ the synthesis of certain essential molecules (e.g., DNA),
(iii) take part in the formation of supporting and protecting structure of the soft parts (e.g., bone).
Following types of organic compounds exist in living substances—A. Carbohydrates, B. Lipids, C. Proteins, D. Nucleotides, E. Vitamins. In addition to these there are organic acids, alcohols and steroids, which are synthesised from other molecules.
These compounds contain carbon, hydrogen and oxygen atoms in their molecules, and the common trend of chemical composition is Cn(H2O)n. The carbohydrates serve as fuel and structural material.
Most of the carbohydrate molecules are very large with a molecular weight of 5,00,000 or more. Each molecule is made up of numerous similar units. Each unit is called sugar. The carbohydrates may be 1. Monosaccharide’s, 2. Disaccharides and 3. Polysaccharides.
Carbohydrate molecules which contain six or lesser number of carbon atoms are included in this group. The best examples are glucose, galactose and fructose (Fig. 2.3). All of them have the same molecular formula and are called isomers.
They differ only in the arrangement of their hydrogen atoms. Of these three, glucose is most important, because it is the, basic transportable form of fuel. It is used as fuel to be utilised during cellular respiration and to supply the energy to the organism for performing its life activities.
These are formed by linking up of two monosaccharides by an oxygen atom between them. The well-known disaccharides are sucrose and maltose (Fig. 2.4). Sucrose is composed of a glucose unit and a fructose unit; maltose results during the breakdown of starch (a polysaccharide).
These are very large carbohydrate molecules, containing series of monosaccharide units. Three important polysaccharides are (a) starch, (b) glycogen and (c) cellulose.
These are storage products in plants and are formed by the conversion of excess sugar (Fig. 2, 5). Starch is insoluble in water. When needed for the body in a watery medium starch is digested in the presence of enzymes called amylase and maltase, into simple sugar.
The breaking down of starch or any other organic compound with the interaction of water is called hydrolysis. Starch provides the richest source of carbohydrate to mankind.
Instead of starch animals store sugar as glycogen. Excess sugar obtained from the plant starch is converted into glycogen which differs in structure from the plant starch. Glycogen is kept stored in muscles and liver. When required, it is quickly broken down into glucose. In lower animals, glycogen serves as the only source of reserve energy but in higher animals major energy reserves are the fats.
This is a very important polysaccharide which is responsible for the formation of structural elements in plants. It is generally absent in animals except in a few cases (small quantities of cellulose are reported to be present in the skin of man). These are very long molecules, each of which may contain three thousand simple sugar units.
Cellulose is digested only by an enzyme called cellulase, which is produced by certain organisms. In ruminants, the cellulose in the ingested plant material is digested by cellulase producing bacteria, which reside within their alimentary canal. Within the gut of termite, a flagellate, Triconympha, performs similar function.
Lipids are the most common energy reserves in animals. It is stored as round droplets in special kind of tissue called adipose tissue and serves the following important functions: (1) as reserved potential energy, (2) as heat insulating layer beneath the skin, (3) as protector of vital organs from mechanical damages and (4) as to meet the water requirements in many animals.
Each molecule contains carbon, hydrogen and oxygen atoms, but their arrangements are entirely different from that of carbohydrates (Fig. 2.6).
Here, in each molecule hydrogen atoms are in greater proportion to oxygen than that in the carbohydrates and thus are concentrated source of potential energy. Moreover, during its burning (oxidation), more water is produced. The animals living in arid zones and un-hatched chicks meet their water requirements from this water produced as a by-product of the breakdown of lipids.
Each lipid, molecule is made up of one alcohol molecule and three molecules of fatty acid. The three fatty acids in one molecule of fat may be identical or may be different. The number of carbon atoms in fatty acid varies from 4-24, and the number is always even.
Lipids may be of three types:
(2) Compound and
1. Simple lipids:
Fatty acids are combined with alcohols to form simple lipids. When the alcohol is glycerol it is called true fat and when it is other than glycerol it is called wax. The common examples of waxes are (a) Beeswax—Fatty acids are combined with myricil. (b) Lanoline—Fatty acids are united with cholesterol.
2. Compound lipids:
When fats are combined with other non-fatty groups like phosphates, sulphates, sugar and amino acids. The examples are (a) Phospholipids—Fats with phosphoric acid and nitrogenous base, (b) Glycolipids—Fats with sugar and nitrogenous base, (c) Amino lipids—Fats united with amino acids, (d) Sulpholipids—Fats united with sulphur.
3. Derived lipids:
These are products which are obtained from the breakdown of simple and compound lipids.
Fats which are eaten as food, are first emulsified by the action of bile salts produced in the liver. It is then hydrolysed by the action of an enzyme lipase which converts it into fatty acids and glycerol. As fats are insoluble in water, emulsification is a necessary prerequisite for transporting it through a watery medium.
The water-soluble form is also obtained by replacing one of the three fatty acid molecules with a phosphorus-containing molecule. The resulting substance is called phospholipid. It may be mentioned here that in man and some other animals, sometimes the carbohydrates are converted into fats.
Proteins are the most important compounds which provide the building blocks of the living body. From hair to the nail of the toe, each and every part is made up of protein. It remains dissolved or suspended, either singly or with others in the living substance. When united with other kinds of molecules, they are known as conjugated proteins.
Before entering into the details of protein structure, it is important to note that the diversity of protein in a living body is unique. Each structure in a living body is made up of a specific kind of protein.
Nature of protein not only differs in each species but also no two individuals (excepting identical twins) possess proteins of identical structure. Such uniqueness of proteins in each individual is believable only when the complexities and possibilities of variety of protein molecules are understood.
Each protein molecule contains nitrogen in addition to carbon, hydrogen and oxygen atoms. Other elements, i.e., sulphur, phosphorus, iron and copper may also be present. The structure of protein molecule is very large and is folded into three dimensional shapes. The two simpler proteins—insulin and beta-lacto-globulin have the molecular formulae—C254 H377 N65 O75 S6 and C1864 H3012 O576 N463 S21.
In spite of their large size the protein molecules are built up in an orderly fashion. Each chain is built up with simpler units called amino acids. Out of nearly eighty known amino acids, twenty are common in all living organisms.
These twenty amino acids are built up in the following pattern. An amino group (-NH2) unites with the acid group (COOH), by removing a molecule of water. Figure 2.7 shows the general plan of amino acids.
The letter R represents the particular chemical group which remains associated with the amino acid. The structure of R varies in different amino acids (Fig. 2.8). In a molecule of protein the amino acids are linked together in such a way that amino end unites with the acid end of another after removing the water molecules between them.
When it is a combination of two amino acids it is called dipeptide and when many amino acids are united they form a polypeptide. Thus, like the twenty- six alphabets making a voluminous dictionary, innumerable combinations of amino acids form the diverse kinds of proteins.
During the breaking down of protein, by the action of proteolytic enzymes, the long chains of amino acids are broken into shorter chains. Finally, the shorter chains are broken into constituent amino acids.
A water molecule is inserted at the broken end. The chain of protein molecules which remain folded is extremely sensitive to various physical and chemical agents. When in contact with these agents they lose their characteristic folding. It is called denaturation.
Each nucleotide consists of a pentose sugar, a phosphate and one of the four bases. The pentose sugar is either ribose or deoxyribose. The ribose contains one oxygen atoms more than deoxyribose.
The phosphate is derived from phosphoric acid. Four bases which are present in the nucleic acids are nitrogenous and two of them are in the group called Purines and two are Pyrimidines. The pruines are adenine and guanine, the pyrimidines are cytosine and thymine or uracil.
Thus, there are four types of nucleotides, each type is characterised by a particular base. When one base unites with one pentose unit, they form a nucleoside, e.g., thymine with ribose = Thymidine; adenine with ribose = Adenosine.
The sugar end of a nucleoside unites with the phosphate group to form a nucleotide unit, e.g., Adenosine monophosphate or AMP, Thymidine monophosphate or TMP. Nucleotides act as (1) components of genetic system, (2) as energy conveyor and (3) as coenzymes.
1. Nucleotides in Genetic system:
Here nucleotides unite to form complex macro- molecules called nucleic acids. Two types of nucleic acids are found (a) Deoxyribonucleic acid (DNA) and (b) Ribonucleic acid (RNA).
(a) Deoxyribonucleic acid:
Deoxyribonucleic acid or DNA is the most important chemical substance present in the living system. The chemical basis of heredity depends upon the working of this substance which is called the key molecule of life. The DNA is localised in the nucleus (on the chromosomes) and sometimes is seen in other parts, i.e., mitochondria.
Each molecule of DNA is made up of two strands or chains, each of which is formed by the alternate arrangement of deoxy sugar and phosphate groups (Fig. 2.9).
The two chains are helically coiled as in a spiral staircase. This brings the two bases in between the two strands and in front of each other. Space between the two chains is such that one purine and one pyrimidine can fit together by the force of a weak hydrogen bond.
This again is possible when adenine pairs with thymine and guanine pairs with cytosine. Though the sequence of base in one strand varies in different organisms, the pairing of base is always the same. From microbes to man, everywhere adenine fits with thymine and guanine couples with cytosine.
Thus the sequence of base in one strand acts as a template of the other strand. The findings of molecular biology in recent years have established that in the arrangement of pairing of purine and pyrimidine bases lies the code of blue print of ‘building and working’ of the living system. These codes are first of all transcribed into the nitrogen base sequence of RNA.
That instruction is again translated to arrange different amino acids in proper sequence to form protein. The DNA can multiply and this involves self-replication. This property serves as the basis of reproduction in all living organisms.
Another important property of DNA is that it is mutable. The sequence of nitrogen base pairing may undergo change. This results into the production of a code of different kind which results into the appearance of changed traits.
(b) Ribonucleic acid:
In most living forms ribonucleic acid or RNA is responsible for the synthesis of proteins. In a group of viruses, it acts like DNA to serve as the material basis of inheritance.
Structure of RNA:
Long, thread-like molecules are arranged usually in single strand but it may be coiled in several places to form helices. Sugar in the nucleotide is ribose sugar and of the pyrimidine bases the thymine is replaced by uracil, but the plan of pairing is same as in DNA, i.e., adenine with uracil and. guanine with cytosine.
According to their functional role in the process of protein synthesis, RNAs are classified into— messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).
Nucleotides as Conveyors of Energy:
Nucleotides exhibit a tendency to couple with additional phosphate group. For example, Adenosine monophosphate (AMP) with a second phosphate group becomes Adenosine diphosphae (ADP), while the addition of a third phosphate group makes Adenosine triphosphate (ATP).
This addition or union of additional phosphate groups requires a large quantity of energy which is available from respiratory fuels. This linking energy is called high energy bond. When ATP breaks into ADP and finally into AMP, this bounded form of energy is released and utilised by the cell.
Nucleotides as Coenzymes:
The nucleotides which work as coenzymes, are complex substances which accompany the activity of an enzyme. In a chemical reaction within the body certain atoms are often transferred from one compound to another. An enzyme hastens the process and a coenzyme actually helps in transfer.
Most of the coenzymes are produced from nucleotides, e.g., Flavin mononucleotide (FMN) and Flavin adenine dinucleotide (FAD). Both are formed by the union of flavin parts of vitamin B, Riboflavin with nucleotide derivatives. They work in transferring hydrogen atoms.
These organic substances are never produced from carbohydrates, fats, proteins or nucleotides, but mostly taken directly from external sources. Some vitamins are of course synthesised in the body or are supplied secondarily by the micro-organisms living inside.
The vitamins are required in very small quantities but are essential for the individual. Till the chemical nature of the vitamins was unknown, these substances were called in alphabetical names like, Vitamins A, B and so on.
Formerly, it was detected that lack of vitamins in the diet produces various kinds of deficiency diseases. In recent years, it has been established that vitamins act by uniting with the protein part of the enzymes. It is also understood that vitamin requirement is not same in all organisms.
A particular vitamin which is essential for a particular organism, may not be required by some other forms. The latter groups may be capable of synthesising it within their body. A list of vitamins with their chemical names, sources and deficiency diseases produced by them are mentioned in Table 2.1.