Basic Chemical Constituents of Cell :
Carbohydrates :
Carbohydrates are the basic components of our food. They are organic compound produced in the chlorophyII containing-cells during photosynthesis. Carbohydrates are compounds of carbon, hydrogen and oxygen atoms. The proportion of hydrogen and oxygen is the same as in water, i.e. 2:1. The general formula CnH2nOn for simple and (C6H10O5)n, for complex carbohydrates. There are some organic compounds having same empirical formula as that of carbohydrate but are not carbohydrates, e.g. formaldehyde (HCHO) and lactic acid (CH3CHOH.COOH) Rhamnose (C6H12O5) and digitoxose (C6H12O4) are carbohydrates, but do not follow general formula of carbohydrates. Simple carbohydrates are commonly known as sugars (glucose, fructose, etc) which participate in metabolic reactions. Complex carbohydrates like starch, cellulose, etc. form storage and structural units.
Carbohydrate molecules are also characterized by the presence of either aldehyde (-CHO) or ketone (C=O) group and two or more hydroxyl (-OH) groups.
Classification of carbohydrates
The carbohydrates are classified into three types on the basis of number of sugar units they contain. The three types are-
1) Monosaccharides
2) Disaccharides and
3) Polysaccharides
1) Monosaccharides :- Monosaccharides are compounds, which cannot be further hydrolyzed into still smaller molecules. These are basic units of complex carbohydrates. Monosaccharides are further classified as aldoses and ketoses, depending on the presence of aldehyde or ketone as functional group. They consist of 3 to 6 carbon atoms. They are crystalline, soluble in water and sweet to taste.
Monosaccharides with ketone group are called ketose sugars e.g. ribulose, fructose while those with aldehyde group are called aldose sugars e.g. glucose, xylose, etc. On the basis of number of carbon atoms which they possess, the monosaccharides can be called, trioses (glyceraldehyde), tetroses (erythrose), pentoses (ribose, deoxyribose), hexoses (glucose, fructose), heptoses (sedoheptulose), etc.
2) Disaccharides :- Disaccharide is a carbohydrate made up of two monosaccharide units. They can be hydrolyzed into monosaccharides. These are soluble in water, sweet to taste and crystalline. The covalent bond that joins monosaccharide units is called glycosidic bond. During formation of a disaccharides from monosaccharides (condensation) one water molecules is released. The most familiar disaccharides are sucrose (composed of glucose and fructose), lactose (composed of glucose and galactose), and maltose (compound of two glucose units).
3) Polysaccharides :- These are complex carbohydrate formed by the condensation of large number of monosaccharides . A single polysaccharide may consist of thousands of unit of monosaccharides. Polysaccharides can be easily hydrolyzed. These are amorphous, tasteless and insoluble (or slightly soluble) in water. A polysaccharides may contain one type of monosaccharides (homopolysaccharide) or different types of monosaccharides (heteropolysaccharide). Examples of homo polysaccharides are cellulose, starch, glycogen, etc. while hyaluronic acid is an examples of heteropolysaccharide. General formula of polysaccharides is (C6H10O5) where 'n' represents the number of monosaccharide links.
The role of carbohydrates is to provide energy for metabolism. The monosaccharide like glucose in main substrate for synthesis of ATP. In mammals, disaccharide lactose present in the milk, provides energy to their babies. The polysaccharides serve as structural components of cell membrane and cell wall (cellulose) and also serve as reserved food material (starch).
Proteins :
Proteins are long chain polymers of amino acids. The term proteins was coined by Berzelius (1830). Proteins are macromolecules with high molecular weight and most abundant organic components of the cell. For example the molecular weight of haemoglobin is 68000 daltons. Proteins serve as an important structural constituent of cells. Most of the proteins are host specific and show slight variations in each species. The differences among the species are due to difference in their proteins components. Number of proteins in a cell varies (in bacterial cell their number is over 2000 while in human cell it is over 1,00,000). All proteins consist of nitrogen in addition to the carbon, hydrogen and oxygen. Some protein molecules contain sulphur and other elements in addition to C, H, O and N.
General structure of protein :
In a long chain of amino acids forming a protein, the amino group (-NH2) of one amino acid is linked to the carboxyl (-COOH) group of the other amino acid. Two amino acids are condensed by removal of a water molecule (OH from COOH and H from NH2) to form a peptide linkage. The remainder of each amino acid after removal of a water molecule (H+ and OH-) is called residue. A molecule of a protein made up of two amino acid residues is called dipeptide, of three residues as tripeptide and of many residues as polypeptide.
Each polypeptide i.e. a long chain of amino acids contains free amino group (-NH2) at one end and carboxyl (-COOH) group at he other end, called N-terminal and C-terminal respectively.
During elongation of polypeptide chain a new amino acids can be added at either end due to free amino or carboxyl group.
Amino acids which occur in proteins :
1) Glycine
2) Alanine
3)Serine
4) Cysteine
5) Aspartic acid
6) Glutamic acid
7) Asparagine
8) Glutamine
9) Methionine
10) Threonine
11) Valine
12) Leucine
13) Isoleucine
14) Lysine
15) Histidine
16) Arginine
17) Phenylalanine
18) Tyrosine
19) Tryptophan
20) Proline
A protein molecule may consist of onw, two or more polypeptide chains. These polypeptide chains give characteristic structure to protein molecule due to their folding. The folded structure is held and maintained with the help of additional special bonds like disulphides bonds (-S-S)
Classification of Proteins :
Proteins can be classified on the basis of their nature or composition into two types as
1) Simple proteins : These are composed of only amino acids or their derivatives. e.g. histones, zein (from maize) etc.
2) Conjugated proteins : These are simple proteins (amino acids) with some non protein part called prosthetic group. e.g. Lipoproteins - proteins + lipid
Nucleoprotein - Proteins + Nucleic acid
Glycoproteins - Proteins + Carbohydrates
Chromo-protein - Proteins + Pigment
Proteins play important role as -
Enzymes - Most of the enzymes are proteins (but all proteins are not enzymes) e.g. amylase.
Hormones - Hormones like insulin, growth hormones, etc. are proteins.
Structural Proteins - These proteins form parts of cells or tissues e.g. keratin is present in hair and skin while elastin occurs in connective tissue. Lipoprotein is present in cell membrane.
Contractile proteins - They are useful for transportation of certain materials e.g. Haemoglobin in blood for transport of oxygen, myoglobin for transport of oxygen in muscles, etc.
Defensive Proteins - They are useful for protection of the body against disease. e.g. Immunoglobulins (antibodies), thrombin, for blood clotting.
Lipids :
Lipids are a group of organic components having oily or greasy consistency. The term lipid was coined by Bloor (1943). Lipids are a group of heterogenous compounds like fats, oils, steroids, waxes etc. Animal fat is often solid while in plants there are liquid oils. Waxes are produced by both, plants and animals. Lipids are insoluble in water but freely soluble in organic or non-polar solvents like benzene, chloroform, etc.
similar to carbohydrates lipids are composed of C, H, O atoms but the proportion of H:O is greater than 2:1 unlike that in carbohydrates (number of oxygen is very less). Compound lipids contain N, S and P in addition to C, H and O.
Lipids differ from carbohydrates and proteins in having two types of monomers such as fatty acids and long chain alcohols, commonly glycerol.
Classification of lipids :
Lipids are classified into 3 main types as -
1) Simple lipids
2) Compound lipids
3) Derived lipids
1) Simple lipids : These are esters of fatty acids with alcohol. Glycerol is a three-carbon alcohol with three -OH groups. Fatty acid is a long straight chain of carbon atoms with a carboxyl (-COOH) group at one end. Depending upon the number of fatty acid molecules attached to the glycerol molecule the esters are called mono-, di or tri-glycerides. Tri-glycerides are the neutral fats.
The fatty acids are of two types as saturated fatty acids and unsaturated fatty acids.
Saturated fatty acids - These fatty acids do not have double bond between carbon atoms of its chain and consist of maximum possible hydrogen atoms e.g. palmitic acid, stearic acid, etc.
Unsaturated fatty acids - These fatty acids contain one or more double bonds between carbon atoms of its chain e.g. oleic acid, linoleic acid, linolenic acid, palmitoleic acid, etc. These acids are not fully saturated with hydrogen atoms.
Fats containing unsaturated fatty acids are liquid at room temperature and are called oils. Most of the plant fats are unsaturated fatty acids while animal fats like butter are saturated fats and are solid at room temperature.
2) Compound lipids : These lipids contain some additional elements or groups in addition to fatty acids and alcohol such as nitrogen, phosphorous, sulphur protein, etc. e.g. phospholipids, glycerophospholipids, glycolipids. Phospholipids are the most important compound lipid and are major constituents of cell membrane.
3) derived lipids : These lipids are the hydrolytic products of lipids. They include - Steroids, waxes, carotenoids, essential oils, etc.
a. Steroids : Steroids are structurally quite different from other lipids. Each molecule of steroid has carbon atom arranged in four interlocking rings. Some of the biologically important steroids are cholesterol, bile salts, male and female sex hormones (testosterone, oestrogen), etc.
b. Waxes : Plant waxes are esters of saturated fatty acids with long chain alcohols (other than glycerol) and ketone. These are secreted by epidermis and form a covering on stem, fruits and leaves. In animals, fur and feathers are coated with wax.
c. Carotenoids : Carotenoids are pigments composed of two, six-carbon rings with a highly unsaturated straight chain of hydrocarbons. Carotenoids occur in the thylakoids of chloroplasts and chromoplasts of almost all higher plants. For example, alpha and beta carotene, xanthophylls, etc.
Lipids acts as high energy reserve food material e.g. oil seeds store oils. They are important components of cell membrane of eukaryotes e.g. phospholipids. Some of the lipids act as components of some enzyme systems. Subcutaneous tissue contains fat which acts as insulator for heat. Wax provides water proofing and checks the rate of transpiration in plants. Steroids like cholic acid are constituents of bile. Cholesterol takes part in the synthesis of vitamin D and is precursor molecule of many sex hormones.
Nucleic Acids :
From the chemical analysis of nucleus it is clear that it consists of two types of nucleic acids i.e. DNA (Deoxyribose nucleic acid) and RNA (Ribose nucleic acid). Nucleic acid were first discovered by a Swiss biochemist Fredrick Miescher in 1869 who called then 'nuclein'.
Nucleic acids are macromolecules composed of many small units or monomers called nucleotides. Each nucleotide is found of three components i.e. a pentose (5c) sugar, a nitrogenous base and a phosphate (phosphoric acid).
Nucleotide = Sugar + Nitrogenous base + Phosphoric acid.
When, a sugar combines with the nitrogenous base it is called nucleoside.
Nucleoside = Sugar + Nitrogenous base
Nucleotide can also be called nucleoside phosphate and the nucleotides are phosphoric esters of nucleosides.
Different components of nucleic acids are :
I) Sugar - It is a pentose (5c) sugar. The sugars can be one of the two types i.e. ribose sugar or deoxyribose sugar. A nucleotide which contains ribose sugar is called ribo-nucleotide while the one which contains de-oxyribose sugar is called de-oxyribo-nucleotide.
II) Nitrogenous bases - Each nucleotide has one of the four types of nitrogenous bases out of which two are purines and remaining two are pyrimidines. Purine bases are double ring compounds further distinguished into adenine and guanine while pyrimidine bases are single ring compounds further distinguished into thymine, cytosine and uracil.
III) Phosphoric acid - Phosphate is present in nucleic acid as phosphoric acid (H3PO4) and therefore nucleic acids are acidic in nature.
Structure of De-oxyribose Nucleic Acid (DNA)
DNA is double-stranded helix in which each strand is made up of thousands of deoxyribose nucleotides. Deoxyribose nucleotides are linked with each other by phospho-di-ester bonds.
The two strands of DNA molecule are antiparallel, complementary and are joined by weak H-bonds. The N-bases are complementary i.e. adenine always pairs with thymine and guanine pairs with cytosine and vice versa.
There are two H-bonds between adenine and thymine while three H-bonds between guanine and cytosine. Total number of purine bases is equal to that of pyrimidine bases. Thus purine : pyrimidine ratio is 1:1.
Watson and Crick (1953) proposed the double helix structure of DNA molecule with the help of X-ray diffraction studies of Wilkins and were awarded Nobel Prize in 1962 along with Wilkins.
In eukaryotic cell, DNA is found mainly in the nucleus but also occurs in mitochondria and chloroplast. It is the genetic material and contains all the information needed for development and existence of an organism. The quantity of DNA in each diploid cell is constant.
Structure of Ribose Nucleic Acid (RNA)
RNA is single stranded and the strand may be straight or variously folded upon itself. The nitrogenous base thymine is replaced by uracil in RNA. The RNA is of two types -
1) Genetic RNA : RNA is the genetic material in most of the plant viruses and some animal viruses. H. Fraenkel- Conrat showed that RNA is genetic material in TMV [Tobacco Mosaic Virus].
2) Non-genetic RNA : This type of RNA is present in organisms in which the genetic material is DNA. The synthesis of non-genetic RNA occurs on DNA molecule. Non-genetic RNA is of three types such as -
I) m-RNA (Messenger RNA) : It is a linear molecule. Its synthesis takes place on DNA by the process called 'Transcription'. It carries genetic information from nucleus to the site of protein synthesis. It forms about 5% of the total cell RNA.
II) r-RNA (Ribosomal RNA) : It is a linear molecule folded at certain regions due to complementation of nitrogenous bases. It is associated with ribosome. It forms about 80% of the total cell RNA.
III) t-RNA (Transfer RNA) : It is soluble RNA with hair pin of clover leaf like structure. This is the smallest among three types. It forms about 10-15% of the total cell RNA. It carries activated amino acids to ribosomes and helps in elongation of polypeptide chain during the process of translation.