Life Component

Various chemical substances are the basic constituents of living organisms. These substances are essential for growth, development and maintenance of life. Such substances are called life components.

The life components are of the following types:

1. Inorganic life components
2. Organic life components

A) Inorganic Life Components

The simple chemical substances are called inorganic life components. They are also essential for living organisms. The inorganic life components are of the following types:

1. Water
2. Mineral nutrients

Water

It is the most abundant inorganic compound in the cells. It makes 60–70% of body weight and 75–90% of the protoplasm. It does not yield energy, but it is vital in the maintenance of life.

It is formed by the combination of two hydrogen atoms and one oxygen atom, which are connected by covalent bond. The molecule behaves as a dipole. Hydrogen bond is formed among water molecules. The bond angle between hydrogen and oxygen is 104.5°.

Biological Functions of Water

• Solvent: It is the medium in which soluble materials are dissolved.
• Dispersion medium: It serves as a good dispersion medium for the colloids of cells.
• Temperature stabilizer: It causes elimination of excess heat by evaporation of sweat, which gives a cooling effect.
• Maintenance of pH: Water also helps in maintaining the constant pH of cells.
• Surface tension: Due to high surface tension and cohesion, conduction of water through xylem vessels in plants is possible.

Mineral Nutrients

The mineral nutrients are such chemical elements which are essential for growth and development of organisms. About 27 chemical elements are necessary for the growth and development of living organisms.

The mineral nutrients are basically of two types:

1. Macro elements or major elements
2. Micro elements or trace elements

I) Macro Elements

Those chemical elements which are required by living organisms in a large quantity are called macro elements.

Some of them are Calcium (Ca), Nitrogen (N), Phosphorus (P), Sulphur (S), Magnesium (Mg), Potassium (K), Sodium (Na), Chlorine (Cl), etc.

II) Micro Elements

Those chemical elements which are required by living organisms in very small or minute quantity are called micro elements. Some of them are Iron, Zinc, Manganese, Iodine, Molybdenum, etc.

B) Organic Life Components

Those complex organic compounds which are essential for growth, development and maintenance of life are called organic life components. They are carbohydrate, protein, lipids, nucleic acid, hormones, enzymes, etc.

Biomolecules

Carbohydrates

Carbohydrates are the complex organic compounds made up of carbon, hydrogen and oxygen. Chemically, carbohydrates are hydrates of carbon, i.e. carbon having hydroxide. Structurally, carbohydrates are polyhydroxy aldehyde or polyhydroxy ketone.

Green plants are capable of synthesizing carbohydrates by photosynthesis, while non-green organisms obtain them from others.

Carbohydrates are classified according to the complexity of chemical substances of which they are formed:

1. Monosaccharides
2. Oligosaccharides
3. Polysaccharides

A) Monosaccharides

These are simple sugars having empirical formula C_n(H₂O)_n, containing 3–7 carbon atoms. They cannot be hydrolyzed into smaller carbohydrates. The common examples of monosaccharides are glucose, fructose and galactose. The sugars containing ketone group are called ketoses.

According to the number of carbon atoms present, they are classified as follows:

B) Oligosaccharides

The carbohydrates having 2–10 molecules of monosaccharides joined by glycosidic linkage, which further can be split on hydrolysis, are called oligosaccharides. A common example is disaccharide.

• Disaccharides: e.g. sucrose, maltose.
• Trisaccharides: e.g. raffinose; the second example in the scan is not fully clear.
• Tetrasaccharides: the scan gives examples including stachyose; the remaining word is not fully legible.

C) Polysaccharides

Polysaccharides are carbohydrates having more than 10 molecules of simple sugars or monosaccharides. These are polymers made up of monomers joined together by glucose linkage. Common examples are starch, cellulose and glycogen. They are insoluble in water and not sweet in taste.

Functions of Carbohydrates

• About 60% of the total energy is provided by the breakdown of carbohydrates.
• Monosaccharides act as building blocks.
• Carbohydrates are stored in the form of starch and glycogen.
• They help in synthesis of fats and amino acids.
• Monosaccharides like ribose and deoxyribose sugars are structural components of RNA and DNA respectively.

Amino Acids

Amino acids are the building blocks of proteins. Many amino acids are linked to one another to form protein, where R stands for variety of chemical combination. Each amino acid has an amino group (-NH₂) and one or more carboxyl groups (-COOH). There are 20 types of amino acids which are categorized into two categories.

1) Essential Amino Acids

They are not synthesized in the body; therefore, they must be included in diet. Examples given in the scan are leucine, isoleucine, lysine, tryptophan, valine, methionine, etc.

2) Non-Essential Amino Acids

These are synthesized in the body. They may or may not be present in the diet. The scan lists alanine, glycine, glutamine, tryptophan, valine, methionine, aspartic acid, glutamic acid, etc.

Proteins

Proteins are complex organic compounds made up of carbon, hydrogen and nitrogen. Thus, they are nitrogenous organic compounds. Proteins are macromolecules having very high molecular weight. The molecular weight of protein ranges from a few thousands to many millions. Proteins control and regulate all the activity of cells.

Proteins are polymers made up of monomers called amino acids. The amino acids are linked together by peptide bonds. A peptide bond is established between the amino group (-NH₂) of one amino acid and carboxylic group (-COOH) of another amino acid with removal of one molecule of water.

When two amino acids are linked together by peptide bond, a dipeptide molecule is formed. Similarly, tripeptide, tetrapeptide and polypeptide molecules are formed. When at least 50 amino acids are linked together by peptide bonds, then only it starts showing the property of proteins.

A) Classification of Proteins on the Basis of Structure

1) Globular Proteins

Proteins which are spherical, oval or globular in shape are called globular proteins, e.g. globulin, albumin, etc.

2) Fibrous Proteins

Protein molecules which are elongated and have hair- or thread-like structure are called fibrous proteins, e.g. collagen, keratin, etc.

B) Classification of Proteins on the Basis of Chemical Composition

1) Simple Proteins

Protein molecules which are made up of polypeptide chain of amino acids only are known as simple proteins, e.g. albumin, globulin, histones, etc.

2) Conjugated Proteins

Protein molecules which are made up of amino acids and non-amino-acid substances are called conjugated proteins. These non-amino-acid substances are also called prosthetic groups.

3) Derived Proteins

Protein molecules which are formed by partial hydrolysis of simple and conjugated proteins are called derived proteins, e.g. peptones, proteoses, etc.

C) Classification of Proteins on the Basis of Arrangement of Polypeptide Chain

1) Primary Proteins

Protein molecules which consist of only one polypeptide chain of amino acids are called primary proteins, e.g. insulin.

2) Secondary Proteins

Protein molecules which consist of two or more polypeptide chains of amino acids linked together by hydrogen bonds are called secondary proteins, e.g. keratin.

3) Tertiary Proteins

Protein molecules which consist of two or more polypeptides of molecules linked together by sulphur bond are called tertiary proteins, e.g. hemoglobin.

4) Quaternary Proteins

Protein molecules which contain two or more polypeptide chains of amino acids linked together by weak covalent bond are called quaternary proteins, e.g. phosphorylase.

Functions of Proteins

• Proteins are building blocks; many proteins act as structural proteins and take part in building and repairing body tissues.
• Proteins provide energy.
• Proteins act as enzymes or biocatalysts.
• Proteins act as hormones; some hormones are proteins, e.g. insulin.
• Proteins act as defensive substances; some proteins neutralize foreign bodies and develop immunity.

Lipids

Lipids are a group of fat and fat-like substances. These are insoluble in water and soluble in organic solvents like acetone, ether, alcohol, etc. They need a large amount of oxygen for their oxidation to release energy.

Lipids are grouped into three categories:

1. Simple lipids
2. Complex or conjugated lipids
3. Derived lipids

A) Simple Lipids

Lipids which are made up of lipid molecules only are called simple lipids. They are esters of fatty acid and alcohol. The simple lipids are of the following types: oils, fats and waxes.

Oils

Oils are esters of fatty acid and glycerol. The fatty acid is unsaturated, i.e. having one or more double bonds in their structures. They are liquid at normal room temperature. The scan states that oils can be recommended by physicians to persons suffering from cardiovascular diseases. Sources include mustard and sunflower, etc.

Fats

Fats are also esters of fatty acids and glycerol. The fatty acid is saturated, i.e. without any double bond in their structure. They are solid or semi-solid at normal temperature. The scan states that fats cannot be recommended by physicians to persons suffering from cardiovascular diseases. Sources include ghee, butter, etc.

Waxes

Waxes are esters of fatty acid and alcohol but not glycerol. They consist of long-chain fatty acid and long-chain alcohol other than glycerol. They act as waterproof materials on the skin of human beings, exoskeleton of insects and cuticle of plants, etc.

B) Complex Lipids

Complex lipids are lipids which are made up of lipids and some non-lipid compounds.

• Glycolipids: lipids + carbohydrates.
• Lipoprotein: lipids + protein molecules.
• Phospholipids: lipids + phosphate.

C) Derived Lipids

Lipids which are derived from either simple or complex lipids by hydrolysis are called derived lipids. Structurally, the derived lipids are composed of a fused hydrocarbon ring and hydrocarbon side chain, e.g. steroids.

Functions of Lipids

• Lipids are an efficient source of energy.
• Lipids act as heat insulators and are deposited in subcutaneous tissues.
• Some lipids act as carriers of fat-soluble vitamins like vitamins A, D, E and K.
• Vitamin D is synthesized from cholesterol on exposure to direct sunlight.
• Some lipids act as structural components of cell membranes and cell organelles, e.g. lipoprotein.
• Waxes act as waterproof materials on human skin, exoskeleton of insects and epidermis/cuticle of plants as written in the scan.

Nucleic Acids

Nucleic acids are made up of a number of nucleotides. The nucleotides are linked together by phosphodiester bonds to form a polynucleotide, which forms nucleic acids.

Nucleotide is composed of three components:

1. Pentose sugar
2. Nitrogenous bases
3. Phosphoric acid

Pentose Sugar

Two types of nucleic acids are distinguished on the basis of pentose sugar which they possess. One possesses ribose sugar, called RNA, and the other contains deoxyribose sugar, called DNA.

Nitrogenous Bases

Two types of nitrogenous bases are found in nucleic acids, i.e. purines and pyrimidines.

Purines

Purines have two rings in their structure.

Pyrimidines

Pyrimidines have one ring in their structure.

Types of Nucleic Acids

• Deoxyribonucleic acid (DNA)
• Ribonucleic acid (RNA)

Deoxyribonucleic Acid (DNA)

DNA is the genetic material and is capable of self replication. The purine and pyrimidine bases of DNA carry genetic information, whereas the sugar and phosphate groups perform a structural role. DNA is found mainly in the nucleus, but it also occurs in chloroplasts and mitochondria.

The scan states that DNA was isolated from pus cells by F. Miescher and called nuclein.

A molecule of DNA consists of pentose sugar (deoxyribose), phosphate and nitrogen bases (purines and pyrimidines).

According to the base pairing rule of Chargaff, pairing is A = T and G ≡ C by hydrogen bonds, and the A+T/G+C ratio is constant. The amount of purine and pyrimidine is always equal.

Purine and pyrimidine bases are joined with deoxyribose sugar at the first carbon position by β-glycosidic bonds. Nitrogen base and sugar form nucleosides, while nucleosides and phosphoric acid join to form nucleotides, which are structural units of DNA. Purine and pyrimidine nucleotides are linked serially by phosphate groups.

Watson & Crick Double Helix Model

Watson and Crick gave the double helix model of DNA structure. According to this model, two strands of DNA are coiled upon itself like spiral staircase steps. The sugar and phosphate units are like railings and the hydrogen-bonded base pairs act as the steps.

Two complementary strands of DNA are coiled in right-hand direction. Diameter of the double helix is 20 Å. The bases are 3.4 Å apart along the helix axis and are related by a rotation of 36 degrees. Therefore, the helical structure repeats after 10 residues on each chain, i.e. at intervals of 34 Å. Each turn of the helix contains 10 nucleotide residues.

The two chains are held together by hydrogen bonds between pairs of bases. Adenine always pairs with thymine by 2 hydrogen bonds and guanine with cytosine by 3 hydrogen bonds.

Ribonucleic Acid (RNA)

RNA is the polymer of ribonucleotides. The ribonucleotides are of four types, written in the scan as RAMP, RGMP, RCMP and UMP. These nucleotides join together to form single-stranded RNA.

RNA is produced mainly in the nucleus but moves out into the cytoplasm. The prime role of RNA is protein synthesis, but in organisms having only RNA it acts as genetic material. The scan attributes this fact to Frankel Conrat.

An RNA molecule consists of a single strand of polynucleotide. Here thymine is replaced by uracil (U), and the pentose sugar is ribose.

Genetic RNA

RNA which acts as genetic material in most plant viruses, viroids, virus-like agents and bacteriophages is called genetic RNA. It can be single- or double-stranded.

Non-Genetic RNA

When DNA is genetic material, RNA is non-genetic and helps in protein synthesis. Non-genetic RNA is of three types:

1. m-RNA
2. t-RNA
3. r-RNA

1) Transfer RNA (10–15%)

It is also called labourer of the cell. t-RNA carries specific amino acids from the cytoplasm to the ribosome.

2) Messenger RNA (5–10%)

It is also called contractor of all cell. m-RNA carries genetic information contained in DNA to the ribosome for protein synthesis.

3) Ribosomal RNA (80%)

The most stable RNA is r-RNA. It is found in ribosome.

Difference Between DNA and RNA

Enzymes

The term enzyme was coined by Kuhne (1878) and the scan notes Buchner (1897). Enzymes are biocatalysts that enhance the rate of biochemical reactions but do not affect the nature of the final product.

According to the International Union of Biochemistry (IUB), enzymes are divided into six categories:

1. Oxidoreductases
2. Transferases
3. Hydrolases
4. Lyases
5. Isomerases
6. Ligases

Functions of Enzymes

• ATPase helps in the formation of energy in the form of ATP by the process of phosphorylation, as written in the scan.
• Transcription, translation and DNA replication are controlled by RNA polymerase, peptidyl transferase and DNA polymerase respectively.
• Oxidoreductase catalyzes oxidation reactions.
• Transferases help in transportation of functional groups among acceptor and donor molecules.
• Hydrolases catalyze hydrolysis reactions by adding water to cleave the bond.
• Isomerases catalyze structural shifts present in a molecule, causing a change in the shape of the molecule.
• Ligases catalyze the joining of two molecules.