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Four Macromolecules

We must have heard about macromolecules or polymers. As name suggested, macromolecules are huge and bulky molecules with a large number of atoms. Polymer means a molecule which is composed of many units. These basic units are called as monomers. Monomer units are bonded with each other in a certain manner to form a big macromolecule. We are using many macromolecules in our everyday life such as polythene, PVC, Terylene, nylon etc.

Polythene molecule consists of a large number of ethene molecules which are bonded with each other through additional bonds by shifting of double bonds between carbon atoms. There are many macromolecules which are formed by condensation reactions between monomer units such as nylon, polyester etc. The condensation reaction between monomer unit involves the elimination of small molecules such as water, carbon monoxide etc.  There are many macromolecules which are found in living cells and are involve in different biological reactions. They are also called as biomolecules.

Carbohydrates, proteins, lipids, vitamins, hormones etc are biomolecules. These all are macromolecules and many of them are composed of different repeating units. Biological macromolecules or biomolecules are polar in nature. Monomer units of biological molecules have two polar groups at their head and tail part. The polar nature of molecule makes them soluble in water and improves their reactivity. Lipids are not soluble in water due to their complex structure. As the complexity of molecule increases, the solubility of molecule decreases.

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Four Macromolecules and their Subunits

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There are four major types of biological macromolecules in living organisms.
  1. Carbohydrates
  2. Nucleic acids
  3. Proteins
  4. Lipids
All these four macromolecules consist of small subunits which are bonded with each other in a certain manner to form a stable molecule. Let’s discuss the monomer units of these macromolecules. The structures of sub units of these four macromolecules are listed below.

 Monomer unit 
 Structure of monomer unit 
 Macromolecule 
 1. Monosaccharide  Monosaccharides
 Carbohydrate
 2. Amino acid
Amino Acid
 Proteins
 3. Nucleotide
Nucleotide
 Nucleic acids
 4. Fatty acid and glycerol  Fatty Acid
 Lipid

Four Macromolecules and Their Structures

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Let’s have a look on the sub-units of each macromolecule in detail. Monosaccarides are polyhydroxy carbonyl compounds with either aldehyde or ketonic group. For example glucose has aldehyde group whereas fructose has ketonic group. They also exist in ring structures which are formed by hemiacetal linkage between them. The ring structure of monosaccharide molecules can be represented with the help of Haworth projection. The position of –OH group determines the type of ring structure that could be alpha or beta-form. The presence of free anomeric carbons determines the chemical reactivity of carbonyl carbons. Free anomeric carbonyl group can reduce alkaline solutions of cupric salts therefore such sugar molecules are called as reducing sugars.

Monosaccharide molecules are bonded with each other through glycosidic linkage. It is a condensation step which occurs through elimination of water molecule between the anomeric hydroxyl and a hydroxyl. If after formation of glycosidic linkage, the anomeric carbonyl group remains free for reducing reactions, such sugar molecules are called as reducing sugar. In some of carbohydrate molecules, the anomeric carbon atom is not free therefore they cannot show reducing reactions and such carbohydrates are called as no-reducing sugar. Due to presence of hydroxyl groups, branching is possible in these molecules which results the formation of more compact molecule. Another macromolecule is nucleic acid which consists of several nucleotides units. 

Nucleic Acid

Each Nucleotide unit is made of phosphate group, one sugar molecule and one nitrogenous base. These units polymerize to yield nucleic acids. The sugar molecule can be Ribose (in ribonucleotides) or Deoxyribose, which lacks a 2' -OH (in deoxyribonucleotides). There are four types of nitrogenous bases adenine (purine), cytosine (pyrimidine), guanine (purine), uracil (in ribonucleotides) or thymine (in deoxyribonucleotides).  Both uracil and thymine are pyrimidine bases and different from each other by a methyl group. The polymerization of Nucleotides is again a condensation reaction which occurs through elimination of a water molecule. Nucleotides are bonded through ester linkage between the 5'-phosphate and the 3' -OH of another nucleotide. A 3'->5' phosphodiester bond is formed. The product has ends with an end with a free 5' group which is called the 5' end or an end with a free 3' group which is known as the 3' end. The nitrogenous bases are abbreviated by their initials like A, C, G and U or T. Sequences of monomer units are written with the 5' end to the left and the 3' end to the right. Since RNA has a 2' –OH therefore branching is possible in RNA molecules but not possible in DNA. Although the branching occurs only during the RNA modification yet not occurs in any finished RNA species. Proteins are also macromolecules which are formed by the combination of monomer units; amino acids. Amino acids are organic compounds with two functional groups; amino group and carboxyl group. The amino group is bonded at the alpha carbon atom to carboxyl group therefore such amino acids are called as alpha amino acids. There are 20 alpha amino acids which combine in different combinations to form polypeptide chains. These amino acids form amide linkage between amino group and carboxyl group with the elimination of water molecule. Therefore such linkages are called as peptide linkage. All amino acids have different alkyl or R-groups which effects the bonding, chemical and physical properties of the molecule.

Four Macromolecules Chart

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 Monomer unit 
 Structure of monomer unit 
 Macromolecule   Function 
 Monosaccharide  Monosaccharides
 Carbohydrate  Provide energy 
 Amino acid Amino Acid
 Proteins  Building blocks of body 
 Nucleotide  Nucleotide
 Nucleic acids  Hereditary and transfer of characters from one generation to next 
 Fatty acid and glycerol  Fatty Acid
 Lipid  Provide and safety layer

Four Macromolecules and Examples

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So we discussed that there are mainly four macromolecules which take part in biological reactions; carbohydrates, proteins, nucleic acid and lipid. They are polymers in which monomer units are bonded with each other through either addition or condensation process. For example; monosaccharide units are bonded with each other through glycosidic linkage which forms due to condensation process. Similarly amino acids are bonded with each through peptide bonds which are formed by condensation reaction between amino group and carboxyl group.

Four Macromolecules Functions

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Carbohydrates use for energy to living cells such as brains consumes large amounts of glucose and remains most active. Lipids help the body store the energy in the form of fats and oils. Proteins act as building blocks for our body and also workhorses of the body's machinery. They carry out specific functions and act as enzymes which catalyze biochemical reactions.

Nucleic acids are central to the function of living cells and are arranged in a linear sequence. They code for function of the body's proteins.
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