Fundamental Molecules of Life

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If we take the dry mass of any cell that exists on the earth we will mainly end up with four essential class of molecules.  These four essential molecules are classified as proteins, nucleic acids, carbohydrates and lipids.  They provide structure and energy to the cells of all living organisms.  In addition, these molecules are necessary for the various functions in cells, therefore, they are referred to as fundamental molecules of life.

Proteins are essential molecules for the existence of life and are the most abundant molecules found in all living organisms.  They account for ~50% of the dry weight of any cell.  The key role of proteins in cells is to provide structure and to be involved in the various cellular functions such as muscle formation, transport, transfer of information, catalysts and much more, for this reason they are also recognized as building blocks of life.  The fundamental unit of the primary structure of a protein is called amino acids.  There are 20 different amino acids which are joined in various arrangements by peptide bond to form unique proteins.  Some of these amino acids called essential amino acids cannot be produced by our body, they are the part of our diet.  The preliminary sequence of amino acids in proteins tells us about the primary structure of proteins, which then leads to formation of more complex secondary, tertiary and quaternary structures of proteins.  Research indicates that the function of protein is majorly dictated by their structure, therefore determination of structure of proteins offers an exciting area of research.  Nuclear magnetic resonance (NMR), X-ray diffraction, small angle X-ray scattering and cryo electron microscopy are the majorly used techniques for the determination of protein structure.

The next class of fundamental molecules of life are carbohydrates.  Carbohydrates serve as vital source of energy for all living organism.  In addition they are involved in an array of biological processes including structural component and protein modification.  The general formula for carbohydrate is Cn(H2O)n.  Based on the number of monomer unit they are divided into different classes such as monosaccharides, disaccharides, trisaccharides and polysaccharides.  Carbohydrate such as cellulose are the key components of the cell wall of plants and fungi and provide structure and protection to them.  They are the key components of paper, wood and fibre industries.  They constitute major part of our daily diet in the form of sugar, flour and vegetables.  Carbohydrate rich food is also recommended by nutritionists to maintain good health.

Lipids are amphiphilic molecules that consist of a hydrophilic polar head group and hydrophobic chains.  The major lipids found in our body include phospholipids, cholesterol,  and sphingolipids.  Lipids are the major components of biological membrane which gives structure and protection to the other components of cells.  The cellular membrane is mainly made up of lipids.  Many important proteins such as ion channels, G protein-coupled receptors, and proteins involved in signal transduction reside in membranes.  Therefore, these lipids play an important role in cellular physiology.  In addition, they provide a barrier  against pathogen attack by effecting their entry inside the cells.  Biological membranes constitute major portion of our central nervous system, therefore, changes in lipid composition such as cholesterol modulates the function of neuronal receptors in a significant manner.  Cholesterol plays an important role in the cardiovascular system and accumulation of cholesterol is frequently associated with the risk of cardiac abnormalities.  Predominantly our food contains triglycerides, cholesterol, and phospholipids as a major source of fat.  Earlier studies have indicated that dietary fat is associated with the higher risk of weight gain and diabetes, but now we know that it is not the intake, but the accumulation of fats that leads to cardiovascular diseases and higher risk of diabetes.  Therefore, physicians always advise their overweight patients to exercise, to help them burn fat and live a healthy life.

The final molecule in this write up of fundamental molecules of life are the nucleic acids.  They are made up of 3 components: 5 carbon containing sugar, a nitrogenous base and a phosphate group.  The ribose sugar containing polymer is called RNA (ribonucleic acid) and deoxyribose sugar containing polymer is known as DNA (deoxyribonucleic acid).  The four nitrogenous bases found in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T).  In DNA double stranded structure, adenine is paired with thymine (A=T) by two hydrogen bonds and guanine is paired with cytosine (GºC) by three hydrogen bonds.  RNA contains Uracil (U) instead of thymine.  In RNA, adenine is paired with Uracil (A=U) by two hydrogen bonds and guanine is paired with cytosine (GºC) by three hydrogen bonds.  DNA is the major component of genetic material of all cells.  DNA carries all the genetic information from one generation to next, which are necessary for an organism to survive, grow and reproduce.  DNA finger printing a cutting edge technique serves as a powerful tool to solve crime, identifying genetic relationship between two individuals, and characterize disease causing organisms.  RNA is necessary for the synthesis of proteins in all living organism.  According to the central dogma of molecular biology the major role of RNA is to translate the information stored in DNA into proteins. 

Author: Sandeep Shrivastava, Senior Technical Officer, CSIR-Centre for Cellular and Molecular Biology.  Email: sandeeps@ccmb.res.in