In chemistry, the traditional definition of an organic substance is any carbon-containing molecule found in living organisms. This definition has since been modified to include hydrogen and the other fundamental elements of life, such as oxygen and nitrogen.
Scientists previously considered the carbon atom as the primary element of life due to its unique bonding property that allows the element to combine with four other atoms. This characteristic makes it possible for carbon to form complex substances that were inherently found in all types of organisms. However, the discovery of carbon compounds in non-living matter has led scientists to redefine the meaning of "organic."
The four essential organic bio-molecules that are present in living organisms are classified into four major groups: carbohydrates, proteins, lipids and nucleic acids. These molecules are primarily composed of the elements carbon, hydrogen, nitrogen, oxygen, sulfur and phosphorus. Carbohydrates, such as glucose and starch, are the chief sources of usable energy that drive vital life processes. The building blocks of carbohydrates are called monosaccharides. Proteins are built from amino acids, common examples of which include hormones and enzymes. Lipids, which include oils and fats, are used for energy storage. These compounds are the main components in cellular membranes. Nucleic acids, such as deoxyribonucleic acid, or DNA, carry the genetic code of an organism.