Throughout life, all living organisms undergo changes in relation to their size, complexity, function, etc. Almost all of these changes are the result of four types of macromolecules. These are the carbohydrates, lipids, proteins and nucleic acids. The reason for these molecules to be referred to as "macromolecules" is its structure and in general, all these macromolecules are formed by combining two or more much smaller units known as monomers. Although the available monomers in nature are formed by the basic elements such as carbon, hydrogen, oxygen and nitrogen, when the monomers combine, it can give rise to different macromolecular structures that are described in this article.
Carbohydrates are formed with the combination of monomers known as monosaccharides. In its most simplistic form, a single monosaccharide gives rise to a "simple sugar" known as "glucose." As the structure becomes complex, several monosaccharides combine to form di- or poly-saccharides, which are known as "complex sugars." Starch is one example of a polysaccharide. It is one of the stored polysaccharides found in plants, while another polysaccharide known as "cellulose" takes part in forming the cellular wall of plant cells. The main elements that form each of the monosaccharides include carbon, hydrogen and oxygen in a ratio of 1:2:1. The mono- and disaccharides are usually the energy sources (fast-energy) for most living organisms, while humans receive the same through the food that they eat. The plants are able to produce their own carbohydrates through photosynthesis.
Lipids are known as the body’s reserve energy, and most of the unused carbohydrates convert themselves into lipids of various types. The lipid macromolecule is formed from the base elements carbon, hydrogen and oxygen, while its base units are known as "fatty acids." Fatty acid molecules are combined in ester bonds to form the much larger lipid molecules, which do not break down easily (hydrophobic) when reacting with water, unlike the carbohydrate molecules described earlier. As a result, the body tissues are not able to use lipids for their rapid energy needs although lipids contain more energy per unit weight than the carbohydrate molecules. However, as the carbohydrate energy sources become depleted, lipids are broken down to obtain energy to maintain the biological functions. In addition, lipid molecules also support the formation of biological membranes as well.
The difference in the structure of proteins could be greatly attributed to the presence of nitrogen and sulphur as the base elements, in addition to carbon, hydrogen and oxygen. The basic units, which give rise to proteins, are known as amino acids. Around 20 different amino acids exist in nature and through various combinations; it is possible to form different types of proteins to facilitate various functions in the living body. It should also be mentioned that among the groups of various elements that are present in amino acids, the group known as the "R group" defines the characteristics of each amino acid and therefore the function of the proteins that it forms.
As with other macromolecules, nucleic acids are also formed with a basic unit, which is known as a "nucleotide." The basic elements which form the nucleotides include carbon, hydrogen, oxygen, nitrogen and phosphate. The two types of nucleic acids which exist in nature are the DNA and the RNA. Among them, the DNA provides the structure to genes that are present in all living organisms while, in certain instances, the RNA molecules form the genetic material of certain cells. These molecules are responsible for the genetic inheritance and hold the key information necessary to form new cells and other molecules that will perform the functions of active cells.
Together, these four macromolecules define each of the living organisms, including animals and plants. A deficiency of these molecules, or else an imbalance in the same, could give rise to various diseases and deformations, which emphasize the importance of having these molecules adequately in the diet in order to live a healthy life.