The Families of Biological Molecules

Diversity is essential to the sustainability of living systems. This is true for species in ecosystems as it is for molecules in cells, tissues, and organisms. Yet, the same way different species are linked by common ancestors and may be grouped in taxonomic classes according to common characteristics they share, molecules may be grouped in classes and classified according to common chemical and physical characteristics. One of such characteristics is solubility in water (in other words: how polar atoms are distributed in the 3D structure of molecules). One class of biological molecules, the lipids, includes only low water solubility (“hydrophobic”) molecules, this being the characteristic that defines this class. Other classes include molecules that are mostly moderately or highly soluble in water and can be recognized for the dominant presence of specific chemical groups: OH in saccharides (also referred to as “carbohydrates”) and a combination of amino and carboxyl groups in amino acids. Lipids, saccharides, and amino acids may combine with molecules of its own class to form either polymers (molecules formed by successively covalently attaching smaller molecules), such as polysaccharides and proteins, or supramolecular assemblies (organized arrangements of molecules that are in contact but are not covalently attached), such as the lipid bilayer of cell membranes. It is common to find molecules and supramolecular assemblies that combine elements from different classes, such as nucleotides, which contain saccharides. Proteins are extremely versatile in this regard because protein interactions with saccharides, lipids, and nucleic acids (nucleotide polymers) are ubiquitous in virtually all cells.