What are the main classes of biomolecules?

Biomolecules refer to all non-living organic substances that play pivotal roles in the structure and function of living organisms. The four major types of biomolecules include carbohydrates, lipids, nucleic acids, and proteins. Each type has its own unique characteristics and each is designated to perform some specific function that’s essential for life.

Carbohydrates

Carbohydrates are the most abundant type of biomolecules on earth. They are primarily responsible for providing the energy necessary to perform any activity. Carbohydrates are further categorized into three groups based on the number of products formed after hydrolysis: 

• Monosaccharides: These are the simplest unit of carbohydrates. They are composed of carbon, oxygen, and hydrogen atoms and cannot be broken down further as they are already in their simplest form. Monosaccharides are completely soluble in water and are responsible for generating energy that the body needs. Examples include fructose, glucose, and ribose. 
• Disaccharides: These are composed of two monosaccharide units linked together by a glycosidic bond. Disaccharides are essential in carbohydrate metabolism and are an important energy source for animals. Examples include sucrose, lactose, and maltose.
• Polysaccharides: These are long chains of carbohydrate molecules, composed of multiple smaller monosaccharides. They are also known as glycans. Polysaccharides function as an important source of energy in animal cells. They are of two types – homopolysaccharides, which are composed of a single type of sugar unit and heteropolysaccharides, which contain two or more different types of sugar units. Examples include glycogen, starch, and cellulose. 

Proteins

Proteins are unbranched polymers of amino acid residues. They are essential components of all organisms and participate in almost every cellular process including but not limited to cell signaling, DNA replication, construction of cell and tissue structures, catalyzing metabolic reactions, and transportation of molecules. 

Based on their functional properties, proteins may be classified into 8 groups: 

• Enzymes: Also known as biological catalysts, enzymes are globular conjugated proteins that catalyze metabolic reactions by lowering activating energy to increase the reaction rate. Examples include protease, DNA polymerase, and lipase. 
• Structural proteins: These tough, water-insoluble fibrous proteins form the structural component of bones, connective tissues, hair, nails, tendons, cartilages, and horns. Examples include collagen, elastin, and keratin. 
• Hormones: Made up of long chains of linked amino acids, hormones play a pivotal role in regulating critical physiological processes of the body including growth and development, sleep, reproduction, and electrolyte balance among others. Examples include cortisol, insulin, estrogen, and testosterone. 
• Transport proteins: Transport proteins help to transfer select molecules inside the cells by forming channels in the plasma membrane. Examples include channel proteins, carrier proteins, and serum albumin. 
• Storage proteins: Present in seeds, pulses, and eggs, these proteins act as storage reserves of metal ions and amino acids in cells. Examples include ovalbumin, casein, and ferritin. 
• Motor proteins: These proteins play a key role in muscle movement by contracting and relaxing muscles. Examples include myosin, actin, and kinesin. 
• Respiratory pigments: These globular, water-soluble protein pigments perform various functions including distributing blood to the various organs and tissues and providing oxygen to the working muscles. Examples include hemoglobin and myoglobin. 
• Toxins: Generally produced by bacteria, toxins are responsible for creating cytotoxicity, which attacks and kills the host organism. Examples include diphtheria toxin, ribosome-inactivating proteins, and Pseudomonas exotoxin. 

Nucleic Acids

Nucleic acids are macromolecules responsible for storing and transferring genetic information. They may be categorized into two groups based on their structure, function, and nature. 

• Deoxyribonucleic acids (DNA): DNA is the hereditary material that stores all the information to be transferred to the progeny. It is housed within the nucleus and has a double-helix structure composed of two antiparallel strands that interact by hydrogen bonds present between complementary pairs. 
• Ribonucleic acids (RNA): RNA is present in all living cells and exists in both single-stranded and double-stranded forms. There are three types of RNA – mRNA, rRNA, and tRNA – all of which have critical but different roles in developing and maintaining life. 

Lipids

Lipids are hydrophobic organic compounds that are made up of a chain of hydrocarbons. They are soluble in organic solvents such as benzene or chloroform but are insoluble or poorly soluble in water. Lipids are primarily involved in energy storage and membrane structure. Lipids are categorized into 6 classes: 

• Fatty acids: The simplest form of lipids, fatty acids are the building blocks of other types of lipids. Structurally, they may be linear or branched and made up of hydrocarbon chains of 4 - 36 carbons and one acidic group. 
• Phospholipids: Made up of fatty acids, a phosphate, and an alcohol attached to phosphate, phospholipids are a component of the cell membrane.
• Waxes: Waxes are essentially esters of fatty acids and long-chain alcohols. They are synthesized by a wide range of animals and plants, with bee wax being the most well-known among them. They are made up of hydrocarbon chains of 14-36 carbons. 
• Steroids: These complex derivatives of triterpenes are a component of the cell membrane. They function as a precursor for the biosynthesis of bile acids and steroid hormones. 
• Glycolipids: Glycolipids are lipids that contain saccharide groups. They are components of the cell membrane and play a role in signal transductions.
• Eicosanoids: Arising from the 20 carbons of polyunsaturated fatty acids eicosanoids perform a wide range of functions. Some play a role in lowering blood pressure and stimulating uterine contraction and platelet aggregation. Other eicosanoids are involved in inflammation while some act as vasoconstrictors.