Introduction

The human body is composed of roughly 30 trillion cells that collectively perform the essential functions of life. The cells can perform these life-sustaining tasks with the help of several organic molecules present in them. These organic molecules are referred to as biomolecules.

The biomolecules have a wide range of sizes and structures, and they are involved in a vast array of life functions. They are composed of more than 25 naturally occurring elements, with the primary elements being carbon, hydrogen, oxygen, phosphorus, and sulfur. Carbon compounds have major involvement in the formation of biomolecules. They covalently bind with other elements to form several other compounds. Some biomolecules are considered derivatives of hydrocarbons, they’re formed by replacing hydrogen atoms from functional groups like alcohols, amines, aldehydes, ketones, and carboxylic groups.

Given below is a list of small biomolecules and the macromolecules that are formed after the polymerization of these small monomer units.

Small Biomolecules Macromolecules
Sugars Carbohydrates
Fatty acids Fats/lipids
Amino acids Proteins
Nucleotides Nucleic acids

 

This article briefly explains the major biomolecules and the functions they perform in our bodies.

Four Major Types of Biomolecules

Approximately 10,000 to 100,000 molecules are present in a cell to regulate bodily function. But the four major types of biomolecules include carbohydrates, lipids, nucleic acids, and proteins. Most of the other compounds are derivatives of these major primary compounds. Every biomolecule has its characteristics and is designated to perform some specific function essential for life. So, let’s see what they are all about!!!

1. Carbohydrates

Carbohydrates are a vital part of a healthy diet. They provide the energy required to do work. Scientifically, it’s a polyhydroxy aldehyde or polyhydroxy ketone. Carbohydrates are the most abundant biomolecules on earth.

Types of Carbohydrates and Their Functions

Depending on the number of products formed after hydrolysis, carbohydrates are classified into three groups.

A. Monosaccharides: These are composed of a single unit of polyhydroxy aldehyde or ketone. Monosaccharides are colorless, crystalline solids that are completely soluble in water. They are involved in generating energy for the body.  Examples include glucose, fructose, ribose, and arabinose.

B. Disaccharides: These are composed of two units of sugars joined by O-glycosidic bonds. A list of disaccharides with their monomer units and functions is given below.

No. Disaccharide Monomer units Function
1 Sucrose Glucose and Fructose It’s a product of photosynthesis
2 Lactose Galactose and Glucose A major animal energy source
3 Maltose Glucose and Glucose (alpha-1,4 linkage) Important intermediate in starch and glycogen digestion
4 Trehalose Glucose and Glucose (alpha-1, alpha-1 linkage) An energy source for insects
5 Cellobiose Glucose and Glucose (beta-1,4 linkage) Essential in carbohydrate metabolism
6 Gentiobiose Glucose and Glucose (beta-1,6 linkage) A constituent of plant glycosides and some polysaccharides

C. Polysaccharides: These consist of more than two sugar monomer units. They are also known as glycans. They are of two types:

  • Homopolysaccharides: They are composed of only a single type of sugar unit. Based on the function they perform, homopolysaccharides are classified into two groups:

Structural polysaccharide: They provide mechanical stability to cells, organs, and organisms. Examples are chitin and cellulose. Chitin is involved in the construction of a fungal cell wall, while cellulose is an important constituent of diet for ruminants.

Storage polysaccharides: They serve as carbohydrate stores that release sugar monomers when required by the body. Examples include starch, glycogen, and inulin. Starch stores energy for plants. In animals, it is catalyzed by the enzyme amylase (found in saliva) to fulfill the energy requirement. Glycogen is a polysaccharide food reserve of animals, bacteria, and fungi.

  • Heteropolysaccharides: They contain two or more different types of sugar units. It includes glycosaminoglycans like hyaluronic acid, heparan sulfate, keratan sulfate, and murein. These polysaccharides have diverse functions. For example, heparin is an anticoagulant (prevents blood clotting, it’s also known as blood thinners), hyaluronic acid is a shock absorber and lubricant, while peptidoglycans or mureins are present in the bacterial cell wall.

2. Proteins

Proteins are unbranched polymers of amino acid residues. There are about 22 amino acids that are involved in the synthesis of proteins according to their location and function. Proteins are categorized into four groups depending on their structural organization:

  • Primary structure: It is formed by the formation of a peptide bond between amino acids.
  • Secondary structure: It is a folded structure within a polypeptide that’s due to the formation of hydrogen bonds between amide hydrogen and the carbonyl oxygen of the peptide backbone. It includes structures like alpha-helix and beta-sheet.
  • Tertiary structure: It is a three-dimensional conformation that’s formed due to the interaction between R-groups or side chains of the amino acids that make up the proteins. Bonds that contribute to the formation of this structure include hydrophobic interaction, electrostatic interactions, hydrogen bonds, and Van der Waals forces of interaction.
  • Quaternary structure: This structure forms between two or more polypeptide chains. Each polypeptide chain is called a subunit. The quaternary structures may occur between identical or different polypeptide chains. The bonds involved in the formation of these structures include hydrophobic bonds, electrostatic bonds, hydrogen bonds, and covalent cross-links.

Functions of Proteins

Proteins are essential components of organisms. It participates in almost every process within cells. It is involved in the processes of DNA replication, cell signaling, catalyzing metabolic reactions, construction of cell and tissue structures, and transportation of molecules from one place to another.

Given below are eight groups of proteins that are categorized based on their functional properties.

  1. Structural proteins: These proteins are fibrous proteins that are tough and insoluble in water. They form the structural component of connective tissues, bones, tendons, cartilages, nails, hairs, and horns. Examples of structural proteins are collagen, elastin, and keratin.
  2. Enzymes: These are globular conjugated proteins that are also known as biological catalysts. They catalyze metabolic reactions by reducing the activation energy that increases the rate of the reaction. Some examples of protein enzymes are DNA polymerase, lysozyme, nitrogenase, and lipase.
  3. Hormones: These are long polypeptides composed of long chains of linked amino acids.  They play critical roles in regulating the physiological processes of the body, these processes include reproduction, growth and development, electrolyte balance, sleep, etc. Some examples of these hormones are growth hormone (GH) and follicle-stimulating hormone (FSH).
  4. Respiratory pigments: These are globular protein pigments that are usually soluble in water. Examples include myoglobin which provides oxygen to the working muscles and hemoglobin which transfers blood to all the tissues and organs through the blood.
  5. Transport proteins: These are structural components of the cell membrane. They form channels in the plasma membrane to transfer selective molecules inside the cells. Some of them also form components of blood and lymph in animals. Examples of transport proteins are serum albumin (transport hemin and fatty acids), channel proteins, and carrier proteins.
  6. Motor proteins: These proteins are involved in the contraction and relaxation of the muscle (muscle movement). It includes actin, myosin, kinesin, and dynein.
  7. Storage proteins: These proteins are the storage reserve of amino acids and metal ions in cells. They are present in eggs, seeds, and pulses. Examples of storage proteins include ferritin, ovalbumin, and casein.
  8. Toxins: These proteins are generally produced by bacteria. They include diphtheria toxin, Pseudomonas exotoxin, and ribosome-inactivating proteins. They help bacteria to attack and kill their host organism by creating cytotoxicity.

3. Nucleic Acids

Nucleic acids are macromolecules present in cells and viruses, and they are involved in the storage and transfer of genetic information. The nucleic acid was first discovered by Friedrich Miesher in the nuclei of leukocytes. Later, further studies showed that it’s a mixture of basic proteins and phosphorus-containing organic acid.

Structurally, nucleic acids are polymers of nucleotides (or polynucleotides) which are phosphate esters of nucleosides. The nucleotides are comprised of three components:

  • Nitrogenous base: These are heterocyclic, planar, and aromatic molecules. It is of two types: purines and pyrimidines. Purines include adenine and guanine, both of which are found in both DNA and RNA. Pyrimidines include thymine (found only in DNA), cytosine (found in both DNA and RNA), and uracil (found only in RNA).
  • Five carbon sugar: The two types of pentose sugar are ribose (present only in RNA) and deoxyribose (present in DNA). These sugars in nucleic acids have the D-stereoisomeric configuration.
  • Phosphoric acid ion: It’s a phosphate group involved in the polymerization of the nucleotides. A phosphodiester bond links two or more nucleotides leading to the formation of polynucleotides.

Types of Nucleic Acids and Their Functions

Based on nature, structure, and function, the nucleic acids are categorized into two groups: Deoxyribonucleic acids (DNA) and Ribonucleic acids (RNA).

  • Deoxyribonucleic acids (DNA)

DNAs are the hereditary material that resides inside the nucleus. In 1953, the first structure of DNA double helix (B-form of DNA) was discovered by Watson and Crick. DNA has two other forms as well, A and Z forms. The conformation DNA will adopt depends on the hydration level, DNA sequence, chemical modification of the bases, the type, and concentration of a metal ion in the solution.

The double helix structure represents two polynucleotides DNA coiled around a central helix. The two strands are antiparallel and interact by hydrogen bonds between complementary base pairs. In some cases, like at low pH, the triple helix form of DNA also exists. It’s formed by laying a third strand into the major groove of the DNA.

It is the genetic material that stores all the information required to be transferred to the progeny. It specifies the biological development of all living organisms and viruses.

Do You Know?

It is believed that, around 4 billion years ago, RNA was the first genetic material! Scientists say it is largely because of its self-replicating ability and enzymatic activity. This hypothetical period is known as the RNA world. But when the protein-forming enzymes came into existence, DNA became the most dominating and stable form of genetic material.

The DNA structure is more stable than RNA because of the absence of a 2’ hydroxyl group. The other advantage DNA has is that its double-stranded structure allows for the correction of mutations as well.

  • Ribonucleic acids (RNA)

RNA is present in all living cells. It has different roles to play in different organisms. It acts as genetic material in some viruses and has enzymatic activity in other organisms (where it is called ribozyme). Three types of RNA are present among organisms: rRNA, mRNA, and tRNA. All three have essential roles in the development and maintenance of life.

The importance of RNA and DNA is incomparable. DNA carrying the genetic information can’t leave its home, the nucleus, and this is why RNA exists. They are involved in the transfer of genetic information for protein synthesis via the processes of transcription and translation (outside the nucleus), and they control gene expression as well.

Structurally, RNA exists in both single-stranded (primary structure) and double-stranded (secondary structure) forms. The double-helical structure of RNA is present in the A form.

Do You Know?

RNA duplexes are more stable than DNA duplexes.

At physiological pH, RNA duplexes require a higher temperature for denaturation than DNA duplexes. Though, the physical basis for this difference is still a mystery.

4. Lipids

Lipids are organic compounds that are insoluble or poorly soluble in water but soluble in organic solvents (like dissolves like) such as ether, benzene, or chloroform. They are hydrophobic and structurally composed of a chain of hydrocarbons. They are chemically more diverse than other biomolecules, and they are primarily involved in membrane structure and energy storage.

Classes of Lipids and Their Functions

Different classes of lipids include:

  1. Fatty acids: These are the simplest forms of lipids. They are composed of hydrocarbon chains of 4-36 carbons and one acidic group. They can be linear or branched. Moreover, fatty acids are the building blocks of other types of lipids.
  2. Waxes: These are esters of fatty acids and long-chain alcohols. They are composed of hydrocarbon chains of 14-36 carbons. They are synthesized by many plants and animals. However, the best-known wax is bee wax which is composed of an ester of palmitic acid with triacontanol alcohol.
  3. Phospholipids: These are composed of fatty acids, an attachment platform for fatty acids, a phosphate, and an alcohol attached to phosphate. They are part of the cell membrane of the organisms.
  4. Glycolipids: These are lipids containing saccharide groups. They are constituents of the cell membrane and are involved in signal transductions.
  5. Steroids: These are complex derivatives of triterpenes. For example, cholesterol is a constituent of the cell membrane and acts as a precursor for the biosynthesis of steroid hormones and bile acids.
  6. Eicosanoids: They arise from the 20 carbons of polyunsaturated fatty acids. They perform several functions. For example, prostaglandins stimulate uterine contraction and lower blood pressure, leukotrienes are involved in chemotaxis and inflammation, and thromboxanes act as vasoconstrictors and stimulate platelet aggregation.

Other than these lipid molecules, some plasma lipoproteins also exist that are structurally a lipid-protein complex. These complexes function as lipid transport systems in blood. Some examples of lipoproteins are chylomicrons, low-density lipoproteins, and high-density lipoproteins.

Conclusion

Biomolecules are vital for life as it aids organisms to grow, sustain, and reproduce. They are involved in building organisms from single cells to complex living beings like humans, by interacting with each other. The diversity in their shape and structure provides diversity in their functions.

The study of these biomolecules is known as biochemistry. Biochemistry deals with the study of their structures, functions, interactions, and reactions. Several functions of these biological molecules are still a mystery and current advanced techniques are being used to discover more molecules and understand their role in life-sustaining processes.

References

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  3. Heteropolysaccharides. Retrieved from https://www.britannica.com/science/carbohydrate/Heteropolysaccharides
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  7. Hormones. Retrieved from https://courses.lumenlearning.com/suny-ap2/chapter/hormones/#
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