Biomolecules In Living Organisms

Introduction

Living cells comprise crucial chemical substances including water and a wide variety of carbon-based organic compounds called biomolecules. Biomolecules are organic compounds that are produced by living organisms. These biomolecules primarily consist of carbon, hydrogen, oxygen, phosphorus and sulfur.

Classification of Biomolecules

Biomolecules may be classified based on their weight into micromolecules and macromolecules. Micromolecules are low-weight molecules, such as monosaccharides, amino acids, nucleotides, fatty acids, etc. These micromolecules act as monomers that polymerise to form macromolecules. The macromolecules include carbohydrates, proteins, and nucleic acids.

Types of Biomolecules

Carbohydrates

Carbohydrates are polyhydroxy aldehydes (aldoses) or ketones (ketoses), or those compounds that upon hydrolysis yield aldoses or ketoses. Carbohydrates are further classified into −

• Monosaccharides − simple sugars consisting of a single unit of polyhydroxy aldehyde or polyhydroxy ketone.
• The general formula for monosaccharides is C_nH_2nO_n.
• All hexoses and pentoses in aqueous solutions exist as cyclic structures, as hemiacetals or hemiketals

Examples of monosaccharides include glucose (an aldohexose), fructose (an aldopentose), and galactose (an aldohexose).

• Disaccharides − formed when two monosaccharide units are linked by dehydration reactions, yielding an O-glycosidic bond between them.
  Examples: Sucrose (glucose + fructose), maltose (glucose + glucose), and lactose (galactose + glucose).
• Polysaccharides − several hundred to thousands of monosaccharide units. They may be homopolysaccharides (like glycogen, cellulose, chitin, etc.) or heteropolysaccharides.

Functions of Carbohydrates

• The breakdown of simple sugars such as glucose is the primary source of energy which can be used to drive cellular reactions.

• They also serve as precursors for the synthesis of other cellular material

• Storage polysaccharides (glycogen and starch) can be broken down later on to serve the energy requirements of the cell

• Structural polysaccharides (cellulose, peptidoglycan, etc) provide mechanical stability by forming structural components of the cells

• Polysaccharides are involved in cell signalling activities

• Carbohydrates also serve as an important dietary component

Proteins

Proteins comprise repeating amino acid units, which are joined together via peptide linkages between the amino group of one and the carboxyl group of the other amino acid. 22 amino acids combine in different sequences to result in a wide variety of proteins.

The structure of a protein is described under the following four levels −

• Primary structure − this is the amino acid sequence that makes the polypeptide molecule, specified by the genetic codes carried on the DNA.

• Secondary structure − the localized spatial arrangement of the amino acids of the main chain of the polypeptide.
  The most common secondary structures are −

• Alpha-helices, formed when the polypeptide chain twists into a helical conformation via hydrogen bonds between the -CO and -NH groups, and

• Beta-pleated sheets, formed when two or more segments of a polypeptide chain, or several polypeptide chains, line up side by side.

• Tertiary structure − results from the interaction between the side chains of the polypeptides, causing the folding of the polypeptide chain globular domains.

• Quaternary structure − refers to the arrangement of constituent polypeptide chains of multimeric proteins, which are held together by covalent and non-covalent bonds.

Functions of Proteins

• Structural proteins (keratin, collagen, etc.) provide the body and the cells support and shape

• Catalytic proteins (aka enzymes) are imperative in all the biological reactions that occur in the cell

• Regulation of metabolic processes as well.

• Transport of material across cell membranes (e.g. aquaporins) and also across the body (e.g. haemoglobin)

• Hormonal proteins transmit messages between cells and regulate metabolic processes

• Defence proteins like immunoglobulins are imperative in protecting the body from pathogenic diseases

• Contractile proteins (actin and myosin) facilitate muscle movement and also the movement across cells

Nucleic acids

Nucleic Acids are polymers of units called nucleotides. Hence they are also called polynucleotides.

• Nucleic acids are of two kinds- the double-stranded molecule called the Deoxyribonucleic Acid (DNA) and, the single-stranded molecule known as the Ribonucleic Acid (RNA).

• Each monomer in a DNA or RNA molecule contains −

• A nitrogenous base (also known as a nucleobase), maybe a derivative of a purine (adenine and guanine) or a pyrimidine (cytosine, thymine and uracil)

• A ribose sugar (deoxyribose in DNA, ribose in RNA)

• A phosphate group

• These monomeric nucleotides are joined to each other via phosphodiester linkages, forming polynucleotides.

• DNA exists as a double helix molecule, comprised of two strands of polynucleotides that run antiparallel to each other. Contrastingly, the RNA is a single-stranded molecule

Functions of Nucleic Acids

• Nucleic acids carry genetic information that is encoded in the form of a triplet code, within the specific sequence of nucleotides in their structure. The genetic information is converted to the specific proteins which perform various metabolic functions in the body.

• The nucleic acids are also used in conducting fingerprint analysis.

Lipids

These are a group of diverse organic compounds that are insoluble in water. Their hydrophobicity is due to their structure- lipids predominantly contain hydrocarbon chains

Lipids may be of different types, depending upon their structural features −

• Fatty acids − the simplest lipids, containing long chains of hydrocarbons (usually 12-24 C long). They may be unsaturated or saturated.

• Triacylglycerols − triesters of three fatty acids and glycerol.

• Phospholipids − an amphipathic lipid consisting of two hydrophobic) fatty acid tails and a hydrophilic “head”, esterified to a phosphate group.

• Glycolipids − lipids containing saccharide groups, attached via glycosidic bonds

• Lipoproteins − these are lipid-protein complexes, including LDL and HDL

Functions of lipids

• Triacylglycerols serve as an important form of stored energy
• Phospholipids are a structural component of all biological cell membranes
• Steroids serve as hormones, namely estrogen and testosterone
• They also serve as pigments, signalling molecules, cofactors, detergents, etc.

Conclusion

• Biomolecules are organic compounds found in living organisms and are of four types- carbohydrates, proteins, nucleic acids and lipids

• Carbohydrates are polyhydroxy aldehyde or ketone molecules. They are classified as monosaccharides, disaccharides, or polysaccharides, based on the number of monomeric units

• Proteins are polypeptide molecules of repeating units called amino acids. They serve as enzymes, hormones, immunoglobulins, and transporters, and also serve structural functions

• Nucleic acids are polymers of nucleotides. They are of two kinds- DNA and RNA. They carry genetic information

• Lipids are hydrophobic hydrocarbon molecules which serve as energy sources, hormones, structural components of biomembranes, etc.

FAQs

Q1. Which are the simplest monosaccharides?

Ans. The simplest monosaccharides are trioses (3C sugars), like dihydroxyacetone and glyceraldehyde

Q2. What are some examples of heteropolysaccharides?

Ans. Chondroitin sulphate, keratan sulfate, heparin, hyaluronic acid, etc

Q3. Why doesn't RNA exist as a double helix?

Ans. RNA has ribose in its structure, which contains a hydroxyl group at the 2’ position. Hence, making a bond with another nucleotide would lead to steric hindrance and instability of the molecule.

Q4. What is the difference between fats and oils?

Ans. At room temperature, fats exist as solids and contain a high amount of saturated fatty acids, while oils exist as liquids at room temperature and contain more unsaturated fatty acids

Q5. Which protein is most abundant in mammals?

Ans. Collagen