1.2. Describe the classification of biological molecules

This guide will help you answer 1.2. Describe the classification of biological molecules.

Biological molecules, often referred to as biomolecules, are the essential building blocks of life. They form the structure of cells, carry out metabolic processes, and store or transmit genetic information. These molecules are large compounds made of smaller, repeating units and play a crucial role in the functioning of living organisms. Their classification is primarily based on their chemical composition and role in the body. In this guide, we will categorise the four main types of biological molecules: carbohydrates, lipids, proteins, and nucleic acids.

Carbohydrates

Carbohydrates are organic molecules made up of carbon, hydrogen, and oxygen atoms. The general formula for carbohydrates is (CH₂O)ₙ, where “n” represents the number of carbon atoms. They are a primary energy source for the body and also serve structural purposes in some organisms.

Carbohydrates can be classified into three subcategories based on their size and composition:

Monosaccharides

• These are the simplest carbohydrates and consist of a single sugar molecule.
• Examples include glucose, fructose, and galactose.
• Glucose is a significant energy source, especially for the brain and muscles.

Disaccharides

• Disaccharides are made of two monosaccharide units joined by a glycosidic bond.
• Common examples include sucrose (table sugar), lactose (found in milk), and maltose.
• Enzymes in the body break these bonds to release monosaccharides for energy.

Polysaccharides

• These are long chains of monosaccharides linked together.
• Examples include starch, glycogen, and cellulose.
• Starch is a storage form of carbohydrate in plants.
• Glycogen is the storage form in animals, mainly found in the liver and muscles.
• Cellulose provides structural support in plant cell walls and is a major dietary fibre.

Lipids

Lipids are a diverse group of hydrophobic (water-insoluble) molecules composed mainly of carbon, hydrogen, and oxygen. They are an important energy reserve and form the structural components of cell membranes.

Lipids are classified into the following types:

Triglycerides

• These are the most common lipids and consist of one glycerol molecule bonded to three fatty acid chains.
• They store energy and provide insulation and protection for vital organs.
• Fatty acids can be saturated (no double bonds) or unsaturated (one or more double bonds).

Phospholipids

• These contain two fatty acids and a phosphate group attached to glycerol.
• Phospholipids are key components of cell membranes, forming a bilayer that controls substances entering and leaving the cell.

Steroids

• Steroids are lipid molecules with a structure of four fused carbon rings.
• Cholesterol is a well-known steroid; it stabilises cell membranes and is a precursor for hormones like oestrogen and testosterone.

Waxes

• Waxes are long-chain fatty acids linked to alcohols.
• They provide protection and waterproofing, for example, on plant leaves or in earwax.

Proteins

Proteins consist of long chains of amino acids, which are organic compounds made up of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. Proteins are vital for almost every biological process, as they perform a wide range of functions, including structural roles, enzymatic actions, and immune responses.

Proteins are classified based on their structure and function:

Structural Proteins

• These provide support and strength to tissues.
• Examples include keratin (in hair and nails) and collagen (found in tendons and ligaments).

Enzymatic Proteins

• Enzymes speed up chemical reactions by lowering the activation energy required for the reaction.
• Examples include amylase (digests carbohydrates) and lipase (breaks down lipids).

Transport Proteins

• These carry molecules around the body or within cells.
• Haemoglobin, for instance, transports oxygen in the blood.

Hormonal Proteins

• Hormones regulate physiological processes and behaviour.
• Insulin is a protein that controls blood sugar levels.

Defence Proteins

• Antibodies are proteins that help the immune system target and destroy invading pathogens.

Contractile Proteins

• These enable movement in muscles.
• Actin and myosin are key examples.

Nucleic Acids

Nucleic acids are large molecules that store and transfer genetic information. They are composed of smaller units called nucleotides, which consist of three components: a sugar, a phosphate group, and a nitrogenous base.

There are two main types of nucleic acids:

DNA (Deoxyribonucleic Acid)

• DNA stores the genetic instructions used in the growth, development, functioning, and reproduction of all living organisms.
• It is found in the nucleus and has a double-helix structure.
• The nitrogenous bases are adenine (A), thymine (T), cytosine (C), and guanine (G).
• Each base pairs with its complementary partner: A with T, and C with G.

RNA (Ribonucleic Acid)

• RNA is involved in protein synthesis and transfers genetic information from DNA to ribosomes.
• It is usually single-stranded.
• The nitrogenous bases in RNA are adenine (A), uracil (U), cytosine (C), and guanine (G).
• Uracil replaces thymine in RNA.

Other Types of Biological Molecules

Although carbohydrates, lipids, proteins, and nucleic acids are the core biomolecules, there are others that don’t fit neatly into these categories but play an important role in biological processes:

Vitamins

• Vitamins are organic molecules that the body needs in small amounts for various metabolic processes.
• They include water-soluble vitamins like vitamin C and fat-soluble vitamins like vitamin D.

Minerals

• Minerals are inorganic elements like calcium and iron.
• They are involved in bone formation, oxygen transport, and many enzymatic reactions.

Functions of Biological Molecules

Biological molecules are central to human health and survival:

• Energy Provision: Carbohydrates and lipids are the primary sources of energy.
• Structural Support: Proteins and lipids contribute to the maintenance of cell structure, skin, hair, and connective tissues.
• Information Storage: DNA and RNA store and pass on genetic information.
• Regulation: Proteins, such as enzymes and hormones, regulate processes like digestion, metabolism, and growth.

Summary of Classifications

The classification of biological molecules is based on their structure and role. Each group plays a unique part in supporting life:

• Carbohydrates: Simple and complex sugars for energy and structure.
• Lipids: Energy storage and membrane formation.
• Proteins: Diverse roles including structural, enzymatic, and transport functions.
• Nucleic Acids: Genetic information storage and protein synthesis.

These building blocks work together to sustain all living organisms.