Chapter 2: Biological Molecules


Carbohydrates are the sugar polymers. Their molecules contain C, H and O atoms. The H atoms are twice as many as C or O atoms. They are long chain of sugar molecule called saccharides. There are three types of saccharides and they are monosaccharide, disaccharide and polysaccharide. Monosaccharide are single unit that can join to form polysaccharides and disaccharides by condensation reaction.


These are the simplest Carbohydrates. They have varying number of carbon atom as;

C = 3 = tirose     C = 4 = tetrose

C = 5 = pentose     C = 6 = hexose

Glyceraldehyde is a tirose used in metabolic reaction.

Ribose is a pentose sugar.

Glucose is a monosaccharide containing six carbon atoms in each molecule, it is the main substrate for respiration therefore it is of great importance. It has two isomers – alpha and beta glucose.

Function of monosaccharide

  • They are used as the source of energy in respiration.
  • They are important as building block for larger molecules.




A disaccharide is a sugar molecule consisting of two monosaccharide joined together by a glycosidic bond. They are formed by the condensation reaction between monosaccharides.

Maltose is a disaccharide formed by condensation of two glucose molecule. Sucrose is formed by condensation of glucose and fructose. And lactose is formed by condensation of glucose and galactose.


Polysaccharide is a polymer whose subunits are monosaccharide joined together by glycosidic bond. They are:

  • Starch and glycogen

They are both formed by the condensation of alpha glucose. Starch is the mixture of two substance; amylose and amylopectin. Starch is not found in animal cell instead glycogen is found.

  • Cellulose

They are made of ß- glucose molecule. Cellulose is a component of cell wells in plants and it’s composed of long, unbranched

chains of beta glucose which are joined by glycosidic bonds. Microfibrils are strong threads which are made of long cellulose chains joined together by hydrogen bonds and they provide structural support in plant cells.


They are all organic molecules which are insoluble in water and are only soluble in organic solvents such as alcohol.

There are two types of lipids and they are:

  1. Saturated lipid

Saturated lipids such as those found in animal fats – saturated lipids don’t contain any carbon-carbon double bonds. Too much saturated fat can increase the cholesterol levels in blood thus increasing the risk of coronary heart disease.


  1. Unsaturated lipids

They are those which can be found in plants – unsaturated lipids contain carbon-carbon double bonds and melt at lower temperatures than saturated fats. Unsaturated fats are healthy as they provide essential fatty acids.

Fatty acids

They are series of acids some of which are found in fats. They contain the acidic group known as carboxyl group.


Triglycerides are non-polar and hydrophobic molecules. They are the most common lipids. They are insoluble in water but soluble in organic solvent as ether, chloroform and ethanol. Triglycerides are used as energy reserves in plant and animal cells.


In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group. Phosphate heads are hydrophilic and the tails are hydrophobic and as a result phospholipids form micelles when they are in contact with water as heads are on the outside as they are attracted to water and tails are on the inside as they move away from water.


Proteins are a long chain of amino acids which fold into precise shapes. All the enzymes are proteins. Protein is the essential component of the cell membrane. Proteins are made from a basic monomer called amino acids.

Amino Acids

Amino acids are the monomers from which proteins are made. Amino acids contain an amino group –NH2, carboxylic acid group and a variable R group which is a carbon-containing chain. There are 20 different amino acids with different R groups. Amino acids are joined by peptide bonds formed in condensation reactions.

The structure of protein is as follows:

  • Primary structure: It is the sequence of amino acids in a polypeptide or protein.
  • Secondary structure: It is the structure of a protein molecule resulting from regular coiling or folding of the chain of amino acids.
  • Tertiary structure: It is the compact structure of protein molecule resulting from the three dimensional coiling of the already folded chain of amino acids.
  • Quaternary structure: It is the three dimensional arrangement of two or more polypeptide, or of a polypeptide and a non protein component in a protein molecule.


Collagen is a fibrous protein of great strength due to the presence of both hydrogen and covalent bonds in the structure. Collagen molecules wrap around each other and form fibrils which form strong collagen fibres. Collagen forms the structure of bones, cartilage and connective tissue and is a main component of tendons which connect muscles to bones.


Hemoglobin is a water-soluble globular protein. Hemoglobin carries oxygen in the blood. It has a quaternary structure consisting of 2 alpha subunits and 2 beta subunits. Each polypeptide chain contains a prosthetic (non-protein) group – haem (Fe2+).


Water is a very important biochemical of all. Without it life wouldn’t exist in the planet. It is important as:

  • Water is an excellent solvent for ions and polar molecules because they are attracted to the ions and polar molecules.
  • Water is important within plants and animals, where it forms a large part of the mass of each cell.
  • Water has a relatively large latent heat of vaporisation, meaning evaporation of water provides a cooling effect with little water loss.
  • Water takes part as a reagent in some chemical reactions inside cell as in photosynthesis.
  • Water is a metabolite in metabolic reactions such as condensation and hydrolysis which are used in forming and breaking of chemical bonds.