Biological Molecules

• Polysaccharides

  • Glycogen
  • Starch
  • Cellulose
• Starch stores energy in plants and is a mixture of two polysaccharides called amylose and amylop
• Monomers and polymers

  • Monomers are small units which are the components of larger molecules
  • Polymers are molecules made from monomers joined together
  • Condensation reaction joins monomers by chemical bonds with the elimination of a water molecule
  • Hydrolysis is when water is added to break a chemical bond between two molecules
• Monosaccharides

  • Glucose
• Disaccharides

  • Maltose
  • Sucrose
  • Lactose
• Carbohydrates


  • They consist only of carbon, hydrogen, and oxygen
  • They are long chains of sugar units called saccharides
  • There are three types of saccharides - monosaccharides, disaccharides, and polysaccharides
  • Monosaccharides can join together to form disaccharides and polysaccharides by glycosidic bonds formed in condensation reactions
• Glycogen is the main energy storage molecule in animals and is formed from many molecules of alpha glucose joined together by 1, 4 and 1, 6 glycosidic bonds. It has a large number of side branches for quick energy release and is a relatively large but compact molecule maximizing energy storage
• Water

  • It is a polar molecule due to uneven distribution of charge within the molecule
  • It is a metabolite in metabolic reactions such as condensation and hydrolysis
  • It is a solvent in which many metabolic reactions occur
  • It has a high heat specific capacity meaning that a lot of energy is required to warm water up, acting as a buffer
  • It has a relatively large latent heat of vaporisation, providing a cooling effect with little water loss
  • Strong cohesion between molecules enables effective transport of water in tube-like transport cells
  • Maximum density of water is at 4 degrees, making ice less dense than water and float on top, creating an insulating layer
  • Water is incompressible, providing good support
• Triglycerides are lipids made of one molecule of glycerol and three fatty acids joined by ester bonds formed in condensation reactions
• Amylopectin


  • Branched chain of glucose molecules joined by 1, 4 and 1, 6 glycosidic bonds
  • Rapidly digested by enzymes, energy released quickly
• Starch components

  • Amylose
  • Amylopectin
• Amino acids

  Monomers from which proteins are made, contain amino group, carboxylic acid group, and a variable R group
• Phospholipids formation

  1. One fatty acid of a triglyceride is substituted by a phosphate-containing group

  2. Phosphate heads are hydrophilic, tails are hydrophobic, form micelles in contact with water
• Lipid types

  • Saturated lipids found in animal fats, no carbon-carbon double bonds
  • Unsaturated lipids found in plants, contain carbon-carbon double bonds, melt at lower temperatures than saturated fats
• Cellulose composition

  • Long, unbranched chains of beta glucose joined by glycosidic bonds
  • Microfibrils made of long cellulose chains joined by hydrogen bonds, provide structural support in plant cells
• Amylose


  • Unbranched chain of glucose molecules joined by 1, 4 glycosidic bonds
  • Coiled and compact molecule, can store a lot of energy
• Number of unsaturated bonds

  • Weaker intermolecular bonds, lower melting point
  • Saturated fats solid at room temperature, unsaturated lipids liquid at room temperature
• Triglycerides are used as energy reserves in plant and animal cells
• There are 20 different amino acids with different properties
• Inorganic ions

  • Hydrogen ions determine pH, higher concentration leads to lower pH
  • Iron ions are component of haemoglobin in red blood cells
  • Sodium ions involved in co-transport of glucose and amino acids
  • Phosphate ions are component of DNA and ATP
• Proteins

  • Collagen
  • Haemoglobin
• Formation of proteins

  Amino acids are joined by peptide bonds formed in condensation reactions. Dipeptides contain two amino acids, and polypeptides contain three or more amino acids
• Haemoglobin

  Consists of two alpha and two beta polypeptide chains, carries oxygen in the blood
• Components of DNA and ATP

  • Phosphate ions
• Biuret test

  Used to test for proteins, presence of protein turns the solution lilac as an indicator of peptide bonds
• Starch test

  Tested with iodine, presence of starch changes the colour of potassium iodide solution from yellow to black/blue
• Structure of proteins

  • Primary structure: order and number of amino acids
  • Secondary structure: shape determined by hydrogen bonding, can be alpha helix or beta pleated sheet
  • Tertiary structure: 3D shape, can be globular or fibrous
• Collagen


  Forms the structure of bones, cartilage, and connective tissue, main component of tendons
• Amino acids

  Monomers from which proteins are made, containing an amino group (NH2), carboxylic acid group, and a variable R group
• Emulsion test

  Used to test for lipids, presence of lipids indicated by a cloudy solution due to the formation of an emulsion
• Benedict’s test

  Used for reducing and non-reducing sugars, presence of reducing sugar forms a red precipitate of copper (I) oxide