Biological Molecules: Key Terms (CIE A Level Biology)

• The key molecules that are required to build structures that enable organisms to function are:
  • Carbohydrates
  • Proteins
  • Lipids
  • Nucleic Acids
  • Water

The key biological molecules for living organisms

• Carbohydrates, proteins, lipids and nucleic acids contain the elements carbon (C) and hydrogen (H) making them organic compounds
• Carbon atoms are key to the organic compounds because:
  • Each carbon atom can form four covalent bonds – this makes the compounds very stable (as covalent bonds are so strong they require a large input of energy to break them)
  • Carbon atoms can form covalent bonds with oxygen, nitrogen and sulfur
  • Carbon atoms can bond to form straight chains, branched chains or rings

• Carbon compounds can form small single subunits (monomers) that bond with many repeating subunits to form large molecules (polymers) by a process called polymerisation
• Macromolecules are very large molecules
  • That contain 1000 or more atoms therefore having a high molecular mass
  • Polymers can be macromolecules, however not all macromolecules are polymers as the subunits of polymers have to be the same repeating units

Carbohydrates

• Carbohydrates are one of the main carbon-based compounds in living organisms
• All molecules in this group contain C, H and O
• As H and O atoms are always present in the ratio of 2:1 (eg. water H₂O, which is where ‘hydrate’ comes from) they can be represented by the formula C_x (H₂O)_y
• The three types of carbohydrates are monosaccharides, disaccharides and polysaccharides

Types of carbohydrate table

• The most well-known carbohydrate monomer is glucose
• Glucose has the molecular formula C₆H₁₂O₆
• Glucose is the most common monosaccharide and is of central importance to most forms of life
• There are different types of monosaccharide formed from molecules with varying numbers of carbon atom, for example:
  • Trioses (3C) eg. glyceraldehyde
  • Pentoses (5C) eg. ribose
  • Hexoses (6C) eg. glucose

• Glucose exists in two structurally different forms – alpha (α) glucose and beta (β) glucose and is therefore known as an isomer
  • This structural variety results in different functions between carbohydrates

Straight chain and ring structural formula of alpha & beta glucose

• Different polysaccharides are formed from the two isomers of glucose

Structure of polysaccharides table