Unit 2.1.1: Biological Molecules

Condensation and Hydrolysis

A condensation reaction is a chemical reaction that links biological monomers together.

In condensation reactions:

  • A water molecule is released. 
  • A new covalent bond is formed. 
  • A larger molecule is formed by the bonding of small molecules. 

Hydrolysis splits larger molecules to monomers: 

  • A water molecule is used.
  • A covalent bond is broken.
  • Smaller molecules are formed by the splitting of larger molecules.
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Condensation and Hydrolysis Diagram

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Hydrogen bonding in water

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Hydrogen bonding in water

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Amino Acids

Amino Acids are monomers that make up proteins. The consist of: 

  • An amino group at one end.
  • A carboxyl group at the other end. 
  • An R-Group side chain. 

A condensation reaction between an acid group and an amino group forms a covalent peptide bond as well as producing a water molecule.

  • The new molecule produced is called a: dipeptide.
  • It can be broken by a hydrolysis reaction.

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Amino Acids Diagram

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Primary, Secondary, Tertiary and Quaternary Struct


  • The primary structure is the specific sequence of amino acids that make up the protein. 
  • They are bonded by Peptid bonds.


  • DNA determines the sequence in which amino acids are bonded. 
  • As the chain becomes longer, parts of the chain are stabilised by being coiled up/pleated. 
  • The coils and pleats are held in place by hydrogen bonds. 
  • The amount of coiling/pleating depends on the priamry structure.
  • It is formed when the chain of amino acid coils/folds form an alpha helix or a beta pleated sheet.


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  • Once the final polypeptide is formed, the coil and pleats come together in a specific way to form an overall 3D shape
  • This is the tertiary structure and its specific shape is vital.
  • Tertiary structure in proteins is stabilised by a number of bonds:
    • Disulphide bonds: when two amino acids containing sulphure are found close to each other, a covalent bond can from.
    • Ionic bonds: R-groups sometimes carry a chage, either +ve or -ve. Where oppositely charged amino acids are found close to each other, an ionic bond forms.
    • Hydrogen bonds: in the secondary structure.
    • Hydrophic - Hydrophilic interactions: in a water based environment, htdrophobic amino acids will be most stable if they're held together with water excluded. Hydrophilic amino acids tend to be found on the outside in globular proteins, with hydrophobic amino acids in the centre. These interactions keep chemical groups positioned close to each other. 
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Primary, Secondary, Tertiary and Quaternary Struct

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Collagen is a fibrous protein:

  • Consisting of 3 polypeptide chains wound around each other like a twisted rope.
  • Hydrogen bonds form between the chains, giving it structural strength.
  • This strength is increased further because each collagen molecules forms covalent bonds called cross-links. 
  • These result in a structure called collagen fibril.

Collagen provides mechanical strength in:

  • The walls of arteries.
  • Tendons.
  • Bones.
  • Cartilage and connective tissue.                                    
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Collagen Pt.2

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Carbohydrates contain:

  • Carbon
  • Hydrogen
  • Oxygen

-It can act as a quick source of energy, energy storage or for structural purposes.

  • Monosaccharides are the monomers of carbohydrates. 
  • They contain between 3-6 carbon atoms. 
  • Triose sugar - 3 Carbons.
  • Pentose sugar - 5 Carbons.
  • Hexose sugar - 6 Carbons.

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How does the structure of glucose relate to it's functions in living organisms?

  • Contains a large number of bonds that can be broken down to form simple molecules.
  • The breaking of strong covalent bonds releases a large amount of energy, making it a good energy source.
  • Alpha-Glucose has a shape that is complementary to the active site of enzymes in animals and plants. 
  • As a simple sugar, it is soluble in water.
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  • A triglyceride consists of one glycerol molecules bonded to three fatty acid molecules.
  • A condensation reaction between the acid group of a fatty acid and one of the hydroxyl groups forms a covalent bond, called and Ester Bond.

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Lipids Pt.2

Phospholipids: consist of a glycerol molecule, two fatty acids and a phosphate group.

  • All bonds occur through condensation reactions.

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Chemical tests


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