Proteins & water
Proteins and water are the two molecules life cannot manage without. In this lesson we build a protein from the ground up: first the amino acid, with its central carbon carrying an amino group, a carboxyl group, a hydrogen and a variable R group; then the peptide bond, formed by condensation and broken by hydrolysis. We climb the four levels of protein structure — primary, secondary (the alpha-helix and beta-pleated sheet), tertiary and quaternary — and name the four kinds of bond (hydrogen, ionic, disulfide and hydrophobic interactions) that hold a protein in shape. We compare haemoglobin, a globular protein of two alpha and two beta chains whose haem iron binds oxygen, with collagen, a fibrous protein whose triple-helix molecules bundle into strong fibres, and we learn the biuret test for protein. Finally we turn to water: why it is a polar molecule, how hydrogen bonds form between water molecules, and how that explains water's solvent action, high specific heat capacity and high latent heat of vaporisation.
By the end you should be able to (NSSCAS Biology (AS) 2.4.2):
- describe the structure of an amino acid and the formation and breakage of a peptide bond
- explain the meaning of the terms primary structure, secondary structure, tertiary structure and quaternary structure of proteins
- describe the types of bonding (hydrogen, ionic, disulfide and hydrophobic interactions) that hold these molecules in shape
- describe the molecular structure of haemoglobin as an example of a globular protein, and of collagen as an example of a fibrous protein and relate these structures to their functions (haemoglobin = two alpha (α) chains and two beta (β) chains; distinction between collagen molecules and collagen fibres)
- state the importance of iron in the haemoglobin molecule
- carry out biuret test to identify the content of solutions, food substances and biological specimens
- explain that water is a polar molecule and explain how hydrogen bonding occurs between water molecules
- relate the properties of water to its roles in living organisms (limited to solvent action, specific heat capacity and latent heat of vapourisation)
Miss Rachel and Mike talk through the whole topic — with the figure and working drawn live.