B1.2 ProteinsIB Biology SL: Revision notes
Section 1
Structure of an amino acid
Every amino acid has a central alpha carbon atom bonded to four things: an amine group (–NH₂), a carboxyl group (–COOH), a hydrogen atom and an R-group. The R-group is the only part that differs between amino acids. You should be able to draw this generalized structure with all four groups attached to the alpha carbon.
Section 2
Condensation: dipeptides and polypeptides
Amino acids are joined by condensation reactions. The carboxyl group of one amino acid reacts with the amine group of the next, forming a peptide bond and releasing a molecule of water.
Word equation: amino acid + amino acid → dipeptide + water
Repeating the reaction adds more amino acids to make a polypeptide. A chain of n amino acids contains n − 1 peptide bonds, so n − 1 water molecules are released. The reverse reaction, which uses water, is hydrolysis.
Water comes from the carboxyl and amine groups, not the R-groups. Count peptide bonds, not amino acids, when calculating water released.
Section 3
Dietary requirements for amino acids
Essential amino acids cannot be synthesized by the body and must be obtained from food. Non-essential amino acids can be made from other amino acids. Plant proteins often contain low amounts of one or more essential amino acids, so a vegan diet needs attention: eating a variety of plant proteins together (for example cereals with legumes) means one food supplies what the other lacks.
You do not need to name examples of essential or non-essential amino acids.
Section 4
Infinite variety of polypeptides
The genetic code specifies 20 amino acids. They can be linked in any order, and chains can be any length, from a few amino acids to thousands. For a chain of n amino acids there are 20ⁿ possible sequences, so even a 9-amino-acid chain has 20⁹ = 5.12 × 10¹¹ possibilities. Examples of polypeptides include insulin (51 amino acids, a hormone), oxytocin (9, a hormone) and titin (about 34 000, gives muscle elasticity).
Section 5
Effects of temperature and pH: denaturation
A protein's function depends on its three-dimensional shape, which is held by relatively weak bonds between parts of the chain. High temperatures make the molecule vibrate more and break these bonds. Extremes of pH change the charges on amino acids and disrupt the bonds. The resulting permanent change in shape is denaturation. The sequence of amino acids is not changed, but the protein can no longer carry out its function, and soluble proteins often become insoluble and clump.
Denaturation does not break peptide bonds; the primary sequence remains. Avoid saying proteins are 'killed'.
That's the notes covered.
Carry on to the next subtopic.