A-LEVEL AQA BIOLOGY NOTES
general properties of proteins
Proteins
- Amino acids are the monomer units used to make proteins.
- The 20 naturally occurring amino acids only differ in their R groups
- Dipeptides are formed when two amino acids are joined together by a condensation reaction, forming a peptide bond.
- A polypeptide is a polymer made of many amino acids joined together by peptide bonds.
- A protein may contain one or more polypeptide chains.
Level |
Definition |
Solubility in Water |
Primary |
The specific sequence of amino acids in a polypeptide chain |
Peptide Bonds |
Secondary |
The curling or folding of the polypeptide chain into α-helices and β-pleated sheets due to the formation of hydrogen bonds |
Hydrogen Bonds |
Tertiary |
The overall specific 3-D shape of a protein, which is determined by interactions between R groups and the properties of R groups |
Hydrogen bonds Ionic bonds Disulphide bridges |
Quaternary |
The specific 3-D shape of a protein that is determined by the multiple polypeptide chains and/or prosthetic groups bonded together |
Hydrogen bonds Ionic bonds Disulphide bridges |
Biochemical tests
Molecule |
Reagent |
Positive result |
Reducing sugars |
Benedict’s reagent → Heat |
Red/orange precipitate |
Non-reducing sugars |
Hydrochloric acid → Heat Sodium hydrogencarbonate Benedict’s reagent → Heat |
Red/orange precipitate |
Starch |
Iodine in potassium iodide solution |
Blue/black |
Proteins |
Sodium hydroxide Copper (II) sulphate |
Purple |
Lipids |
Ethanol Water → Shake |
Cloudy white |