| Date | May 2012 | Marks available | 3 | Reference code | 12M.3.SL.TZ1.9 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Explain | Question number | 9 | Adapted from | N/A |
Question
Explain the significance of polar and non-polar amino acids in proteins.
Markscheme
polar amino acids are water soluble/hydrophilic, non-polar amino acids are not/hydrophobic;
distribution of amino acids influences the position of proteins (in membranes);
polar amino acids often found on outside of protein, non polar orientate themselves away from water/ in core of protein;
polar amino acids create hydrophilic channels through membranes;
non-polar amino acids interact with lipid bi-layer/stabilize proteins in membranes;
polarity of amino acids determines the specificity of active sites in enzymes;
polar and non-polar amino acids affect the tertiary and quaternary/3D structure of proteins;
Examiners report
It would seem that many candidates did not fully understand what was required in this question, and the meaning of the word ”significance” may have misled some students. The question was asking about proteins, but many answered solely in terms of membranes. Those who gained marks did so mainly for referring to the hydrophilic/ hydrophobic nature of amino acids and their ability to affect the tertiary/quarternary/3D structure of proteins.
