Question 19M.3.hl.TZ1.b
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.3.hl.TZ1.b |
| Level | hl | Paper | 3 | Time zone | TZ1 |
| Command term | Discuss | Question number | b | Adapted from | N/A |
Opium and its derivatives have been used for thousands of years as strong analgesics.
Discuss how the difference in structure of two opiates, codeine and morphine, affect their ability to cross the blood–brain barrier. Use section 37 of the data booklet.
[2]
«codeine crosses blood–brain barrier more easily» morphine has more hydroxyl/OH «groups than codeine» [✔]
codeine/ether group is less polar
OR
hydroxyl/OH «groups in morphine» H-bond to water [✔]
Note: Award [1 max] if no statement or an incorrect statement about the blood–brain barrier.
The explanation that required a discussion of the difference in structure of codeine and morphine, and how this affects their ability to cross the blood-brain barrier was challenging. Many scored for saying “codeine is less polar”. Some also scored for saying that “morphine has more hydroxyl groups” but others provided less detail and could not be awarded any marks.
