| Date | May 2019 | Marks available | 2 | Reference code | 19M.3.hl.TZ1.23 |
| Level | HL | Paper | 3 | Time zone | TZ1 |
| Command term | Explain | Question number | 23 | Adapted from | N/A |
Question
Opium and its derivatives have been used for thousands of years as strong analgesics.
Explain how opiates act to provide pain relief.
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.
Markscheme
«temporarily» bond/bind to «opioid» receptors in the brain/CNS [✔]
block the transmission of pain impulses [✔]
«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.
Examiners report
The question asked candidates to explain how opiates provide pain relief. This was difficult and was poorly answered by many. As this has been asked many times over the years, it would be an advantage to candidates to practise answering past examination questions.
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.
