| Date | May 2019 | Marks available | 2 | Reference code | 19M.3.sl.TZ1.18 |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | Explain | Question number | 18 | 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
Most candidates correctly remembered the mode of action strong analgesics, though some had difficulty expressing it clearly.
Many candidates identified the relevant structural difference from the data booklet structures and could use these to explain the difference in the ability of the drugs to reach the brain, though some were confused as to whether this was favoured by aqueous solubility. This again was more frequently discussed in terms of polarity than the ability of –OH groups to form hydrogen bonds.
