| Date | May 2012 | Marks available | 3 | Reference code | 12M.3.sl.TZ2.D3 |
| Level | SL | Paper | 3 | Time zone | TZ2 |
| Command term | Explain | Question number | D3 | Adapted from | N/A |
Question
The discovery of penicillin by Alexander Fleming in 1928 is often given as an example of serendipity in science.
Explain how penicillin works and why it is necessary to continue to develop new forms of penicillin with modified side chains.
Markscheme
Two of these for first two marks:
interferes with enzymes/chemicals that bacteria need to make (normal) cell walls / interferes with cell wall formation (in bacteria);
osmotic pressure causes (weakened) cell wall to break/burst / water enters cell causing it to burst / OWTTE;
\(\beta \)-lactam ring is strained/reactive;
Required for final mark:
bacteria become resistant to penicillins/produce the enzyme penicillinase / OWTTE
Examiners report
In (c) many candidates gave detailed descriptions of the interference of penicillin in the formation of cell walls, but it was not rare to see this connected with the bursting of the cell or the statement that this led to increased osmotic pressure without taking water into consideration. It was rather uncommon to find answers including \(\beta \)-lactam ring‘s reactivity. The vast majority of candidates were aware of problems related to bacteria becoming resistant.
