Inhibitors are substances that prevent biological processes in a variety of different ways. Table 5.1 shows some antibiotics, their modes of action and the diseases which they are used to treat. Table 5.1 antibiotic | mode of action | disease --- | --- | --- erythromycin | binds to ribosomes to inhibit translation | cholera penicillin | enzyme inhibitor | tetanus rifampicin | inhibits the function of RNA polymerase in transcription | tuberculosis

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

1. [1] any two from:
2. [1] bacteria, have 70S ribosomes and humans have 80S ribosomes / only have 70S ribosomes ;
3. [1] antibiotic, cannot bind to 80S / human ribosomes
4. [1] or
5. [1] can only bind to 70S / bacterial ribosomes ;
6. [1] antibiotic, binding site/target site, not present on 80S ribosomes / only present on 70S ribosomes ;
7. [1] A ref. to complementary / specfic – alternative to second part of the MP
8. [1] antibiotic cannot enter mitochondria to reach 70S ribosomes (in humans);
9. [1] AVP; different combinations of proteins in ribosomes / different rRNAs in ribosomes
10. [1] any one from:
11. [1] penicillin acts on cell walls and human cells have no, peptidoglycans / cell walls / enzyme(s) that make cell walls; A murein
12. [1] penicillin prevents formation of peptide / cross, bridges / linkages, (between peptidoglycans) in (bacterial) cell wall ;
13. [1] prevents repair of, gaps / holes / AW, in (bacterial) cell walls caused by autolysins;
14. [1] I ref. to penicillinase
15. [1] any one from:
16. [1] rifampicin
17. [1] blocks / fits into / binds to / competes for, the active site of, RNA polymerase / enzyme ;
18. [1] binds, to allosteric site / site away from active site, to, change shape of active site / prevent it forming phosphodiester bonds;
19. [1] attaches to DNA template blocking RNA polymerase moving along it ;
20. [1] can be incorporated into RNA but no nucleotide can be added to it ;
21. [1] AVP; e.g. attaches to the promoter to stop RNA polymerase, binding to DNA / moving along DNA/AW