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Date November 2019 Marks available 3 Reference code 19N.3.sl.TZ0.18
Level SL Paper 3 Time zone TZ0
Command term Explain and Apply Question number 18 Adapted from N/A

Question

Infectious diseases can be caused by bacteria or viruses.

State one difference between bacteria and viruses.

[1]
a.

Discuss two difficulties, apart from socio-economic factors, associated with finding a cure for AIDS.

[2]
b.

The discovery of penicillins contributed to the development of antibiotics.

Explain how the beta-lactam ring is responsible for the antibiotic properties of penicillin. Refer to section 37 of the data booklet.

[3]
c.

Markscheme

Any one of:
bacteria perform living functions «on their own and viruses do not without host cell»
OR
bacteria have cell walls «and viruses do not»
OR
bacteria do not have a capsid «and viruses do»
OR
bacteria larger than viruses
OR
bacteria reproduce by fission/budding «and viruses reproduce within a living host cell»
OR
bacteria affected by antibiotics «while viruses are not» ✔

NOTE: Accept “bacteria have flagella/cytoplasm/ribosome «and virus can have head/protein tail/double stranded RNA/single stranded DNA»”

Accept “asexual reproduction” for bacteria.

Accept other specific structural differences between bacteria and viruses, and examples of living functions that bacteria perform (such as excretion, reproduction etc.) that viruses do not.

Accept “bacteria are living and viruses are not”

a.

Any two of:
HIV difficult to detect/remains dormant ✔

HIV mutates rapidly/quickly ✔
NOTE: Do not accept “AIDS mutates” without mention of the HIV/virus.

HIV replicates rapidly/quickly ✔

HIV destroys «T-» helper cells/white blood cells/lymphocytes
OR
HIV attacks immune system ✔

HIV has several «significantly different» strains/subtypes ✔

NOTE: Accept “virus” for “HIV”.
Penalize the use of “AIDS” for “HIV” once only.
Accept “HIV metabolism linked to that of host cell” OR “drugs harm host cell as well as HIV”.

b.

ring is «sterically» strained
OR
angles of 90° instead of 109.5/109/120° angles
OR
angles smaller than 109.5/109/120°/tetrahedral/trigonal planar/triangular planar angle ✔
NOTE: Accept arguments using correct descriptions of hybridization for M1.


ring breaks up/opens/reacts «easily»
OR
amido/amide group «in ring» is «highly» reactive ✔


«irreversibly» binds/bonds to enzyme/transpeptidase
OR
inhibits enzyme/transpeptidase «in bacteria» that produces cell walls
OR
prevents cross-linking of «bacterial» cell walls ✔

NOTE: Do not accept "breaks/binds to cell walls" – a reference to the enzyme is needed for alternatives 1 and 2 for M3.
Do not accept “cell membrane” for “cell wall” for M3.

c.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.

Syllabus sections

Options » D: Medicinal chemistry » D.2 Aspirin and penicillin
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Options » D: Medicinal chemistry
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