A-LevelBiologyCell structure9700/23 · May/June 2025 · Q315 Marks

Staphylococcus epidermidis is a species of bacterium that lives on human skin. Staphylococcus aureus is a pathogenic bacterium that can infect humans.

(a)S. epidermidis and S. aureus are prokaryotes. Table 3.1 shows some cell features that could apply to typical prokaryotic cells or to typical eukaryotic cells or to both types of cell. Complete Table 3.1 by using a tick (√) if the feature applies to the type of cell or a cross (x) if the feature does not apply to the type of cell. Put a tick (✔) or a cross (x) in every box. Table 3.1 feature | prokaryotic cell | eukaryotic cell ---|---|--- circular DNA | | 80S ribosomes | | a cell diameter of 20 µm | |[3]
(b)One way that S. aureus can infect humans is through wounds (breaks) in the skin. Populations of S. aureus develop on human skin as part of a biofilm. The biofilm contains cells of S. aureus within a mixture of polymers that have been secreted by the cells. S. epidermidis produces a protease enzyme that prevents the growth of S. aureus populations on human skin. Proteases catalyse the breakdown of proteins. Fig. 3.1 is a diagram showing populations of S. epidermidis and S. aureus on human skin cells. [Figure 3.1] Suggest and explain how the protease produced by S. epidermidis cells prevents the growth of an S. aureus population on human skin.[3]
(c)(i)Describe the role of goblet cells in the protection of tissues in the trachea from infection by S. aureus.[2]
(c)(ii)S. aureus cells can infect tissues by passing in between cells that line the lumen of the trachea. State the name of a cell type, other than goblet cells, that lines the lumen of the trachea.[1]
(d)(i)Vancomycin and penicillin are antibiotics that are used to treat infectious diseases caused by S. aureus. Fig. 3.2 shows the mechanism of action of vancomycin. [Figure 3.2] With reference to Fig. 3.2, describe the similarities and differences between the mechanism of action of vancomycin and the mechanism of action of penicillin.[3]
(d)(ii)Some strains of S. aureus are resistant to vancomycin and penicillin. Describe the steps that can be taken to reduce the impact of antibiotic resistance.[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

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

[3] circular DNA: prokaryotic cell (✔), eukaryotic cell (X)
[3] 80S ribosomes: prokaryotic cell (X), eukaryotic cell (✔)
[3] a cell diameter of 20 µm: prokaryotic cell (X), eukaryotic cell (✔)
[3] one mark per correct row ; if no marks gained, check correct column for 1 mark
[3] allow enzyme for protein
[3] any three from:
[3] protease (produced by S. epidermidis) is, secreted / released ; A protease is an extracellular enzyme
[3] protease breaks down proteins (needed for survival) specific to S. aureus ; AW ; explanation ; e.g. protease active site complementary to substrate (of S. aureus) ; protease complementary to substrate and form enzyme-substrate complexes
[3] protease breaks down, protein in biofilm / (biofilm) polymers ;
[3] idea that without biofilm the S. aureus cannot remain attached to skin cells ;
[3] other example of S. aureus protein broken down ; e.g. cell membrane proteins / receptors / binding sites
[3] AVP; e.g. S. aureus cells need substances from biofilm for survival
[2] allow, bacteria/pathogens/microorganisms, for S. aureus
[2] secrete / produce / AW, mucus / mucin;
[2] mucus, traps S. aureus cells / acts as a barrier (to reach cells); R virus
[1] ciliated epithelial cell ; A ciliated epithelium cell ; I ciliated cell
[3] any three from:
[3] prevent formation of crosslinks / cross bridges, (between peptidoglycan chains); if not gained, allow ecf for vancomycin difference
[3] stop/prevent, synthesis / repair, of cell wall ;
[3] differences
[3] penicillin, binds to / is an inhibitor of, enzyme(s) / transpeptidase(s) (that catalyse formation of cross links) ; ora
[3] vancomycin, binds to / acts on / AW, peptidoglycan / cross link, components ; ora
[3] idea that vancomyin, blocks access to peptidoglycan component/may prevent binding of enzyme that joins peptidoglycan subunits together;
[3] if no marks gained because of incorrect knowledge of pencillin mechanism of action, allow one mark as ecf for an answer incorporating a feature of vancomycin mechanism of action shown on Fig. 3.2
[3] any three from:
[3] 1 prescribing / take, antibiotics only when (absolutely) necessary ; A examples e.g. do not use for viral infections ; do not use as preventative medicine
[3] 2 make sure, correct / effective, antibiotic(s), prescribed / used ; A only use antibiotic for the prescribed condition
[3] 3 complete course / follow instructions for use ; A ref. to DOTS
[3] 4 use other antibacterials ; or develop new, drugs / antibiotics ;
[3] 5 reduce/control, antibiotics in, agriculture / animals used for food;
[3] 6 ref. to break transmission cycle / described example ; e.g. vaccines ; good hygiene in hospitals ; quarantine
[3] 7,8 AVP;; e.g check/improve / AW, knowledge of, healthcare professionals / public, qualified ; report patterns of antibiotic resistance / AW ; ref. to monitor to check if antibiotic is effective ; ref. to WHO Global Plan to End TB ; vary antibiotic treatment ; use a number of different antibiotics at the same time ; limit/prevent, antibiotics being sold context is control by prescription