A-LevelBiologyInfectious diseases9700/22 · Oct/Nov 2024 · Q217 Marks

People who become infected with human immunodeficiency virus (HIV) are at risk of developing HIV/AIDs, particularly if antiretroviral therapy (ART) is not available.

(a)In people infected with HIV, the use of ART also helps to reduce transmission of the virus to uninfected people. Outline two control methods, other than ART, that can be used to reduce the transmission of HIV.[2]
(b)In people with HIV/AIDs, a serious lung disease known as pneumocystis pneumonia can result from infection by an opportunistic pathogen known as Pneumocystis jirovecii. Fig. 2.1 shows P. jirovecii cells in one stage of their life cycle, as seen using a light microscope at a magnification of x600. [Figure 2.1] Define magnification.[1]
(c)(i)Fig. 2.1 shows that P. jirovecii is a unicellular organism. Although the cells of many species of bacteria are the same size as those of P. jirovecii, research concluded that the organism is a eukaryote and is not a bacterium. In 1988, analysis of ribosomal RNA (rRNA) resulted in P. jirovecii being classified as a fungus. Studies of the structure of P. jirovecii have identified that the cell wall is made of polysaccharides such as chitin and 1,3-β-D-glucan. Explain why this feature helped scientists to confirm that P. jirovecii is not a bacterium.[1]
(c)(ii)Scientists have identified other features of the cell structure of P. jirovecii. Some of these are listed in Table 2.1. Complete each row of Table 2.1 so that the table shows: • four structural features identified in P. jirovecii • one function for each structural feature • whether the structural feature is present (✔) or absent (x) in bacterial cells. Table 2.1 structural feature of P. jirovecii | function | present (✔) or absent (X) in bacterial cells ---|---|--- ribosomes | protein synthesis | smooth endoplasmic reticulum || Golgi body | modification of proteins and lipids | aerobic respiration ||[3]
(d)(i)P. jirovecii can adhere (attach) to squamous epithelial cells of the alveoli and to the network of fibrous proteins that support the alveolar wall, known as the extracellular matrix (ECM). Examples of proteins in the ECM are elastin and collagen. Adhesion (attachment) of P. jirovecii to alveolar epithelial cells and the ECM stimulates the growth of its population. Cell surface glycoproteins known as gpA glycoproteins are essential in allowing P. jirovecii cells to adhere to alveolar epithelial cells and ECM proteins. Suggest how a gpA glycoprotein is able to adhere to alveolar epithelial cells and ECM proteins.[2]
(d)(ii)One consequence of the pneumonia that results from P. jirovecii infection is a decrease in the quantity of oxygen that is delivered to body tissues. Explain why a severe P. jirovecii infection results in a decrease in the quantity of oxygen that is delivered to body tissues.[3]
(e)P. jirovecii produces an enzyme known as 1,3-β-D-glucan synthase. The enzyme catalyses the synthesis of 1,3-β-D-glucan. The therapeutic drug caspofungin is a non-competitive inhibitor of 1,3-β-D-glucan synthase. With reference to the mechanism of action of caspofungin, explain how the drug may be useful to treat cases of pneumonia caused by P. jirovecii.[5]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.

[2] I protective gear / education about transmission / contact tracing / early diagnosis / testing any two from: practising safe sex / example ; A protected sex / using protection during sex A barrier contraceptives I use contraceptives screening blood, donated for / before, transfusions AW or not taking blood from high risk individuals / AW ; idea of treating donated blood (to inactivate viruses) ; A example e.g. heat / chemical treatment solvent-detergent treatment in context of drug abusers avoid sharing, needles / syringes / drug injecting equipment AW or use, new / sterile, needles / syringes / drug injecting equipment ; A cookers / sterile containers to mix drugs A use needle-exchange schemes / AW A use safe injection sites / AW avoid breast feeding / use baby milk products instead of breast milk ; pre-exposure prophylaxis / PrEP or post-exposure prophylaxis / PEP ; AVP ; e.g. using sterile equipment for, tattooing / surgery / dental procedures / body piercing
[1] the number of times larger an image is than actual ; AW I or smaller A formula stated in words A symbolised I/A to support a weakly worded answer A (in context of describing an image) enlargement/making larger / making bigger / increase in size / AW
[1] any one from: because bacteria have cell walls (only) of, murein / peptidoglycan ; because chitin is not found in bacterial cell walls ; A chitin / β-D-glucans, main/major/AW component of fungal cell walls
[3] structural feature of P. jirovecii | function | present (✔) or absent (x) in bacterial cells ribosomes | protein synthesis | ✔ smooth endoplasmic reticulum | lipid / cholesterol / steroid, synthesis / metabolism AVP ; detoxification I carbohydrate metabolism | X Golgi body | modification of proteins and lipids | X mitochondrion / mitochondria | aerobic respiration | X one mark each correct column ;;; assume a tick that has been crossed is a cross
[2] allow squamous cells or squamous epithelial cells or epithelial cells for alveolar cells any two from: 1 gpA / (P. jirovecii) glycoprotein, binds to a, receptor / protein / glycoprotein on (surface of) alveolar cell or gpA binds to ECM, protein / glycoprotein ; I receptor A idea that ECM proteins are on, surface of / interacting with, (cell surface membrane of) alveolar cells or receptor / protein / glycoprotein on (surface of) alveolar cell is a binding site (for gpA) A same idea for ECM 2 (gpA) complementary shape ; allow ecf mp1 for gpA complementary shape to receptor on ECM or idea of complementary shape to attach / bind / adhere, to, alveolar cell / ECM 3 suggestion of adhering by, attractive charges / named bond types, between, gpA and, proteins / glycoproteins, on surface of alveolar cell / of ECM ; 4 AVP ; e.g. gpA acts as a ligand
[3] any three from: in a person with malaria 1 alveolar cells / ECM, hindered / surrounded by / AW, P. jirovecii cells ; A alveolar wall becomes, thicker / AW 2 diffusion (of oxygen), impaired / decreased / AW ; A distance for diffusion increased A reduced surface area for diffusion ; 3 between, alveolus / alveolar air / alveolar space, and, capillary / blood / red blood cell(s) ; allow mp if oxygen is clearly going in the correct direction 4 less / AW, oxyhaemoglobin formed ; AW e.g. A less oxygen, binds to / taken up by / associates with, haemoglobin A less oxygen forms bonds with, Fe2+/iron ion / iron, (in haemoglobin) 5 idea of ability of elastic fibres to, stretch / recoil, impaired / AW ; A idea of less elastic / decreased elasticity (of alveoli) 6 alveolar air not, refreshed / AW, so decreased diffusion gradient ; 7 AVP ; e.g. presence of macrophages hinders diffusion of oxygen suggestion that P. jirovecii infection, damages / AW, alveolar capillaries / venules / branches of pulmonary vein suggestion that oxygen used up for P. jirovecii, metabolism / activity / AW less ability of collagen to provide support to alveoli / AW
[5] any five from: caspofungin / drug, binds to site on, enzyme / glucan synthase, other than active site ; A binds to allosteric site changes shape of active site ; A enzyme changes, shape / tertiary structure, so active site is changed substrates cannot bind to active site / active site no longer complementary to substrates / enzyme-substrate complexes do not form ; A fewer enzyme-substrate complexes form less / no, (1,3- β-D) glucans / products, synthesised / produced ; cell wall, weakened / synthesis hindered / AW ; A cell wall not formed (leads to osmotic) lysis of, P. jirovecii / cell / fungus ; A bursting / cytolysis reduces / prevents, population growth ; AW e.g. decrease number of P. jirovecii A decreases ability of P. jirovecii to colonise / AW AVP ; e.g. in context of lysis ref. water entry into cell, by osmosis / down water potential gradient / AW (reduced number) increases chance of immune system eliminating the fungus