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Date November 2020 Marks available 7 Reference code 20N.2.SL.TZ0.6
Level Standard level Paper Paper 2 Time zone TZ0 / no time zone
Command term Explain Question number 6 Adapted from N/A

Question

According to the cell theory, living organisms are composed of cells.

Draw the ultrastructure of a prokaryotic cell based on electron micrographs.

[3]
a.

Outline what occurs in cells in the first division of meiosis.

[5]
b.

Explain the role of cells in the defence against infectious disease.

[7]
c.

Markscheme

a. cell wall;

b. plasma membrane; Clearly shown as a separate line under the cell wall or the inner line

c. cytoplasm AND 70S ribosomes; Do not allow (small) circles

d. nucleoid/naked DNA;

e. plasmid
OR
pili
OR
flagella/flagellum;

Structures correctly drawn and labelled.

Award [2 max] if any exclusively eukaryotic structures are shown.

Do not allow cilia as they are not found in prokaryotes.

a.

a. halves the chromosome number/produces haploid cells;

b. at start of meiosis each chromosome consists of two sister chromatids attached by a centromere;

c. prophase (I): pairing of homologous chromosomes;

d. crossing over occurs;

e. chromosomes condense by supercoiling;

f. metaphase (I): pairs of homologous chromosomes/bivalents move to equator of spindle
OR
metaphase (I): orientation of pairs of homologous chromosomes (prior to separation) is random;

g. anaphase (I): centromeres do not divide
OR
anaphase (I): spindle fibre pulls chromosome/whole centromere with two sister chromatids to opposite poles;

h. telophase (I): arrival of centromere with sister chromatids at opposite poles;

Some of these can be awarded for correctly annotated diagrams.

No credit for events in meiosis II.

b.

a. cells of skin provide a physical barrier/produce fatty acids/lactic acid/lysozyme which stops entry of microbes
OR
mucous membranes produce mucus to trap pathogens
OR
stomach cells produce hydrochloric acid which kills microbes;

b. platelets start the clotting process preventing access of pathogens;

c. (two types of) white blood cells fight infections in the body;

d. phagocytes ingest pathogens (by endocytosis/phagocytosis);

e. gives non-specific immunity to diseases / ingest any type of pathogen;

f. production of antibodies by lymphocytes/B cells;

g. in response to particular pathogens/antigens;

h. gives specific immunity;

i. lymphocyte/B cell makes only one type of antibody;

j. plasma cells produce large quantity of (one type of) antibody;

k. some lymphocytes act/remain as memory cells;

l. can quickly reproduce to form a clone of plasma cells if a pathogen carrying a specific antigen is re-encountered;

m. results in faster defence against second exposure to specific antigen/pathogen/disease;

c.

Examiners report

In part (a) many candidates were penalised for drawing clear eukaryotic structures such as mitochondria. The general level of the drawings was better than in the past. However a prokaryote should not be represented as a square. Ribosomes must be shown as dots, not circles.

 

a.

In part (b), those who had been well prepared easily gained the 5 marks, but there was a significant proportion who did not know the difference between mitosis and meiosis.

b.

In part (c), defence against infection was well known. Poor candidates lost marks by using loose, non-biological expressions such as the ‘white blood cells eat the infection/disease’. However, most had a fair idea of the cellular response to infection.

c.

Syllabus sections

Core » Topic 6: Human physiology » 6.3 Defence against infectious disease
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