State the difference between an antigen and an antibody.

(a) State the difference between an antigen and an
antibody.
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(1)
(b) Explain antibody production.
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(3)
(c) State
two
other substances, apart from antibodies, transport
ed by the blood.
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(1)
(Total 5 marks)
9.
(a) Explain how the skin and mucous membranes prev
ent entry of pathogens into the body.
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(3)
(b) Explain why antibiotics are used to treat bacte
rial but not viral diseases.
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(2)
(Total 5 marks)
10.
Describe how human skin and mucous membranes act a
s barriers to pathogens.
(Total 4 marks)
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(3)
(b) Explain why antibiotics are used to treat bacte
rial but not viral diseases.
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(2)
(Total 5 marks)
11.
Explain the cause, transmission and social implica
tions of AIDS.
(Total 8 marks)
(1)
(Total 5 marks)
12.
Antibiotic peptides occur naturally on the surface
of human skin. One of these peptides, called
LL-37, and three other similar synthetic peptides w
ere investigated to assess both their
antibiotic properties and effect on human cells.
The graphs show the antibiotic effect of the peptid
es against two microbes,
S. aureus
and
C
.
albicans
. The technique involves measuring the inhibition o
f growth of the microbes.
10 10
100 100
40
20
5
40
20
5
8
6
4
2
8
6
4
2
S. aureus
C. albicans
Amount of inhibition /
arbitrary units
Peptide concentration /
μ
mol
dm
–3
Key for
peptides:
LLG
GKE
FKR
LL-37
[Source: T Sigurdardottir,
et al
., (2006),
Antimicrobial Agents and Chemotherapy
,
50
(9), pages 2983
2989]
(a) Describe the effect of the FKR on
C. albicans.
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(2)
(b) Compare the effects of the peptide LLG on
S. aureus
and
C. albicans
.
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(2)
(c) Evaluate the effectiveness of LL-37 against the
two microbes.
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(3)
There is concern that the peptides could damage liv
ing human cells. One form of damage is
fragmentation of DNA. Human cells were incubated fo
r 16 hours with each peptide at varying
concentrations. The amount of DNA fragmentation was
measured.
80 80 80 80
20
6
2
20
6
2
20
6
2
20
6
2
25
20
15
10
5
0
Peptide concentration
/ mol dm
μ
–3
DNA fragmentation / x-fold
increase over control
LL-37 LLG
GKE FKR
[Source: T Sigurdardottir,
et al
, (2006),
Antimicrobial Agents and Chemotherapy
,
50
(9), pages 2983
2989]
(d) State which peptide causes the least damage to
DNA.
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(1)
(e) Calculate the percentage increase in DNA damage
that results when the concentration of
LL-37 increases from 2
μ
mol dm
–3
to 6
μ
mol dm
–3
. Show your working.
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(2)
(f) Discuss the hypothesis that synthetic peptides
are most suitable for controlling
S. aureus
infection inside the human body.
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(4)
The shape of the secondary structure of a peptide c
an be predicted from its amino acid
composition. The figure shows the theoretical likel
ihood of the peptide LL-37 forming an
α
-helix, based on the properties and position of its
37 amino acids.
15
10
5
0
Possibility of forming -helix /
arbitrary units
α
Number of amino acids
30
20
10
[Source: T Sigurdardottir,
et al
., (2006),
Antimicrobial Agents and Chemotherapy
,
50
(9), pages 2983–2989]
(g) In addition to the
α
-helix, state a type of shape commonly formed as th
e secondary
structure of proteins.
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(1)
(h) Analyse the data to determine the region of LL-
37 most likely to form a helical shape.
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(2)
(i) Draw and label the structure of a peptide bond
between two amino acids.
(2)
(Total 19 marks)
1.
B
[1]
2.
A
[1]
3.
C
[1]
4.
B
[1]
5.
B
[1]
6.
B
[1]
7.
D
[1]
8.
(a)
Must have both for
[1]
.
antigen is a substance / molecule that causes antib
ody formation;
antibody is a (globular) protein /
molecule that recognizes an antigen;
1
(b) antigen causes an immune response to produce an
tibodies
specific for that antigen;
antibodies produced in B-lymphocytes;
B-lymphocytes produced in bone marrow;
carried in blood;
antigen presenting cell /
helper T cell present antigen to B cell; 3 max
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