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SECTION B
Answer all the questions.
21 Human insulin is a globular protein with a quaternary structure. One insulin molecule has
51 amino acids.
Fig. 21.1 shows the sequence of amino acids in one molecule of human insulin.
GLY- ILE- VAL- GLU- GLN- CYS- CYS- THR- SER- ILE- CYS- SER- LEU- TYR- GLN- LEU- GLU- ASN- TYR- CYS- ASN
PHE- VAL- ASN- GLN- HIS- LE
U- CYS- GLY- SER- HIS- LEU
- VAL- GLU- ALA- LEU-
TYR- LEU- V
AL- CYS-
GLY- G
LU-
G
A R
Y-
HE- GL
-P
- PHE
R O- THR- TYR
-P
THR- LYS
Fig. 21.1
(a) The amino acid cysteine is abbreviated to ‘CYS’ in Fig. 21.1. The side chain (R group) found
in cysteine is shown in Fig. 21.2.
Complete Fig. 21.2 to show the structure of the amino acid cysteine.
H C H
Fig. 21.2
[3]
(b) Explain how Fig. 21.1 shows that insulin has a quaternary structure.
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(c) Insulin is a hormone that regulates blood glucose concentration. People with type 1 diabetes
need to inject insulin, to reduce their blood glucose concentration, as they are unable to
produce their own insulin.
Diabetics need to inject insulin before every meal as insulin has a short half-life. Enzymes in
the liver cells break down insulin, which removes it from the blood.
Insulin glargine is a modified version of human insulin that lasts much longer in the blood.
Fig. 21.3 shows the sequence of amino acids in one molecule of human glargine with the
modifications in bold.
GLY- ILE- VAL- GLU- GLN- CYS- CYS- THR- SER- ILE- CYS- SER- LEU- TYR- GLN- LEU- GLU- ASN- TYR- CYS- GLY
PHE- VAL- ASN- GLN- HIS- LE
U - CYS- GLY- SER- HIS- LEU-
VAL- GLU- ALA- LEU-
TYR- LEU- V
AL- CYS-
GLY- G
LU-
G
R
-A
E- GLY
H
HE- P
- TH R- TYR- P
- PRO
- THR- LYS
ARG- ARG
Fig. 21.3
(i) Suggest why insulin glargine is long-lasting.
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The table shows some of the DNA triplet codes for amino acids.
1st base 2nd base of DNA triplet 3rd base
of DNA of DNA
triplet T C A G triplet
ATT ACT AAT AGT T
(ASN) (SER)
(ILE) Asparagine Serine
ATC ACC AAC AGC C
Isoleucine (THR)
A
Threonine
ATA ACA AAA AGA A
(LYS) (ARG)
(MET) Lysine Arginine
ATG ACG AAG AGG G
Methionine
GTT GCT GAT (ASP) GGT T
Aspartic
GTC GCC GAC acid GGC C
(VAL) (ALA) (GLY)
G
Valine Alanine Glycine
GTA GCA GAA (GLU) GGA A
Glutamic
GTG GCG GAG acid GGG G
In order to produce insulin glargine, the human insulin gene is modified by genetic
engineering. This is a process which can change the genetic code of the gene. The genetic
code of DNA triplet 21 is changed so that the amino acid it codes for is glycine instead of
asparagine.
(ii) With reference to the table, predict how the genetic code of DNA triplet 21 is changed
so that it codes for the amino acid glycine instead of the amino acid asparagine.
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(iii) The modified polypeptides that form insulin glargine are made inside cells.
The process of making the modified polypeptides that form insulin glargine involves
several steps. The process starts with the modified gene for insulin glargine.
Outline the steps involved in the process of making the modified polypeptides that
form insulin glargine, starting with the gene for insulin glargine until when the modified
polypeptides are made.
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23 The diagram shows the change in hydrostatic pressure across a capillary network in muscle
tissue.
2.3 kPa
4.6 kPa
(a) (i) On the diagram, draw an arrow to show the direction of movement of blood through the
capillary network. [1]
(ii) On the diagram, draw an arrow to show the direction in which oncotic pressure is
acting and suggest a value for oncotic pressure.
Write the value for oncotic pressure next to the arrow. [2]
(iii) Name the structure labelled W.
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(b) Describe how oncotic pressure is established.
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24 A student is investigating how sucrose is loaded into phloem sieve tubes by companion cells,
using a model of phloem tissue.
To create the model, the student added distilled water to a bag made from Visking tubing (an
artificial partially permeable membrane). They inserted a capillary tube into one end of the
Visking tubing and connected the other end to a flask containing 2 mol dm–3 sucrose solution.
They placed the Visking tubing and connected flask into a beaker of distilled water, as shown in
the diagram.
Capillary tube with
coloured water inside
Beaker
Rubber band
Flask of 2 mol dm–3
Bung
sucrose solution
Visking tubing containing
distilled water
Tap (closed)
Distilled water
Delivery tube
The capillary tube, the flask and the beaker of distilled water represent plant tissues involved in
the active loading of sucrose. The Visking tubing represents the cell surface membrane of the
phloem sieve tube.
The student opens the tap on the flask and the level of the coloured water in the capillary
remains the same. After a few minutes, the water level in the capillary tube starts to rise. The tap
is then closed after 5 minutes.
(a) (i) Describe what happens inside the model, immediately after the tap on the flask is
opened.
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(ii) Explain why the water level in the capillary tube starts to rise after a few minutes.
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(iii) With reference to the diagram, name the plant cells or tissues that are represented by:
The capillary tube
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The beaker of distilled water
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The flask
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[3]
(b) The student measured the increase in the level of the coloured liquid in the capillary tube
after 2 minutes. It had risen by 8 mm. The capillary tube has a diameter of 1 mm.
Calculate the rate of osmosis as the volume of water moved per second.
Use the formula: Volume of cylinder = π r 2l
Rate of osmosis = ............................................ mm3 s–1 [2]
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25 (a) Water molecules are transported in the stem of a sunflower. Water molecules are polar and
are therefore attracted to each other.
(i) Draw two water molecules and label the bond between the two molecules.
[3]
(ii) Explain how the properties of water are related to the transport role of water in a stem.
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(b) The photomicrograph is of a transverse section of a stem of a sunflower, Helianthus annuus.
(i) On the photomicrograph, label the location of meristem tissue. [2]
(ii) Name the type of microscope used to produce the photomicrograph and explain the
reasons for your choice.
Name of microscope .........................................................................................................
Reasons for your choice ...................................................................................................
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(c) Explain the role of meristem tissue in a stem.
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(d) Name one potential use of stem cells in medicine.
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END OF QUESTION PAPER
© OCR 2022
