Paper 3s (but P1&P2 can come up too!)

Question 1

Describe how the human immunodeficiency virus (HIV) is replicated once inside
helper T cells (TH cells).
[4 marks]

Answer 1

1. RNA converted into DNA using reverse
   transcriptase;
2. DNA incorporated/inserted into (helper T
   cell) DNA/chromosome/genome/nucleus;
3. DNA transcribed into (HIV m)RNA;
4. (HIV mRNA) translated into (new) HIV/viral
   proteins (for assembly into viral particles);

Question 2

Freshwater marsh soils are normally waterlogged. This creates anaerobic conditions.
Use your knowledge of the nitrogen cycle to suggest why these soils contain relatively
high concentrations of ammonium compounds and low concentrations of nitrite ions
and nitrate ions.
[2 marks]

Answer 2

1. Less nitrification
   OR
   Fewer/less active nitrifying bacteria;
   OR
   Nitrification/nitrifying bacteria require
   oxygen/aerobic conditions;
2. (Less) oxidation/conversion of ammonium (ions)
   to nitrite (ions) and to nitrate (ions);
3. More denitrification
   OR
   More/more active denitrifying bacteria
   OR
   Denitrification/denitrifying bacteria do not require
   oxygen
   OR
   Denitrification/denitrifying bacteria require
   anaerobic conditions;
4. (So more) nitrate (ions) reduced/converted to
   nitrogen (gas);

Question 3

The action of endopeptidases and exopeptidases can increase the rate of protein
digestion. Describe how.
[2 marks]

Answer 3

1. Exopeptidases hydrolyse peptide bonds at the
   ends of a polypeptide/protein
   AND endopeptidases hydrolyse internal peptide
   bonds within a polypeptide/protein;
2. More ‘ends’
   OR
   More surface area;

Question 4

As humans age, there is a decrease in body protein.
Give the name of one body protein that could have resulted in:
[2 marks]
reduced muscle power
reduced immunity

Answer 4

1. Actin/myosin/tropomyosin;
2. Antibodies;

Question 5

Name two techniques the scientists may have used when analysing viral DNA to
determine that the viruses were closely related.
[1 mark]

Answer 5

For one mark, accept any two of the following:
* The polymerase chain reaction
* Genetic/DNA fingerprinting
* (Gel) electrophoresis
* DNA/genome sequencing;

Question 6

Determining the genome of the viruses could allow scientists to develop a vaccine.
Explain how.
[2 marks]

Answer 6

1. (The scientists) could identify proteins (that
   derive from the genetic code)
   OR
   (The scientists) could identify the proteome;
2. (They) could (then) identify potential antigens
   (to use in the vaccine);

Question 7

Describe how the B lymphocytes of a frog would respond to vaccination against
Ranavirus.
You can assume that the B lymphocytes of a frog respond in the same way as
B lymphocytes of a human.
Do not include details of the cellular response in your answer.
[3 marks]

Answer 7

1. B cell (antibody) binds to (viral)
   specific/complementary receptor/antigen;
2. B cell clones
   OR
   B cell divides by mitosis;
3. Plasma cells release/produce (monoclonal)
   antibodies (against the virus);
4. (B/plasma cells produce/develop) memory cells

Question 8

A high concentration of sodium in the blood can affect blood volume and cause
hypertension.
Use your knowledge of water potential to suggest how high sodium concentrations in
the medicines taken could affect blood volume.
[3 marks]

Answer 8

1. (Sodium ions) lower the water potential (of
   blood);
2. Water would move into the blood by osmosis
   (from cells/tissue fluid);
3. Increasing the blood volume;

Question 9

Describe the advantage of the Bohr effect during intense exercise.
[2 marks]

Answer 9

1. Increases dissociation of oxygen;
2. For aerobic respiration at the
   tissues/muscles/cells
   OR
   Anaerobic respiration delayed at the
   tissues/muscles/cells
   OR
   Less lactate at the tissues/muscles/cells;

Question 10

EPO is another performance-enhancing drug. It can increase the haematocrit (the
percentage of red blood cells in blood). A heart attack is caused by a lack of glucose and oxygen being delivered to
cardiac muscle via the coronary arteries. The overuse of EPO can increase the risk of
a heart attack.
Suggest how.
[2 marks]

Answer 10

1. (EPO) causes blood to thicken;
2. (The thickened blood) could block the coronary
   arteries
   OR
   (The thickened blood) slows blood flow
   OR
   (The thicker blood) could cause clots;

Question 11

Describe how ultrafiltration occurs in a glomerulus.
[3 marks]

Answer 11

1. High blood/hydrostatic pressure;
2. Two named small substances pass out eg
   water, glucose, ions, urea;
3. (Through small) gaps/pores/fenestrations in
   (capillary) endothelium;
4. (And) through (capillary) basement membrane;

Question 12

Describe the role of saprobionts in the nitrogen cycle.
[2 marks]

Answer 12

1. (They use enzymes to) decompose
   proteins/DNA/RNA/urea;
2. Producing/releasing ammonia/ammonium
   compounds/ammonium ions;

Question 13

One environmental issue arising from the use of fertilisers is eutrophication.
Eutrophication can cause water to become cloudy.
You are given samples of water from three different rivers.
Describe how you would obtain a quantitative measurement of their cloudiness.
[3 marks]

Answer 13

Accept any valid method, for example
1. Use of colorimeter;
2. Measure the absorbance/transmission (of
light);
3. Example of how method can be
standardised eg same volume of water,
zeroing colorimeter, same wavelength of
light, shaking the sample;

Question 14

Give two safety precautions that should be followed when dissecting a heart.
[1 mark]

Answer 14

Accept any two suitable safety precautions for
1 mark, eg;
Use a sharp scalpel/scissors
Wash hands/wear gloves
Disinfect bench/equipment
Cover any cuts
Cut away from self/others/on a hard surface
Safe disposal

Question 15

The acidic pH conditions created by osteoclasts cause the inactive form of the protein
osteocalcin to change into the active form of osteocalcin.
Suggest how.
[2 marks]

Answer 15

1. (Change in pH) changes / breaks ionic /
   hydrogen bonds;
2. Changes tertiary structure;

Question 16

Binding of insulin leads to an increase in the rate of respiration in cells such as
osteoblasts.
Explain how.
[2 marks]

Answer 16

1. (Insulin) leads to more transport proteins /
   channel (proteins) / carrier (proteins) for
   glucose;
2. More glucose (for respiration / glycolysis)
   enters cell;

Question 17

If most of the mitochondria in a cell are faulty, this prevents many important
enzyme-catalysed reactions taking place or slows them down.
Suggest and explain one reason why.
[2 marks]

Answer 17

1. Not enough / little ATP produced;
2. ATP provides energy for (enzyme) reactions
   OR
   ATP phosphorylates substrates / enzymes, so
   making them (more) reactive;

Question 17

If most of the mitochondria in a cell are faulty, this prevents many important
enzyme-catalysed reactions taking place or slows them down.
Suggest and explain one reason why.
[2 marks]

Answer 17

1. Not enough / little ATP produced;
2. ATP provides energy for (enzyme) reactions
   OR
   ATP phosphorylates substrates / enzymes, so
   making them (more) reactive;

Question 18

Binding of one molecule of oxygen to haemoglobin makes it easier for a second
oxygen molecule to bind.
Explain why.
[2 marks]

Answer 18

1. Binding of first oxygen changes tertiary /
   quaternary (structure) of haemoglobin;
2. Creates / leads to / uncovers second / another
   binding site
   OR
   Uncovers another iron / Fe / haem group to
   bind to;

Question 19

When counting, cells that touch top or left lines are counted but cells that touch right
or bottom lines are not counted.
Suggest two reasons for this rule.
[2 marks]

Answer 19

1. To avoid dealing with parts of cells;
2. To avoid counting same cells twice / more than
   once;
3. To be consistent / get comparable results;

Question 20

Explain how the stain allowed the doctor to count the white blood cells amongst all the
red blood cells. [1 mark]

Answer 20

White cells have a nucleus (that stains but red cells
do not);

Question 21

Scientists investigated the role of a protein called CENP-W in mitosis. Their method
involved cell fractionation and ultracentrifugation. The scientists began by lysing (breaking open) cells and organelles using a detergent
that dissolves lipids in water.
Suggest how the detergent releases CENP-W from cells.
[2 marks]

Answer 21

1. Cell membranes made from phospholipid;
2. (Detergent) dissolves membranes /
   phospholipid (bilayer);

Question 22

Explain how ultracentrifugation separates CENP-W from other molecules.
[2 marks]

Answer 22

1. Spin (liquid / supernatant) at (very) high speed /
   high g;
2. Molecules / CENP-W separates depending on
   (molecular) mass / size / density;

Question 23

The scientists set up their cultures in sterile glass bottles.
Suggest one suitable method for sterilising the bottles and explain why it was
necessary to sterilise them.
[2 marks]

Answer 23

1. Suitable method;
   eg in boiling water / steam / autoclave / wash in
   disinfectant / wash in alcohol
2. (Reason) to remove / kill other bacteria /
   organisms that might break down ammonia;