A- level paper 1 june 2017

Question 1

Give the two types of molecule from which a ribosome is made

Answer 1

One of RNA/ribonucleic acid(s)/nucleotide(s)/nucleic
acid(s)/rRNA/ribosomal RNA/ribosomal ribonucleic
acid
and
one of protein(s)/polypeptide(s)/amino acid(s)/
peptide(s)/ribosomal protein;

Question 2

Give the two types of molecule from which a ribosome is made

Answer 2

One of RNA/ribonucleic acid(s)/nucleotide(s)/nucleic
acid(s)/rRNA/ribosomal RNA/ribosomal ribonucleic
acid
and
one of protein(s)/polypeptide(s)/amino acid(s)/
peptide(s)/ribosomal protein;

Question 3

Describe the role of a ribosome in the production of a polypeptide. Do not include
transcription in your answer.
[3 marks]

Answer 3

1. mRNA binds to ribosome;
2. Idea of two codons/binding sites;
3. (Allows) tRNA with anticodons to bind/associate;
4. (Catalyses) formation of peptide bond between
   amino acids (held by tRNA molecules);
5. Moves along (mRNA to the next
   codon)/translocation described;

Question 4

In a eukaryotic cell, the base sequence of the mRNA might be different from the
sequence of the pre-mRNA.
Explain why.
[2 marks]

Answer 4

1. Introns (in pre-mRNA);
2. Removal of sections of (pre-mRNA)/splicing;
   - ‘Introns removed’
   scores 2 marks.

Question 5

In mammals, in the early stages of pregnancy, a developing embryo exchanges
substances with its mother via cells in the lining of the uterus. At this stage, there
is a high concentration of glycogen in cells lining the uterus.
Describe the structure of glycogen.
[2 marks]

Answer 5

1. Polysaccharide of α-glucose;
OR
polymer of α-glucose;
2. (Joined by) glycosidic bonds
 OR
Branched structure;

Question 6

During early pregnancy, the glycogen in the cells lining the uterus is an important
energy source for the embryo.
Suggest how glycogen acts as a source of energy.
Do not include transport across membranes in your answer.
[2 marks]

Answer 6

1. Hydrolysed (to glucose);

2. Glucose used in respiration;

Question 7

Suggest and explain two ways the cell-surface membranes of the cells lining the
uterus may be adapted to allow rapid transport of nutrients.
[2 marks]

Answer 7

1. Membrane folded so increased/large surface
   area;
   OR
   Membrane has increased/large surface area
   for (fast) diffusion/facilitated diffusion/active
   transport/co-transport;
2. Large number of protein channels/carriers (in
   membrane) for facilitated diffusion;
3. Large number of protein carriers (in membrane)
   for active transport;
4. Large number of protein (channels/carriers in
   membrane) for co-transport;

Question 8

Sodium ions from salt (sodium chloride) are absorbed by cells lining the gut. Some
of these cells have membranes with a carrier protein called NHE3.
NHE3 actively transports one sodium ion into the cell in exchange for one proton
(hydrogen ion) out of the cell.
Use your knowledge of transport across cell membranes to suggest how NHE3
does this.
[3 marks]

Answer 8

1. Co-transport;
2. Uses (hydrolysis of) ATP;
3. Sodium ion and proton bind to the protein;
4. Protein changes shape (to move sodium

Question 9

The scientists carried out a statistical test to see whether the difference in the
means was significant. They calculated a P value of less than 0.05.
They concluded that Tenapanor did reduce salt absorption in the gut.
Use all the information provided and your knowledge of water potential to explain
how they reached this conclusion.
[4 marks]

Answer 9

1. Tenapanor/(Group)B/drug causes a significant
   increase;
   OR
   There is a significant difference with
   Tenapanor/drug/between A and B;
2. There is a less than 0.05 probability that the
   difference is due to chance;
3. (More salt in gut) reduces water potential in gut
   (contents);
4. (so) less water absorbed out of gut (contents)
   by osmosis
   OR
   Less water absorbed into cells by osmosis
   OR
   Water moves into the gut (contents) by
   osmosis.
   OR
   (so) water moves out of cells by osmosis;

Question 10

High absorption of salt from the diet can result in a higher than normal
concentration of salt in the blood plasma entering capillaries. This can lead to a
build-up of tissue fluid.
Explain how.
[2 marks]

Answer 10

1. (Higher salt) results in lower water potential of
   tissue fluid;
2. (So) less water returns to capillary by osmosis
   (at venule end);
   OR
3. (Higher salt) results in higher blood
   pressure/volume;
4. (So) more fluid pushed/forced out (at arteriole
   end) of capillary;

Question 11

Bacteria are often used in industry as a source of enzymes. One reason is
because bacteria divide rapidly, producing a large number of them in a short time.
Describe how bacteria divide.
[2 marks]

Answer 11

1. Binary fission;
2. Replication of (circular) DNA;
3. Division of cytoplasm to produce 2 daughter
   cells;
4. Each with single copy of (circular) DNA;

Question 12

Explain the shape of the curves at 50 °C and 60 °C.

[4 marks]

Answer 12

1. Both denatured (by high temperature);
2. Denaturation faster at 60 °C due to more
   (kinetic) energy;
3. Breaks hydrogen/ionic bonds (between amino
   acids/R groups);
4. Change in shape of the active site/active site no
   longer complementary so fewer enzymesubstrate complexes formed/substrate does not
   fit;

Question 13

Some proteases are secreted as extracellular enzymes by bacteria.
Suggest one advantage to a bacterium of secreting an extracellular protease in its
natural environment.
Explain your answer.
[2 marks]

Answer 13

To digest protein;
2. (So) they can absorb amino acids for
growth/reproduction/protein synthesis/synthesis
of named cell component;
OR
(So) they can destroy a toxic substance/protein;

Question 14

Mammals have some cells that produce extracellular proteases. They also have
cells with membrane-bound dipeptidases.
Describe the action of these membrane-bound dipeptidases and explain their
importance.
[2 marks]

Answer 14

1. Hydrolyse (peptide bonds) to release amino
   acids;
2. Amino acids can cross (cell) membrane;
   OR
   Dipeptides cannot cross (cell) membrane;
   OR
   Maintain concentration gradient of amino acids
   for absorption;
   OR
   Ensure (nearly) maximum yield from protein
   breakdown;

Question 15

the reaction catalysed by ATP synthase. Formula

Answer 15

ADP + Pi → ATP + H2O

Question 16

The new antibiotic is safe to use in humans because it does not inhibit the ATP
synthase found in human cells.
Suggest why human ATP synthase is not inhibited and bacterial synthase is
inhibited.

Answer 16

Human ATP synthase has a different tertiary
structure to bacterial ATP synthase
OR
Human ATP synthase has a different shape
active site to bacterial ATP synthase
OR
Antibiotic cannot enter human cells/mitochondria
OR
Antibiotic not complementary (to human ATP
synthase);

Question 17

The scientists suggested that people newly diagnosed with this bladder infection
should be treated with both the current antibiotic and the new antibiotic.
Explain why the scientists made this suggestion.
Use information from Figure 2 and your knowledge of evolution of antibiotic
resistance in bacteria in your answer.
[3 marks]

Answer 17

1. (From Fig 2) New/old antibiotic does not kill
   all bacteria;
   OR
   (From Fig 2) Some bacteria are resistant to
   the new/old antibiotic;
2. Resistant bacteria will reproduce to produce
   (more) resistant bacteria;
3. (Use of both) one antibiotic will kill bacteria
   resistant to the other antibiotic;
   OR
   Unlikely that bacteria are resistant to both the
   new and the old antibiotic;
   OR
   Use of both antibiotics (likely to) kill all/most
   bacteria;

Question 18

Give three environmental variables that should be controlled when growing the
plants before treatment with the different sprays.
[2 marks]

Answer 18

1. Concentration of mineral ion/named mineral
ion in soil;
2. Soil pH;
3. Temperature;
4. Light intensity/wavelength/duration;
5. Distance between seeds/plants;
6. Volume of water given;
7. CO2 concentration;
8. Humidity;

Question 19

Evaluate the use of 2,4-D as a herbicide on a wheat crop that contains wild oats as
a weed. Use all the information provided.
[4 marks]

Answer 19

1. 2,4-D causes an increase in release of ions
   from wild oat cells and 2,4-D does not
   affect/has little effect on the release of ions
   from wheat cells;
2. (For wheat) Difference is less than LSD/7 so
   difference is not significant;
   OR
   (For wild oats) Difference is more than LSD/10
   so difference is significant;
3. Loss of ions from cells (likely to) lead to
   cell/plant death/damage;
   OR
   Disruption of cell membrane (likely to) lead to
   cell/plant death/damage;
4. No evidence here about death of plants as a
   result of this ion loss;
5. No evidence here of other
   ecological/environmental impact;

Question 20

The scientists incubated the flasks containing the leaf discs at 26 °C and gently
shook the flasks.
Suggest one reason why the scientists ensured the temperature remained constant
and one reason why the leaf discs were shaken.
[2 marks]
Temperature ___
Shaken ___

Answer 20

1. (Maintain temperature) so that the rate of
   diffusion (of ions out of cells) remains constant
   OR
   (Maintain temperature) so no change in fluidity
   of phospholipids/kinetic energy of
   phospholipids;
   OR
   (Maintain temperature) so no change in
   shape/structure/denaturation of membrane
   proteins;
2. (Shaking) So all surfaces of the leaf discs are
   exposed (to water)/so all submerged;
   OR
   To maintain diffusion/concentration gradient
   (for ions out of leaf discs);

Question 21

Describe how phagocytosis of a virus leads to presentation of its antigens.
[3 marks]

Answer 21

1. Phagosome/vesicle fuses with lysosome;
2. (Virus) destroyed by lysozymes/hydrolytic
   enzymes;
3. Peptides/antigen (from virus) are displayed on
   the cell membrane;

Question 22

Describe how presentation of a virus antigen leads to the secretion of an antibody
against this virus antigen.
[3 marks]

Answer 22

1. Helper T cell/TH cell binds to the antigen (on
   the antigen-presenting cell/phagocyte);
2. This helper T/TH cell stimulates a specific B
   cell;
3. B cell clones
   OR
   B cell divides by mitosis;
4. (Forms) plasma cells that release antibodies;

Question 23

Collagen is a protein produced by cells in joints, such as the knee.
Scientists have found a virus that produces a protein very similar to human
collagen.
Suggest how the immune response to this viral protein can result in the
development of RA.
[2 marks]

Answer 23

1. The antibody against virus (antigen) will bind
   to collagen;
2. This results in the destruction of the (human)
   cells/collagen;

Question 24

What do these classifications suggest about the evolutionary relationships between
these species of snake?
[2 marks]
Classification X \_\_\_
Classification Y\_\_\_

Answer 24

1. (Without genetic analysis/X) mackloti and
   olivaceus have a more recent common
   ancestor with each other (than with papuana);
2. (Genetic analysis indicates/Y) papuana and
   mackloti have a more recent common
   ancestor with one another (than with
   olivaceus);

Question 25

State three comparisons of genetic diversity that the scientists used in order to
generate Classification Y.
[3 marks]

Answer 25

1. The (base) sequence of DNA;
2. The (base) sequence of mRNA;
3. The amino acid sequence (of proteins);

Question 26

The adult damselfly uses a tracheal system for gas exchange.
Explain three ways in which an insect’s tracheal system is adapted for efficient gas
exchange.
[3 marks]

Answer 26

1. Tracheoles have thin walls so short
   diffusion distance to cells;
2. Highly branched/large number of
   tracheoles so short diffusion distance to
   cells;
3. Highly branched/large number of
   tracheoles so large surface area (for gas
   exchange);
4. Tracheae provide tubes full of air so fast
   diffusion (into insect tissues);
5. Fluid in the end of the tracheoles that
   moves out (into tissues) during exercise
   so faster diffusion through the air to the
   gas exchange surface;
   OR
   Fluid in the end of the tracheoles that
   moves out (into tissues) during exercise
   so larger surface area (for gas exchange);
6. Body can be moved (by muscles) to move
   air so maintains diffusion/concentration
   gradient for oxygen/carbon dioxide;

Question 27

The damselfly larva is a carnivore that actively hunts prey. It has gills to obtain
oxygen from water.
Some other species of insect have larvae that are a similar size and shape to
damselfly larvae and also live in water. These larvae do not actively hunt prey and
do not have gills.
Explain how the presence of gills adapts the damselfly to its way of life.
[2 marks]

Answer 27

1. Damselfly larvae has high(er)
   metabolic/respiratory (rate);
2. (So) uses more oxygen (per unit time/per
   unit mass);

Question 28

Suggest two ways the student could improve the quality of her scientific drawing of
this gill.
[2 marks]

Answer 28

1. Don’t use shading;
2. Only use single lines/don’t use sketching
   (lines)/ensure lines are
   continuous/connected;
3. Add further labels/annotations;
4. Don’t cross label lines;
5. Add magnification/scale (bar);

Question 29

Contrast how an optical microscope and a transmission electron microscope work
and contrast the limitations of their use when studying cells.
[6 marks]

Answer 29

1. TEM use electrons and optical use light;
2. TEM allows a greater resolution;
3. (So with TEM) smaller organelles/named cell
   structure can be observed
   OR
   greater detail in organelles/named cell structure
   can be observed;
4. TEM view only dead/dehydrated specimens and
   optical (can) view live specimens;
5. TEM does not show colour and optical (can);
6. TEM requires thinner specimens;
7. TEM requires a more complex/time consuming
   preparation;
8. TEM focuses using magnets and optical uses
   (glass) lenses;

Question 30

Explain the appearance of W and Z.

[4 marks]

Answer 30

1. W has 4 cells/nuclei since it is at the (end of) 2
   nd
   division (of meiosis);
2. Z has 2 cells/nuclei since it is at the (end of) 1
   st
   division (of meiosis);
3. W shows haploid cells/cells containing n
   chromosomes;
4. (Cells in) W contain half the (mass of) DNA of
   (Cells in) Z;
   OR
   (between Z and W) chromatids have separated;
   OR
   In Z homologous chromosomes have separated;

Question 31

An environmental scientist investigated a possible relationship between air
pollution and the size of seeds produced by one species of tree.
He was provided with a very large number of seeds collected from a population of
trees in the centre of a city and also a very large number of seeds collected from a
population of trees in the countryside.
Describe how he should collect and process data from these seeds to investigate
whether there is a difference in seed size between these two populations of trees.
[5 marks]

Answer 31

1. Use random sample of seeds (from each
   population);
2. Use (large enough) sample to be representative
   of whole population;
3. Indication of what size was measured eg mass;
4. Calculate a mean and standard deviation (for
   each population);
5. Use the (Student’s) t-test;
6. Analyse whether there is a significant difference
   between (the means of) the two populations;