spec paper 2

Question 1

For the first 10 minutes, the tap attached to tube A was left open and the syringe from tube B was removed.
Suggest three reasons why the apparatus was left for 10 minutes. (3)

Answer 1

1. Equilibrium reached;
2. Allow for expansion/pressure change in apparatus;
3. Allow respiration rate of seeds to stabilise;

Question 2

Suggest and explain why the chosen temperature was 20 °C for this experiment.
[2 marks]

Answer 2

1. Optimum temperature/temperature for normal growth of seeds;
2. (Optimum temperature) for enzymes involved in respiration;

Question 3

Describe the roles of calcium ions and ATP in the contraction of a myofibril. [5 marks]

Answer 3

1. Calcium ions diffuse into myofibrils from (sarcoplasmic) reticulum;
2. (Calcium ions) cause movement of tropomyosin (on actin);
3. (This movement causes) exposure of the binding sites on the actin;
4. Myosin heads attach to binding sites on actin;
5. Hydrolysis of ATP (on myosin heads) causes myosin heads to bend;
6. (Bending) pulling actin molecules;
7. Attachment of a new ATP molecule to each myosin head causes myosin heads to detach (from actin sites);

Question 4

GgNn crossed with ggnn
Grey body, long wings 975
Black body, short wings 963
Grey body, short wings 186
Black body, long wings 194
Use your knowledge of gene linkage to explain these results. [4 marks]

Answer 4

1. GN and gn linked;
2. GgNn individual produces mainly GN and gn gametes;
3. Crossing over produces some/few Gn and gN gametes;
4. So few(er) Ggnn and ggNn individuals;

Question 5

when do you use the chi-squared test?

Answer 5

for categoric data

Question 6

Explain how applying pressure to the Pacinian corpuscle produces the changes in membrane potential recorded by microelectrode P. [3 marks]

Answer 6

1. (Pressure causes) membrane/lamellae to become deformed/stretched;
2. Sodium ion channels in membrane open and sodium ions move in;
3. Greater pressure more channels open/sodium ions enter;

Question 7

The membrane potential at Q was the same whether medium or heavy pressure
was applied to the finger tip. Explain why.
[2 marks]

Answer 7

1. Threshold has been reached;

2. (Threshold or above) causes maximal response / all or nothing principle;

Question 8

Suggest two reasons why it was important that the spider gene was expressed only in
the silk glands of the silkworms. [2 marks]

Answer 8

1. So that protein can be harvested;

2. Fibres in other cells might cause harm;

Question 9

Chloroplasts contain chlorophyll a and chlorophyll b. Scientists found tobacco plants with a mutation that caused them to make more chlorophyll b than normal tobacco plants. They investigated the effect of this mutation on the rate of photosynthesis.
The scientists carried out the following investigation.
• They grew normal and mutant tobacco plants. They grew some of each in low light intensity and grew others in high light intensity.
• They isolated samples of chloroplasts from mature plants of both types.
• Finally, they measured oxygen production by the chloroplasts they had isolated
from the plants.
Explain why the scientists measured the rate of production of oxygen in this investigation.
[2 marks]

Answer 9

1. Oxygen produced in light-dependent reaction;

2. The faster (oxygen) is produced, the faster the light-dependent reaction;

Question 10

Explain how the methylation of tumour suppressor genes can lead to cancer.[3 marks]

Answer 10

1. Methylation prevents transcription of gene;
2. Protein not produced that prevents cell division/ causes cell death/apoptosis;
3. No control of mitosis;

Question 11

Mitochondrial disease (MD) often causes muscle weakness (lines 1–3). Use your knowledge of respiration and muscle contraction to suggest explanations for this
effect of MD.
[3 marks]

Answer 11

1. Reduction in ATP production by aerobic respiration;
2. Less force generated because fewer actin and myosin interactions in muscle;
3. Fatigue caused by lactate from anaerobic respiration;

Question 12

Suggest how the change in the anticodon of a tRNA leads to MD (lines 10–13).
[3 marks]

Answer 12

1. Change to tRNA leads to wrong amino acid being incorporated into protein;
2. Tertiary structure (of protein) changed;
3. Protein required for oxidative phosphorylation/the Krebs cycle, so less/no ATP made;

Question 13

A small amount of DNA can be extracted from mitochondria and DNA sequencing used to try to find a mutation (lines 18–19).
From this sample:
• how would enough DNA be obtained for sequencing?
• how would sequencing allow the identification of a mutation?
[2 marks]

Answer 13

1. Enough DNA using PCR;

2. Compare DNA sequence with ‘normal’ DNA;

Question 14

During vigorous exercise, the pH of skeletal muscle tissue falls. This fall in pH leads to a reduction in the ability of calcium ions to stimulate muscle contraction.
Suggest how.
[3 marks]

Answer 14

1. Low pH changes shape of calcium ion receptors
2. Fewer calcium ions bind to tropomyosin
3. Fewer tropomyosin molecules move away;
4. Fewer binding sites on actin revealed;
5. Fewer cross-bridges can form
   OR
   Fewer myosin heads can bind;

Question 15

Atrazine binds to proteins in the electron transfer chain in chloroplasts of weeds, reducing the transfer of electrons down the chain.
Explain how this reduces the rate of photosynthesis in weeds.
[4 marks]

Answer 15

1. Reduced transfer of protons across thylakoid membrane
   OR
   Reduced chemiosomotic gradient/proton gradient across thylakoid membrane;
2. (So) less ATP produced;
3. (So) less reduced NADP produced;
4. (So) light-independent reaction slows/stops
   OR
   Less reduction of GP to triose phosphate;

Question 16

More than 99% of biological molecules are reabsorbed from the filtrate in the proximal convoluted tubule.
Despite this, the concentration of fluid in this tubule remains constant.
Explain why.
[1 mark]

Answer 16

Water is also reabsorbed;

Question 17

What is the evidence in Figure 4 that this person was secreting antidiuretic hormone (ADH)?
Explain your answer.
[2 marks]

Answer 17

1. Concentration rises in collecting duct because it loses water by osmosis;
2. ADH increases permeability (of walls of collecting duct) to water;

Question 18

Define what is meant by epigenetics.

[2 marks]

Answer 18

1. Heritable changes in gene function;

2. Without changes to the base sequence of DNA;

Question 19

Explain how increased methylation could lead to cancer.

[3 marks]

Answer 19

1. Methyl groups (could be) added to (both copies of) a tumour suppressor gene;
2. The transcription of tumour suppressor genes is inhibited;
3. Leading to uncontrolled cell division;

Question 20

The stream eventually recovered to reach a climax community.

Give two features of a climax community. [2 marks]

Answer 20

1. Same species present (over long time) / stable community (over long time);
2. Abiotic factors (more or less) constant (over time);
3. Populations stable (around carrying capacity);

Question 21

After obtaining copies of the HGH gene, the geneticist will attempt to insert them into plasmid vectors.
Describe how the geneticist would attempt to insert copies of the HGH gene into these plasmids.
[3 marks]

Answer 21

1. Cut the plasmid with a restriction endonuclease;
2. (So that) both have complementary/sticky ends;
3. (Mix together) and add ligase to join the complementary/sticky ends;

Question 22

Suggest two reasons why observed ratios are often not the same as expected ratios.
[2 marks]

Answer 22

1. Fertilisation is random
OR
Fusion of gametes is random;
2. Small/not large population/sample;
3. Selection advantage/disadvantage/lethal alleles;

Question 23

Suggest how defensive enzymes produced by plants destroy bacteria (lines 8–9).
[2 marks]

Answer 23

1. Hydrolyse;

2. murein/glycoprotein (in cell wall);

Question 24

The signal proteins secreted into the air by a plant being attacked by a pathogen act as stimuli leading to the expression of genes for defensive enzymes in other plants (lines 9–11).
Suggest how they lead to the expression of these genes.
[3 marks]

Answer 24

1. Bind to receptor (on target plant);
2. Acts as/leads to production of a transcription factor;
3. (Which) binds to promoter
   OR
   stimulates transcription of genes
   OR
   production of mRNA (for defensive enzymes);