Paper 2

Question 1

Explain why base pairs are a suitable way of measuring the length of a piece of DNA

Answer 1

DNA made of base pairs;
Each base pair is same length / occupies same distance (along molecule);

Question 2

The trained mice were able to exercise for a longer time period than control mice (not exercised).

Explain why.

Answer 2

1. (More aerobic respiration) produces more ATP;
2. Anaerobic respiration delayed;
3. Less or no lactate;

Question 3

Explain why an increase in shoot biomass can be taken as a measurement of net primary productivity.

Answer 3

1. Represents dry mass / mass of carbon;
2. Represents gross production minus respiratory
   losses;

Question 4

Suggest how these two species of palm tree (with different flowering times) arose by sympatric speciation.

Answer 4

1. Occurs in the same habitat/environment/population;
2. Mutation/s cause different flowering times;
3. Reproductive separation/isolation
   OR
   No gene flow
   OR
   Gene pools remain separate;
4. Different allele/s passed on/selected
   OR
   Change in frequency of allele/s;
5. Disruptive (natural) selection;
6. Eventually different species cannot (inter)breed
   to produce fertile offspring;

Question 5

Suggest and explain two reasons why there is a high frequency of the E280A mutation in Yaramul (historically isolated region)

Answer 5

1. Isolated so inbreeding/low genetic diversity/small gene pool;
2. Allele inherited (through generations) from (common) ancestor;

Question 6

Explain the role of the loop of Henle in the absorption of water from the filtrate.

Answer 6

1. In the ascending limb, sodium(ions) actively removed
2. Ascending limb is impermeable to water.
3. Causes high conc of ions in the medulla / tissue fluid - so lowers water potential. –> creates gradient with highest water potential at cortex + progressively lower into medulla.
4. Descending limb is permeable to water - water moves out by osmosis
5. The longer the loop of Henle / the deeper into medulla, the lower the water potential in medulla/tissue fluid
6. Ascending limb + descending limb flow in opposite directions - allows counter-current multiplier - water potential gradient maintained down whole length of loop
7. Water moves out by osmosis / down water potential gradient + absorbed into blood.

Question 7

State a difference between the phosphorus and nitrogen cycle.

Answer 7

NO GASEOUS PHASE in the phosphorus cycle

Question 8

Describe a process carried out by microorganisms which adds ammonium ions to soil.

Answer 8

1. Protein / amino acids / nucleic acids (NAME NITROGEN-CONTAINING COMPOUND) broken down to ammonium ions / ammonia;
2. By saprobionts / saprobiotic (microorganisms).
   Neutral: decomposers
   Reject: answers where incorrect type of bacteria given as saprobionts e.g. Nitrogen fixing bacteria

Question 9

Suggest three ways in which crop rotation may lead to higher crop yields

Answer 9

1. Grow crops / plants with nitrogen-fixing (bacteria);
   Accept: grow legumes / named example e.g. peas, beans, clover
2. Different crops use different minerals / nutrients / ions (from the soil);
3. Different crops have different pests / pathogens / diseases.

Question 10

Describe how alterations to tumour suppressor genes can lead to the development of tumours. (3)

Answer 10

1. Increased methylation (of tumour suppressor
   genes);

2.Mutation (in tumour suppressor genes);

3. Tumour suppressor genes are not
   transcribed/expressed
   OR
   Amino acid sequence/primary structure altered;
4. (Results in) rapid/uncontrollable cell division;

Question 11

How would you explain a figure to show succession is taking place (it shows mean percentage cover of different species over the age of a sanddune)?

Answer 11

1. Beach grass is the pioneer (species);
2. Pioneers/named species change the abiotic environment/habitat/conditions/factors;
3. (So) less hostile for named species
   OR
   (So) more suitable for named species;
4. Conifer/hardwood trees represent climax
   community;

Question 12

Furosemide inhibits the absorption of sodium and chloride ions from the filtrate produced in the nephrons.

Explain how furosemide causes an increase in the volume of urine produced.

Answer 12

1. Water potential of filtrate/tubule decreased; (or reference to water potential gradient)
2. Less water (reabsorbed) by osmosis (from
   filtrate/tubule);
3. Collecting duct (is where osmosis occurs);

Question 13

In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios.

Suggest two reasons why.

Answer 13

1. Small sample size;
2. Fusion/fertilisation of gametes is random;
3. Linked Genes; (accept crossing over / sex-linkage)
4. Epistasis;
5. Lethal genotypes;

Question 14

GABA is a neurotransmitter released in some inhibitory synapses in the brain. GABA causes negatively charged chloride ions to enter postsynaptic neurones.

Explain how this inhibits postsynaptic neurones.

Answer 14

1. (Inside of postsynaptic) neurone becomes more negative/hyperpolarisation/inhibitory
   postsynaptic potential;
2. More sodium ions required (to reach threshold)
   OR
   Not enough sodium ions enter (to reach
   threshold);
3. For depolarisation/action potential;

Question 15

Suggest two features of the structure of different proteins that enable them to be separated by gel electrophoresis.

Answer 15

1. Mass/number of amino acids/polypeptides; (or length of polypeptide chain)
2. Charge;
3. R groups differ;

Question 16

Lactase persistence (continuation of producing lactase) is caused by a mutation in DNA. This mutation does not occur in the gene coding for lactase.
Suggest and explain how this mutation causes LP.

Answer 16

1. Mutation in promoter (DNA/gene) for
   transcription factor
   OR
   Mutation in promoter (region/DNA) for the gene
   OR
   Mutation in gene for transcription factor;
2. Lactase gene continues to be transcribed/active;

Question 17

Suggest how iPS cells could correct red-green colour blindness.

Answer 17

1. (iPS cells) DIVIDE;

2.(iPS cells) develop/differentiate into green
sensitive cones;