Learning from mistakes

Question 1

describe the part played by soil bacteria in making nitrogen compounds from dead organisms available to plants.

Answer 1

• saprobiontic bacteria convert the nitrogen compounds in dead organisms to ammonium ions (ammonification)
• nitrifying bacteria oxidise ammonium to nitrite and then nitrate, which can be absorbed by plants via the roots.

Question 2

advantages of stocking a pond with fish that feed on algae rather than carnivorous fish

Answer 2

• fewer trophic levels in food chain so greater yield of fish, less energy lost between stages du to respiration.

Question 3

advantages of growing leguminous plants

Answer 3

leguminous plants have nitrogen fixing bacteria in their root nodules. these convert nitrogen gas to ammonium , which is converted to nitrates. less fertiliser required.

Question 4

how could releasing sterile male insects reduce population size?

Answer 4

sterile males compete with fertile for resources

• intraspecific competition
• cannot reproduce

Question 5

explain why shoot biomass can be taken as a measurement of NPP

Answer 5

biomass represents dry mass, so is GPP - respiratory losses

Question 6

how could chemicals which inhibit the decolourisation of DCPIP slow the growth of weeds?

Answer 6

less light-dependent reaction, so less ATP and reduced NADP produced
less GP reduced to TP in the light independent reaction
(needed to make glucose etc. needed for growth.)

Question 7

suggest how transcription factors reprogramme cells to form iPS cells

Answer 7

• attach to promoter region of gene

- stimulate or inhibit transcription of target genes

Question 8

why may a variety of primers be produced for PCR?

Answer 8

base sequences of DNA strands differ, different complementary primers are required.

Question 9

why is it better to use Rf values than distance moved to determine the identity of pigments?

Answer 9

• pigments move different distance on different chromatograms
• Rf = dist moved/ dist moved by solvent front
• Rf is constant for each pigment
• Rfs can be compared to known values.

Question 10

why do you measure to the centre of the pigment spot in chromatography?

Answer 10

standardises readings, allows for comparison, pigment is spread out but centre is mean distance moved

Question 11

explain how enzymes can be used to obtain a gene from mRNA (3)

Answer 11

Reverse transcriptase;
makes single strand of DNA/cDNA from RNA;
Double strand then formed;
DNA polymerase;

Question 12

explain the meaning of the term ‘sticky ends’ (2)

Answer 12

Cut ends of DNA;
one strand longer than the other / staggered cut / unpaired bases;
Can attach to complementary DNA / bases;

Question 13

explain the function of a vector (2)

Answer 13

Transfers / carries (foreign) gene / DNA;

Into bacteria / carrot cell / host cell;

Question 14

explain the advantage of showing data using standard deviations rather than ranges (2)

Answer 14

SD is spread of data around the mean
SD reduces effect of anomalies
SD can be used to determine if differences in results are significant

Question 15

evaluate this conclusion - data analysis key points

Answer 15

• correlation but does not prove causation
• other (named) factors may be involved
• variation in number/ overlap
• sample size

Question 16

what is the role of ATP in contraction of a myofibril? (2)

Answer 16

1. (Reaction with ATP) breaks/allows binding
   of myosin to actin/ actinomyosin bridge;
2. Provides energy to move myosin head;

Question 17

Why is it important that mammals maintain a constant core body temperature? (3)

Answer 17

1. (Maintain) optimum temperature for
enzymes;
2. If temperature rises (above
normal/optimum), enzyme activity falls;
3. If temperature falls (below normal), then low
kinetic energy/fewer enzyme-substrate
collisions;
4. Maintains (high) rate of (biochemical)
reactions/metabolic rate/respiration;

Question 18

why would the binding of a drug molecule to a neurotransmitter receptor not lead to the production of an action potential?

Answer 18

1. (Binding) does not lead to opening of
sodium ion channels;
2. (So) no depolarisation / threshold not
reached / sodium ions do not diffuse in;
OR
3. Opens chloride ion channels;
4. Causing hyperpolarisation / preventing
depolarisation

Question 19

explain the role of the loop of henle in producing concentrated urine

Answer 19

1. salt/(sodium) ions diffuse into descending limb;
2. water moves out of descending limb;
3. salt/(sodium) ions actively removed from ascending limb;
4. ascending limb impermeable to water;
5. low water potential/ high concentration of ions in medulla/tissue fluid;
6. water leaves collecting duct / distal tubule;
7. due to difference in water potential / by osmosis;

Question 20

Importance of water as a solvent

Answer 20

Allows metabolic reactions to take place in solution (universal solvent)
Allows transports of substances such as glucose, ions etc in solution in blood plasma

Question 21

Why would a mutation in an intron have little impact on the functioning of an organism?

Answer 21

Introns are regions of DNA which do not code for protein,
Introns are removed during splicing (so do not affect protein produced),
Exon are only parts of DNA that code for protein

Question 22

Describe and explain how modern agricultural techniques reduce biodiversity

Answer 22

Removal of hedgerows - reduced plant diversity, fewer food sources, reduced habitats, niches
Monoculture - reduced plant diversity and variety of food for insects
Draining ponds - reduces habitat diversity
Overgrazing - prevents regeneration of woodland
Pesticides - may kill organisms other than pests. Kill pests so reduce animal diversity, may also reduce diversity of organisms which feed on pests
Herbicides - kill weeds so directly reduce plant diversity, may reduce diversity of organisms which feed on weeds.

Question 23

role of atp and Ca2 in contraction of a myofibril

Answer 23

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 24

how is atp suitable as an energy source for cells to use?

Answer 24

1. Releases relatively small amount of energy /
   little energy lost as heat;
2. Releases energy instantaneously;
3. Phosphorylates other compounds, making them
   more reactive;
4. Can be rapidly re-synthesised;
5. Is not lost from/does not leave cells;

Question 25

explain how the resting potential of -70 mv is maintained when no pressure is applied to a pacinian corpuscle

Answer 25

1. Membrane more permeable to potassium ions
   and less permeable to sodium ions;
2. Sodium ions actively transported/pumped out
   and potassium ions in;

Question 26

explain how methylation of tumour suppressor genes can lead to cancer

Answer 26

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