Exam Questions I've Gotten Wrong

Question 1

Describe the structure of proteins.

Answer 1

.Polymer of amino acids
.Peptide bonds
.Condensation reactions
.Primary structure is order of amino acids
.Secondary structure is folding of polypeptide chain due to hydrogen
bonding (alpha helix /
beta pleated sheet)
.Tertiary structure is 3D folding due to hydrogen bonding, ionic bonds, and
disulfide bridges
.Quaternary structure is two or more polypeptide chains joined

Question 2

Describe how proteins are digested in the human gut

Answer 2

.Hydrolysis of peptide bonds
.Endopeptidases break polypeptides into smaller peptide chains
.Exopeptidases remove terminal amino acids
.Dipeptidases hydrolyse dipeptides into amino acids

Question 3

The scientists used kittens (newborn cats) as model organisms in their laboratory investigation instead of babies (for an investigation into breast milk and enzymes).
Other than ethical reasons, suggest two reasons why they chose to use cats as model organisms.

Answer 3

.Are mammals so likely to have same physiology or reactions as humans
.Small enough to keep in laboratory
.Produce enough milk to extract
.Can use a large number

Question 4

Straw consists of three main organic substances – cellulose, hemicellulose and lignin. Cellulose molecules form chains which pack together into fibres. Hemicellulose is a small
molecule formed mainly from five-carbon (pentose) sugar monomers. It acts as a cement
holding cellulose fibres together.

State ways in which the structure of a hemicellulose molecule differs from the structure of a cellulose molecule

Answer 4

Hemicellulose vs Cellulose

Small vs Long Chains
Pentose vs Hexose

Question 5

Explain why the following step was necessary in finding the water content of straw:

not heating the straw above 90°C

Answer 5

.only water given off below 90 °C

.Above 90°C other substances in the straw burnt and lost as gas which produces a loss in mass

Question 6

Describe the structure of a cellulose molecule and explain how cellulose is adapted for its function in cells.

Answer 6

.Made from β-glucose
.Condensation reaction forming a glycosidic bond
.1 : 4 link
.“flipping over” of alternate molecules
.Hydrogen bonds linking long straight chains
.Cellulose makes cell walls strong as cellulose fibres are strong
.Can resist osmotic pressure
.Bond difficult to break
.Resists digestion

Question 7

Cow’s milk contains the sugar lactose. Many cats are unable to digest cow’s milk because they are lactose intolerant.
Cow’s milk can be made suitable for these cats by treating it with the enzyme lactase to hydrolyse lactose. This makes the cow’s milk lactose-free. Beads are coated with lactase and placed in a tube, as shown in the diagram below. Cow’s milk flows over the beads and the lactose is hydrolysed.

(a) Attaching lactase to the beads is a more efficient use of lactase than adding the lactase directly to cow’s milk.
Suggest three reasons why it is more efficient to attach lactase to the beads.

(b) Monosaccharides and disaccharides taste sweet.
The lactose-free milk made after hydrolysis with lactase tastes sweeter than the cow’s milk containing lactose. Suggest why.

Answer 7

(a)

.Lactase can be reused as not washed away
.No need to remove from milk
.Allows continuous process
.The enzyme is more stable
.Avoid end-product inhibition

(b)

.Lactose hydrolysed to galactose and glucose
.So more sugar molecules

Question 8

Explain the decrease in gross productivity as the woodland matures. (2)

Answer 8

1. Less light / more shading / more competition for light

2. Reduced/no photosynthesis

Question 9

(b) Use the information in the graph and your knowledge of net productivity to explain why biomass shows little increase after 100 years (2)

graph shows increase with decreasing rate of biomass over time and increase with decreasing rate until a decrease of gross productivity over time

Answer 9

1. Net productivity = gross productivity minus respiratory loss
2. Decrease in gross productivity / photosynthesis / increase in respiration.

Question 10

Describe the role of microorganisms in producing nitrates from the remains of dead organisms. (3)

Answer 10

1. Saprobiotic (microorganisms / bacteria) break down remains / dead into ammonia / ammonium;
2. Ammonia / ammonium ions into nitrite and then into nitrate;
3. (By) Nitrifying bacteria / nitrification

Question 11

Upwelling is a process where water moves from deeper parts of the sea to the surface. This water contains a lot of nutrients from the remains of dead organisms.

(b) Upwelling often results in high primary productivity in coastal waters.
Explain why some of the most productive fishing areas are found in coastal waters.

Answer 11

1. Nitrate / phosphate / named ion / nutrients for growth of / absorbed /
   used by plants / algae / producers;
2. More producers / consumers / food so more fish / fish reproduce more /
   fish grow more / fish move to area

Question 12

(d) One farming practice used to maintain high crop yields is crop rotation. This involves growing a different crop each year in the same field.
Suggest two ways in which crop rotation may lead to high crop yields. (2)

Answer 12

1. Grow crops / plants with nitrogen-fixing (bacteria)
2. (Different crops use) different minerals / salts / nutrients / ions
3. (Different crops have) different pests / pathogens / diseases.

Question 13

(b) Leguminous crop plants have nitrogen-fixing bacteria in nodules on their roots. On soils with a low concentration of nitrate ions, leguminous crops often grow better than other types of crop. Explain why. (2)

Answer 13

1. (Nitrogen) to ammonia / NH3 / ammonium;

2. Produce protein / amino acids / named protein / DNA / RNA;

Question 14

Even when the stomata of the leaf is sealed there is still some uptake of carbon dioxide.
Suggest how this uptake of carbon dioxide continues.

Answer 14

(Carbon dioxide uptake) through the upper surface of the leaf / through
cuticle.

Question 15

(b) Large areas of tropical forest are still found on some Caribbean islands. The concentration of carbon dioxide in the air of these forests changes over a period of 24 hours and at different heights above ground.
Use your knowledge of photosynthesis and respiration to describe and explain how the concentration of carbon dioxide in the air changes:
• over a period of 24 hours
• at different heights above ground. (5)

Answer 15

1. High concentration of / increase in carbon dioxide linked with respiration
   at night / in darkness
2. No photosynthesis in dark / night / photosynthesis only in light / day;
3. In light net uptake of carbon dioxide / use more carbon dioxide than
   produced / (rate of) photosynthesis greater than rate of respiration
4. Decrease in carbon dioxide concentration with height;
   More carbon dioxide absorbed higher up
5. (At ground level)
   less photosynthesis / less photosynthesising tissue / more respiration /
   more micro-organisms / micro-organisms produce carbon dioxide.

Question 16

The respiratory substrate and ADP added after 5 minutes (Q) were part of a buffered isotonic solution.
What other substance would the buffer or solution have to contain for respiration? (1)

Answer 16

Phosphate

Question 17

Describe how you could use cell fractionation to isolate chloroplasts from leaf tissue. (3)

Answer 17

1. How to break open cells and remove debris;
2. Solution is cold / isotonic / buffered;
3. Second pellet is chloroplast.

Question 18

Silkworms secrete silk fibres, which are harvested and used to manufacture silk fabric.
Scientists have produced genetically modified (GM) silkworms that contain a gene from a spider.
The GM silkworms secrete fibres made of spider web protein (spider silk), which is stronger than normal silk fibre protein.

(a) Suggest why the plasmids were injected into the eggs of silkworms, rather than into the silkworms. (2)
(d) Suggest two reasons why it was important that the spider gene was expressed only in the silk glands of the silkworms. (2)

Answer 18

1. (If injected into egg), gene gets into all / most of cells of silkworm;
2. So gets into cells that make silk.
3. So that protein can be harvested;
4. Fibres in other cells might cause harm.

Question 19

Describe how you could make a temporary mount of a piece of plant tissue to observe the position of starch grains in the cells when using an optical (light) microscope.

Answer 19

1. Add drop of water to (glass) slide;
2. Obtain thin section (of plant tissue) and place on slide / float on drop of
   water;
3. Stain with / add iodine in potassium iodide.
4. Allow any appropriate method that avoids trapping air
   bubbles
5. Lower cover slip using mounted needle.

Question 20

) A mutation of a tumour suppressor gene can result in the formation of a tumour.
Explain how.
(2)

Answer 20

1. (Tumour suppressor) gene inactivated / not able to control / slow down
   cell division;
   Ignore: references to growth
2. Rate of cell division too fast / out of control.
   1 and 2 Accept: mitosis
   1 and 2 Reject: meiosis

Question 21

MM is caused by a faulty receptor protein in cell-surface membranes. Cells in MM tumours can be destroyed by the immune system.
Suggest why they can be destroyed by the immune system.
(3)

Answer 21

1. Faulty protein recognised as an antigen / as a ‘foreign’ protein;
2. T cells will bind to faulty protein / to (this) ‘foreign’ protein;
3. (Sensitised) T cells will stimulate clonal selection of B cells;
4. (Resulting in) release of antibodies against faulty protein.

Question 22

(g) E. coli has no cholesterol in its cell-surface membrane. Despite this, the cell maintains a constant shape. Explain why. (2)

Answer 22

1. Cell unable to change shape;
2. (Because) cell has a cell wall;
3. (Wall is) rigid / made of peptidoglycan / murein.

Question 23

A mutation of a tumour suppressor gene can result in the formation of a tumour.
Explain how.
(2)

Answer 23

1. (Tumour suppressor) gene inactivated / not able to control / slow down
   cell division;
   Ignore: references to growth
2. Rate of cell division too fast / out of control.
   1 and 2 Accept: mitosis
   1 and 2 Reject: meiosis

Question 24

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)

Answer 24

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 25

(c) There was a time lag between the introduction of Bt crops and the appearance of the first insect species that was resistant to the Bt toxin. Explain why there was a time lag. (3)

Answer 25

1. Initially one / few insects with favourable mutation / allele;
2. Individuals with (favourable) mutation / allele will have more offspring;
3. Takes many generations for (favourable) mutation / allele to become the
   most common allele (of this gene).

Question 26

Give three ways in which courtship behaviour increases the probability of successful mating. (3)

Answer 26

1. Recognise / identify / attract same species;
   Ignore: references to letting them produce fertile offspring
2. Stimulates / synchronises mating / production / release of gametes;
3. Recognition / attraction of mate / opposite sex;
   Accept finding a mate
   Accept: gender
4. Indication of (sexual) maturity / fertility / receptivity / readiness to mate;
5. Formation of a pair bond / bond between two organisms (to have / raise
   young).

Question 27

Explain what is meant by a phylogenetic group (1)

Answer 27

(Grouped according to) evolutionary links / history / relationships /
common ancestry;

Question 28

(a) Explain how the resting potential of –70 mV is maintained in the sensory neurone when no pressure is applied. (2)

Answer 28

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 29

A biologist investigated the stimulation of a Pacinian corpuscle in the skin of a fingertip. She used microelectrodes to measure the maximum membrane potential of a Pacinian corpuscle and its sensory neurone when different pressures were applied to the fingertip.

Microelectrode P is in the pacinian corpuscle, and Q is along the sensory neurone.

(b) Explain how applying pressure to the Pacinian corpuscle produces the changes in membrane potential recorded by microelectrode P. (3)

Answer 29

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 30

A biologist investigated the stimulation of a Pacinian corpuscle in the skin of a fingertip. She used microelectrodes to measure the maximum membrane potential of a Pacinian corpuscle and its sensory neurone when different pressures were applied to the fingertip.

Microelectrode P is in the pacinian corpuscle, and Q is along the sensory neurone.

(c) The membrane potential at Q was the same whether medium or heavy pressure was applied to the finger tip. Explain why. (2)

Answer 30

1. Threshold has been reached;

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

Question 31

(d) Multiple sclerosis is a disease in which parts of the myelin sheaths surrounding neurones are destroyed. Explain how this results in slower responses to stimuli.

Answer 31

1. Less / no saltatory conduction / action potential / impulse unable to
   ‘jump’ from node to node;
2. More depolarisation over length / area of membranes.

Question 32

Serotonin is a neurotransmitter released in some synapses in the brain. It is transported back out of the synaptic gap by a transport protein in the pre-synaptic membrane.

Serotonin diffuses across the synaptic gap and binds to a receptor on the post-synaptic membrane.

(b) It is important that a neurotransmitter such as serotonin is transported back out of synapses. Explain why. (2)

Answer 32

1. (If not removed) keeps binding (to receptors);
   Accept answers based on what happens if it is transported
   out − ie what should happen
2. Keeps causing action potentials / depolarisation (in post-synaptic
   membrane);
3. Accept keeps Na +
   channels open(ing)

Question 33

Scientists investigated the response of the roots of pea seedlings to gravity.
They took three samples of seedlings, A, B, and C, and placed them so that their roots were growing horizontally. The root tips of each sample had been given different treatments. After a set time, the scientists recorded whether the roots of the seedlings had grown upwards or downwards and the amount of curvature. The table shows the treatment they gave to each sample and their results.

(a) The pea seedlings were kept in the dark after each treatment. Explain why this was necessary. (1)

Answer 33

1. (Seedlings) respond to light / are phototropic;
   Reject: roots are positively phototropic / grow towards light
   OR
   Neutral: ‘to control a variable’
2. (Only) measuring the effect of gravity / response to gravity;
   Neutral: light affects growth / results

Question 34

Normal tomato plants have an enzyme that softens tomatoes as they ripen. Genetically engineered tomatoes ripen and soften more slowly. A gene was inserted which reduces the amount of softening enzyme produced.
The diagram shows matching parts of the base sequences for the mRNA produced by the gene for the softening enzyme and that produced by the inserted gene.
Softening gene mRNA …AAUCGGAAU…
Inserted gene mRNA …UUAGCCUUA…
Suggest how the inserted gene reduces the production of the softening enzyme.
(3)

Answer 34

) inserted gene / mRNA complementary to normal gene / mRNA;
binds to it to prevent protein synthesis / form double strand / prevents
mRNA binding to ribosomes;
will not stop all translation, some mRNA reaches ribosomes /
because not all mRNA is bound by inserted gene mRNA