A-Level Biology Past Papers (Specimen)

Explain why determination of dry mass was an appropriate method to use in this investigation.

1. Dry mass measures/determines increase in biological/organic material; 2. Water content varies;

how would enough DNA be obtained for sequencing? • how would sequencing allow the identification of a mutation?

1. Enough DNA using PCR; 2. Compare DNA sequence with 'normal' DNA;

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.

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

Explain why monitoring the pH of the mixture could show whether the cat's milk contained lipase.

1. Hydrolysis of lipids produces fatty acids; 2. Which lower pH of mixture;

In fruit flies, the genes for body colour and wing length are linked. Explain what this means.

(Genes/loci) on same chromosome;

Figure 9 shows how the amount of DNA per cell changed during interphase and meiosis in an animal. Explain how the behaviour of chromosomes causes these changes in the amount of DNA per cell between F and G.

(Increase) 1. Chromosomes/DNA replicates; (First decrease) 2. Homologous chromosomes separate; (Second decrease) 3. Sister chromatids separate;

This test only detects the presence of HIV antibodies. Give two reasons why it cannot be used to find out if a person has AIDS.

(To diagnose AIDS, need to look for/at) 1. (AIDS-related) symptoms; 2. Number of helper T cells;

Use Figure 4, Figure 5 and your knowledge of enzyme action to explain why the arrows point in opposite directions.

1. (Figure 4 shows) DNA has antiparallel strands/described; 2. (Figure 4 shows) shape of the nucleotides is different/nucleotides aligned differently; 3. Enzymes have active sites with specific shape; 4. Only substrates with complementary shape/only the phosphate end (of the developing strand) can bind with active site of enzyme/active site of DNA polymerase;

Suggest why the plasmids were injected into the eggs of silkworms, rather than into the silkworms.

1. (If injected into egg), gene gets into all/most of cells of silkworm; 2. So gets into cells that make silk;

Suggest how the KDR minus allele gives resistance to DDT.

1. (Mutation) changes shape of sodium ion channel (protein) / of receptor (protein); 2. DDT no longer complementary / no longer able to bind;

Use the information in Figure 7 to explain how tissue fluid is formed.

1. (Overall) outward pressure of 3.2 kPa; 2. Forces small molecules out of capillary;

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

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;

Describe the difference between active and passive immunity.

1. Active involves memory cells, passive does not; 2. Active involves production of antibody by plasma cells/memory cells; 3. Passive involves antibody introduced into body from outside/named source; 4. Active long term, because antibody produced in response to antigen; 5. Passive short term, because antibody (given) is broken down; 6. Active (can) take time to develop/work, passive fast acting;

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.

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. Lower cover slip using mounted needle;

Explain why antibody A attaches only to the protein found in species of Plasmodium.

1. Antibody has tertiary structure; 2. Complementary to binding site on protein;

Describe the roles of calcium ions and ATP in the contraction of a myofibril.

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);

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

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

Suggest how the change in the anticodon of a tRNA leads to MD

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;

Which statistical test could the scientist use to determine whether his observed results were significantly different from the expected results? Give the reason for your choice of statistical test.

1. Chi squared test; 2. Categorical data;

A mother who was infected with HIV gave birth to a baby. The baby tested positive using this test. This does not prove the baby is infected with HIV. Explain why.

1. Children receive (HIV) antibodies from their mothers/maternal antibodies; 2. (So) solution will always turn blue/will always test positive (before 18 months);

Describe and explain the appearance of one of the chromosomes in cell X.

1. Chromosome is formed of two chromatids; 2. (Because) DNA replication (has occurred); 3. (Sister) chromatids held together by centromere;

Describe what has happened during division 1 in Figure 3.

1. Chromosomes in homologous pair; 2. One of each into daughter cells / haploid number;

The student concluded that the cell in Figure 1 was in the anaphase stage of mitosis. Was she correct? Give two reasons for your answer.

1. Chromosomes/chromatids are (in two groups) at poles of spindle/at ends of spindle; 2. V-shape shows that (sister) chromatids have been pulled apart at their centromeres/that centromeres of (sister) chromatids have been pulled apart;

Describe and explain the mechanism that causes forced expiration.

1. Contraction of internal intercostal muscles; 2. Relaxation of diaphragm muscles/of external intercostal muscles; 3. Causes decrease in volume of chest/thoracic cavity; 4. Air pushed down pressure gradient;

Describe how you would test a piece of food for the presence of lipid.

1. Dissolve in alcohol, then add water; 2. White emulsion shows presence of lipid;

The ecologists captured insects from a number of sites on the island. Suggest how they decided where to take their samples.

1. Draw grid over (map of) area; 2. Select squares/coordinates at random;

Contrast the processes of facilitated diffusion and active transport.

1. Facilitated diffusion involves channel or carrier proteins whereas active transport only involves carrier proteins; 2. Facilitated diffusion does not use ATP / is passive whereas active transport uses ATP; 3. Facilitated diffusion takes place down a concentration gradient whereas active transport can occur against a concentration gradient;

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.

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;

Use your knowledge of gene linkage to explain these results.

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;

The solution will remain yellow if a person is not infected with HIV. Explain why

1. HIV antibody is not present; 2. (So) second antibody/enzyme will not bind/is not present;

Messenger RNA (mRNA) is used during translation to form polypeptides. Describe how mRNA is produced in the nucleus of a cell.

1. Helicase; 2. Breaks hydrogen bonds; 3. Only one DNA strand acts as template; 4. RNA nucleotides attracted to exposed bases; 5. (Attraction) according to base pairing rule; 6. RNA polymerase joins (RNA) nucleotides together; 7. Pre-mRNA spliced to remove introns;

High blood pressure leads to an accumulation of tissue fluid. Explain how.

1. High blood pressure = high hydrostatic pressure; 2. Increases outward pressure from (arterial) end of capillary/reduces inward pressure at (venule) end of capillary; 3. (So) more tissue fluid formed /less tissue fluid is reabsorbed;

A transmission electron microscope was used to produce the image in Figure 1. Explain why

1. High resolution; 2. Can see internal structure of organelles;

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

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

Describe how proteins are digested in the human gut.

1. Hydrolysis of peptide bonds; 2. Endopeptidases break polypeptides into smaller peptide chains; 3. Exopeptidases remove terminal amino acids; 4. Dipeptidases hydrolyse/break down dipeptides into amino acids;

Describe the mass flow hypothesis for the mechanism of translocation in plants.

1. In source/leaf sugars actively transported into phloem; 2. By companion cells; 3. Lowers water potential of sieve cell/tube and water enters by osmosis; 4. Increase in pressure causes mass movement (towards sink/root); 5. Sugars used/converted in root for respiration for storage;

Name two ways in which meiosis produces genetic variation.

1. Independent segregation (of homologous chromosomes); 2. Crossing over / formation of chiasmata;

Describe and explain the differences between the two curves.

1. Initial rate of reaction faster at 37 °C; 2. Because more kinetic energy; 3. So more E-S collisions/more E-S complexes formed; 4. Graph reaches plateau at 37 °C; 5. Because all substrate used up

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.

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);

Explain one way in which starch molecules are adapted for their function in plant cells.

1. Insoluble; 2. Don't affect water potential; OR 3. Helical; 4. Compact; OR 5. Large molecule; 6. Cannot leave cell;

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.

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

Explain how cellulose molecules are adapted for their function in plant cells.

1. Long and straight chains; 2. Become linked together by many hydrogen bonds to form fibrils; 3. Provide strength (to cell wall);

What reducing sugar, or sugars, would you expect to be produced during chewing? Give a reason for your answer.

1. Maltose; 2. Salivary amylase breaks down starch;

Explain how the resting potential of -70 mV is maintained in the sensory neurone when no pressure is applied.

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

Explain how the methylation of tumour suppressor genes can lead to cancer.

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

If someone has MD, the concentration of lactate in their blood after exercise is usually much higher than normal. Suggest why.

1. Mitochondria/aerobic respiration not producing much/any ATP; 2. (With MD) increased use of ATP supplied by increase in anaerobic respiration; 3. More lactate produced and leaves muscle by (facilitated) diffusion;

Explain how an asthma attack caused the drop in the mean FEV shown in Figure 4

1. Muscle walls of bronchi/bronchioles contract; 2. Walls of bronchi/bronchioles secrete more mucus; 3. Diameter of airways reduced; 4. (Therefore) flow of air reduced;

Suggest an explanation for the results in Figure 6.

1. Mutation produced KDR minus/resistance allele; 2. DDT use provides selection pressure; 3. Mosquitoes with KDR minus allele more likely (to survive) to reproduce; 4. Leading to increase in KDR minus allele in population;

One farmer stated that the increase in the use of Bt crop plants had caused a mutation in one of the insect species and that this mutation had spread to other species of insect. Was he correct? Explain your answer.

1. Mutations are spontaneous/random; 2. Only the rate of mutation is affected by environment; 3. Different species do not interbreed/do not produce fertile offspring; 4. So mutation/gene/allele cannot be passed from one species to another;

When DDT binds to a sodium ion channel, the channel remains open all the time. Use this information to suggest how DDT kills insects.

1. Neurones remain depolarised; 2. So no action potentials / no impulse transmission;

Give two assumptions made when using the mark-release-recapture method.

1. No emigration/immigration; 2. No losses to predation; 3. Marking does not affect survival; 4. Birth rate and death rate equal; 5. (In this case) all belong to one population;

Suggest why the scientists used a marker gene and why they used the EGFP gene.

1. Not all eggs will successfully take up the plasmid; 2. Silkworms that have taken up gene will glow;

Suggest the advantage of using the fluorescent powder in this experiment.

1. Only glows brightly with UV, so doesn't make insects more visible; 2. So doesn't affect/increase predation; OR 1. Glows brightly with UV making visible; 2. So makes it easy to pick out labelled insects;

A control well is set up every time this test is used. This is treated in exactly the same way as the test wells, except that blood plasma is replaced by a salt solution. Use information from Figure 5 to suggest two purposes of the control well.

1. Only the enzyme/nothing else is causing a colour change; 2. Washing is effective/all unbound antibody is washed away;

Suggest and explain why the chosen temperature was 20 °C for this experiment.

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

HSV infects nerve cells in the face (line 1). Explain why it infects only nerve cells

1. Outside of virus has antigens/proteins; 2. With complementary shape to receptor/protein in membrane of cells; 3. (Receptor/protein) found only on membrane of nerve cells;

Explain why the scientists measured the rate of production of oxygen in this investigation.

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

During the experiment, the coloured liquid in the tubing moved towards tube B. Explain what caused this.

1. Oxygen taken up/used by seeds; 2. CO2 given out is absorbed by KOH (solution); 3. Volume/pressure (in B) decreases;

Stimulation of osmoreceptors can lead to secretion of the hormone ADH. Describe and explain how the secretion of ADH affects urine produced by the kidneys.

1. Permeability of membrane/cells (to water) is increased; 2. More water absorbed from/leaves distal tubule/collecting duct; 3. Smaller volume of urine; 4. Urine becomes more concentrated;

Describe how you would use the student's results in Table 3 to find the water potential of the potato tissue.

1. Plot a graph with concentration on the x-axis and percentage change in mass on the y-axis; 2. Find concentration where curve crosses the x-axis/where percentage change is zero; 3. Use (another) resource to find water potential of sucrose concentration (where curve crosses x-axis);

Describe the structure of proteins.

1. Polymer of amino acids; 2. Joined by peptide bonds; 3. Formed by condensation; 4. Primary structure is order of amino acids; 5. Secondary structure is folding of polypeptide chain due to hydrogen bonding; 6. Tertiary structure is 3-D folding due to hydrogen bonding and ionic/disulfide bonds; 7. Quaternary structure is two or more polypeptide chains;

Other than hunting, suggest two reasons why populations might show very low levels of genetic diversity.

1. Population might have been very small/genetic bottleneck; 2. Population might have started with small number of individuals / by one pregnant female / founder effect; 3. Inbreeding;

"Push down hard on the cover slip, but do not push the cover slip sideways." Explain why she was given this instruction.

1. Push hard - spread/squash tissue; 2. Not push sideways - avoid rolling cells together/breaking chromosomes;

Mitochondrial disease (MD) often causes muscle weakness. Use your knowledge of respiration and muscle contraction to suggest explanations for this effect of MD.

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;

Use Figure 6 and your knowledge of enzyme action and DNA replication to explain why new nucleotides can only be added in a 5' to 3' direction.

1. Reference to DNA polymerase; 2. (Which is) specific; 3. Only complementary with/binds to 5' end (of strand); 4. Only complementary with/binds with phosphate end (of the developing strand);

ATP is an energy source used in many cell processes. Give two ways in which ATP is a suitable energy source for cells to use.

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;

Describe how you would plot a suitable graph of these data. Explain your choice of type of graph.

1. Scatter graph; 2. Fat on x axis and death rate on y axis; 3. (Because) looking at relationship between two discrete/independent variables;

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

1. So that protein can be harvested; 2. Fibres in other cells might cause harm;

Explain the difference in the structure of the starch molecule and the cellulose molecule shown in Figure 3.

1. Starch formed from α-glucose but cellulose formed from β-glucose; 2. Position of hydrogen and hydroxyl groups on carbon atom 1 inverted;

Deletion of the sixth base (G) in the sequence shown in Figure 3 would change the nature of the protein produced but substitution of the same base would not. Use the information in Table 1 and your own knowledge to explain why.

1. Substitution would result in CCA/CCC/CCU; 2. (All) code for same amino acid/proline; 3. Deletion would cause frame shift/change in all following codons/change next codon from UAC to ACC;

Suggest how this student would obtain data to give a more precise value for the index of diversity of this habitat.

1. Take more samples and find mean; 2. Method for randomised samples described;

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

1. Threshold has been reached; 2. (Threshold or above) causes maximal response / all or nothing principle;

Describe how oxygen in the air reaches capillaries surrounding alveoli in the lungs. Details of breathing are not required.

1. Trachea and bronchi and bronchioles; 2. Down pressure gradient; 3. Down diffusion gradient; 4. Across alveolar epithelium; 5. Across capillary endothelium/epithelium;

When a vaccine is given to a person, it leads to the production of antibodies against a disease-causing organism. Describe how.

1. Vaccine contains antigen from pathogen; 2. Macrophage presents antigen on its surface; 3. T cell with complementary receptor protein binds to antigen; 4. T cell stimulates B cell; 5. (With) complementary antibody on its surface; 6. B cell secretes large amounts of antibody; 7. B cell divides to form clone all secreting/producing same antibody;

The water potential of the blood plasma is more negative at the venule end of the capillary than at the arteriole end of the capillary. Explain why.

1. Water has left the capillary; 2. Proteins (in blood) too large to leave capillary; 3. Increasing/giving higher concentration of blood proteins (and thus wp);

Describe how a high pressure is produced in the leaves.

1. Water potential becomes lower/becomes more negative (as sugar enters phloem); 2. Water enters phloem by osmosis; 3. Increased volume (of water) causes increased pressure;

When a person is dehydrated, the cell volume of an osmoreceptor decreases. Explain why.

1. Water potential of blood will decrease; 2. Water moves from osmoreceptor into blood by osmosis;

Explain the change in mass of potato tissue in the 0.40 mol dm-3 solution of sucrose.

1. Water potential of solution is less than/more negative than that of potato tissue; 2. Tissue loses water by osmosis;

Suggest what the scientists should have done during the drying process to be sure that all of the water had been removed from the plant samples.

1. Weigh samples at intervals during drying; 2. To see if weighings became constant (by 3 days);

The importance of movement in living organisms.

3.1.4.2 Enzyme-catalysed reactions 3.1.5.2 DNA replication 3.1.6 ATP 3.2.2 Cell division 3.2.3 Transport across membranes 3.2.4 Immune response 3.2.2 Gas exchange 3.3.3 Digestion and absorption 3.3.4.1, 4.2 Mass transport 3.4.2 DNA and protein synthesis 3.4.3 Meiosis 3.5.1 Photosynthesis 3.5.2 Respiration 3.6.1 Survival and response 3.6.1.2 Receptors 3.6.1.3 Control of heart rate 3.6.2.1 Nerve impulses 3.6.2.2 Synapses 3.6.2.2 Synaptic transmission 3.6.3 Skeletal muscle 3.6.4.2 Control of blood glucose concentration 3.6.4.3 Control of blood water potential 3.7.3 Evolution (population isolation and movement between) 3.8.2.2 Regulation of transcription and translation 3.8.2.3 Gene expression and cancer

The importance of receptors in living organisms.

3.1.4.2 Enzymes 3.2.1.2 Structure of prokaryotic cells and of viruses 3.2.3 Transport across cell membranes 3.2.4 Cell recognition and the immune system 3.3.4.1 Mass transport in animals 3.4.2 DNA and protein synthesis 3.5.1 Photosynthesis 3.5.2 Respiration 3.6.1.1 Survival and response 3.6.1.2 Receptors 3.6.1.3 Control of heart rate 3.6.2.1 Nerve impulses 3.6.2.2 Synaptic transmission 3.6.3 Skeletal muscles 3.6.4.1 Principles of homeostasis 3.6.4.2 Control of blood glucose concentration 3.6.4.3 Control of blood water potential 3.8.2.2 Regulation of transcription and translation 3.8.2.3 Gene expression and cancer

Explain the difference in the initial rate of reaction at 60 °C and 37 °C.

At 60 °C: 1. More kinetic energy; 2. More E-S complexes formed;

The scientists suggested that mutant plants producing more chlorophyll b would grow faster than normal plants in all light intensities. Explain how these data support this suggestion.

At all light intensities, chloroplasts from mutant plants: 1. Have faster production of ATP and reduced NADP; 2. (So) have faster/more light-independent reaction; 3. (So) produce more sugars that can be used in respiration; 4. (So) have more energy for growth; 5. Have faster/more synthesis of new organic materials;

Explain why the data in Table 3 are described as processed results.

Calculations made (from raw data)/raw data would have recorded initial and final masses;

Explain the difference in the rates of reaction at 60 °C and 37 °C between 20 and 40 minutes.

Different times: 1. Higher temperature / 60 °C causes denaturation of all of enzyme; 2. Reaction stops (sooner) because shape of active site changed; Different concentrations of product (at 60 °C) 3. Substrate still available (when enzyme denatured); 4. But not converted to product;

Aspartic acid and proline are both amino acids. Describe how two amino acids differ from one another. You may use a diagram to help your description.

Have different R group;

Give the location of osmoreceptors in the body of a mammal.

Hypothalamus;

The hydrostatic pressure falls from the arteriole end of the capillary to the venule end of the capillary. Explain why.

Loss of water/loss of fluid/friction (against capillary lining);

Explain how this microRNA allows HSV to remain in the body for years.

MicroRNA binds to cell's mRNA (no mark) 1. (Binds) by specific base pairing; 2. (So) prevents mRNA being read by ribosomes; 3. (So) prevents translation/production of proteins; 4. (Proteins) that cause cell death;

Name two structures in a eukaryotic cell that cannot be identified using an optical microscope.

Mitochondrion/ribosome/endoplasmic reticulum/lysosome/cell-surface membrane

Creatinine is a breakdown product of creatine found in muscle tissues. Apart from age and gender, give two factors that could affect the concentration of creatinine in the blood.

Muscle/body mass Ethnicity Exercise Kidney disease

In this investigation, what is meant by genetic diversity?

Number of different alleles of each gene;

What would the scientists have inserted into the plasmid along with the spider gene to ensure that the spider gene was only expressed in the silk glands of the silkworms?

Promoter (region/gene);

Suggest one advantage of the different percentage of cholesterol in red blood cells compared with cells lining the ileum.

Red blood cells free in blood/not supported by other cells so cholesterol helps to maintain shape;

Species richness and an index of diversity can be used to measure biodiversity within a community. What is the difference between these two measures of biodiversity?

Species richness measures only number of (different) species / does not measure number of individuals;