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A-Level · Biology

ARP-Bio A-Level Biology: Required Practicals and Data Analysis

AQA 7402 · Calculator allowed · about 150 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

A student calibrates a light microscope using an eyepiece graticule and a stage micrometre scale, then uses the calibration to measure palisade mesophyll cells from a privet leaf.

(a)At x400 magnification, 25 divisions of the stage micrometre scale (each division = 10 micrometres, um) aligned exactly with 100 eyepiece graticule units (egu). Calculate the value of 1 egu in micrometres at this magnification.(2)

(b)Using the same objective lens, a palisade mesophyll cell measures 18 egu across. Calculate its actual width in micrometres.(1)

(c)The student repeats the measurement on five different palisade cells, obtaining readings of 16, 18, 17, 19 and 15 egu. Using the calibration from part (a), calculate the mean actual cell width and the standard deviation of the actual widths, using SD = √ sum(x - mean)² / (n - 1) . Give both answers to 3 significant figures.(4)

(Total for Question 1 is 7 marks)

A student at Oakfield Academy near Nottingham used a systematic belt transect across a 20 m gradient from the edge of a small woodland into open meadow, recording light intensity and the percentage cover of white clover (Trifolium repens) using 0.25 m² quadrats every 4 m.
Distance from woodland edge (m): 0, 4, 8, 12, 16, 20
Light intensity, mean of 3 readings (klux): 2.1, 5.4, 9.0, 13.8, 17.6, 20.9
Percentage cover of clover (%): 4, 11, 28, 54, 69, 82

(a)Identify the sampling technique used and explain why it is suitable for investigating a change along an environmental gradient such as this.(2)

(b)State a suitable null hypothesis for this investigation.(2)

(c)A Spearman's rank correlation coefficient calculated for this data gives rs = +0.94. The critical value at p = 0.05 for n = 6 is 0.886. Explain what the value of rs indicates about the relationship between light intensity and clover cover, and comment on its statistical significance.(3)

(d)Calculate the standard error of the mean for the light intensity readings, given a mean of 11.5 klux and a standard deviation of 7.3 klux (n = 6). Use SE = SD / √n. Give your answer to 3 significant figures.(2)

(e)Evaluate the reliability of using percentage cover as a measure of clover abundance in this investigation, and suggest two improvements to the method.(6)

(Total for Question 2 is 15 marks)

A student investigated the rate of the enzyme-catalysed breakdown of hydrogen peroxide (H2O2) by catalase in potato tissue, using a gas syringe to collect the oxygen produced over 60 seconds at 25 degC, for a dilution series of H2O2 concentrations.
H2O2 concentration (%): 0.5, 1.0, 1.5, 2.0, 2.5, 3.0
Volume of O2 collected in 60 s (cm³): 6, 13, 19, 24, 26, 27

(a)Identify the independent variable in this investigation.(1)

(b)State one variable that should be controlled in this investigation.(1)

(c)Calculate the rate of reaction at 1.5% H2O2, in cm³ s^-1, to 3 significant figures.(2)

(d)Explain the shape of the graph of rate against H2O2 concentration between 2.0% and 3.0%.(3)

(e)Suggest why the student used a water bath at a constant temperature rather than carrying out the reaction at room temperature.(2)

(Total for Question 3 is 9 marks)

A student investigated water potential in potato tissue using a dilution series of sucrose solution. Potato cylinders (4 cm length, cut with the same cork borer) were blotted, weighed, placed into sucrose solutions of 0.0 to 1.0 mol dm^-3 for 24 hours, then reweighed.
Sucrose concentration (mol dm^-3): 0.0, 0.2, 0.4, 0.6, 0.8, 1.0
Percentage change in mass (%): +8.2, +3.1, -1.5, -6.0, -10.4, -14.9
Water potential of each sucrose solution (kPa): 0, -540, -1120, -1780, -2520, -3350

(a)One potato cylinder had an initial mass of 4.85 g and a final mass of 4.50 g. Calculate the percentage change in mass, to 3 significant figures, stating whether this is a gain or a loss.(2)

(b)Using the data table, estimate by interpolation the sucrose concentration at which the percentage change in mass would be zero, and use the water potential values given to estimate the water potential of the potato tissue.(2)

(c)Explain, in terms of water potential, why the potato cylinder placed in distilled water increased in mass.(2)

(d)State two variables that should be controlled in this investigation to ensure valid results.(2)

(Total for Question 4 is 8 marks)

A student investigated the effect of temperature on the rate of respiration of yeast, measuring the volume of carbon dioxide produced per minute using a respirometer, with five replicates at each of two temperatures.
Rate of respiration at 25 degC (cm³ CO2 min^-1): 2.1, 2.4, 2.0, 2.3, 2.2
Rate of respiration at 35 degC (cm³ CO2 min^-1): 3.5, 3.2, 3.8, 3.4, 3.6

(a)Calculate the mean rate of respiration at 25 degC and at 35 degC.(2)

(b)Calculate the standard deviation of the rate of respiration at 25 degC, using SD = √ sum(x - mean)² / (n - 1) . Give your answer to 3 significant figures.(3)

(c)Error bars representing ± 1 standard deviation are plotted on a bar chart of the two means, and the error bars for the two temperatures do not overlap. Explain what this indicates about the difference between the means.(2)

(d)A t-test on the two data sets gives a calculated t-value of 9.83. The critical value of t at p = 0.05 for the appropriate number of degrees of freedom is 2.31. State the number of degrees of freedom used, and explain, with reference to both values, whether the difference in respiration rate between the two temperatures is statistically significant.(4)

(Total for Question 5 is 11 marks)

A student investigated the effect of light intensity on the rate of photosynthesis of Cabomba pondweed by counting the number of oxygen bubbles produced per minute at different distances from a lamp.
Distance from lamp (cm): 10, 20, 30, 40, 50
Bubbles produced per minute: 48, 22, 11, 7, 5

(a)Light intensity is proportional to 1/d², where d is the distance from the lamp. Calculate the ratio of light intensity at 20 cm to light intensity at 40 cm.(2)

(b)The number of bubbles produced per minute is 22 at 20 cm and 7 at 40 cm. Comment on whether this is consistent with the prediction from part (a).(2)

(c)Suggest two variables that should be controlled to ensure a valid comparison at each distance.(2)

(d)Explain one limitation of using the rate of bubble production as a measure of the rate of photosynthesis, and suggest how this could be improved.(2)

(Total for Question 6 is 8 marks)

A student separated leaf pigments from spinach using paper chromatography. The solvent front travelled 9.6 cm. Distances travelled by the pigment spots were: chlorophyll a = 6.5 cm, chlorophyll b = 5.6 cm, carotene = 9.1 cm, xanthophyll = 7.8 cm.

(a)State the formula used to calculate an Rf value.(1)

(b)Calculate the Rf value for chlorophyll a, to 2 decimal places.(2)

(c)Calculate the Rf value for carotene, to 2 decimal places, and use it to explain the relative solubility of carotene in the solvent compared with chlorophyll a.(3)

(d)Explain why the same species of plant might give a different Rf value for chlorophyll a if the practical is repeated using a different solvent.(2)

(Total for Question 7 is 8 marks)

A student investigated the effect of temperature on the permeability of beetroot cell-surface membranes. Equal-sized beetroot discs were placed in water baths at different temperatures for 10 minutes, then the absorbance of the resulting pigment (betalain) solution was measured using a colorimeter.
Temperature (degC): 10, 20, 30, 40, 50, 60
Absorbance (arbitrary units): 0.05, 0.08, 0.15, 0.42, 0.71, 0.88

(a)Explain why a colorimeter, rather than visual comparison of colour, was used to measure pigment leakage.(2)

(b)Calculate the percentage increase in absorbance between 30 degC and 40 degC.(2)

(c)A Pearson's product-moment correlation coefficient calculated for temperature against absorbance across the full data set gives r = +0.89. State what this value shows about the relationship, and explain, in terms of membrane structure, why absorbance increases so sharply between 30 degC and 50 degC.(3)

(d)State two variables that must be standardised between the beetroot discs used at each temperature to ensure a valid comparison.(2)

(Total for Question 8 is 9 marks)

In a genetics investigation, a dihybrid cross between two heterozygous pea plants was expected to produce offspring in a 9:3:3:1 ratio of round yellow : round green : wrinkled yellow : wrinkled green seeds. From 160 offspring, the observed and expected numbers were:
Phenotype: round yellow, round green, wrinkled yellow, wrinkled green
Observed (O): 96, 28, 25, 11
Expected (E): 90, 30, 30, 10
Use the formula: chi-squared = sum( (O - E)² / E )

(a)State the null hypothesis for this cross.(1)

(b)Calculate the value of chi-squared for this data, showing your working, to 3 significant figures.(3)

(c)The critical value of chi-squared at p = 0.05 with 3 degrees of freedom is 7.82. State, with a reason, whether the null hypothesis should be accepted or rejected.(2)

(d)Suggest one reason why chi-squared, rather than a t-test, is the appropriate statistical test for this data.(2)

(Total for Question 9 is 8 marks)

A student used a choice chamber with four quadrants (damp/dark, damp/light, dry/dark, dry/light) to investigate the effect of humidity and light on the distribution of woodlice (Porcellio scaber). After 10 minutes, 40 woodlice were distributed as follows:
Damp/dark: 16, Damp/light: 9, Dry/dark: 8, Dry/light: 7

(a)Explain why a choice chamber is a suitable piece of apparatus to investigate the effect of humidity and light on the distribution of woodlice.(2)

(b)State the null hypothesis for this investigation.(1)

(c)If there is no preference, an even distribution of woodlice among the four quadrants would be expected. Calculate the expected number per quadrant and the value of chi-squared for this data, using chi-squared = sum( (O - E)² / E ), and state the number of degrees of freedom.(3)

(d)The critical value of chi-squared at p = 0.05 for 3 degrees of freedom is 7.82. Interpret the result of this test, referring to both values.(2)

(Total for Question 10 is 8 marks)

A student investigated the antimicrobial effect of tea tree oil on the growth of a named non-pathogenic bacterium, using paper discs soaked in different concentrations of tea tree oil placed on an agar plate inoculated with a lawn of bacteria, incubated at 25 degC for 48 hours.
Tea tree oil concentration (%): 0 (control, distilled water), 25, 50, 75, 100
Diameter of zone of inhibition (mm): 0, 6, 11, 16, 20

(a)State two aseptic technique steps that should be used when inoculating the agar plate, and explain the purpose of one of them.(2)

(b)Calculate the area of the zone of inhibition produced by the 100% concentration disc, which has a diameter of 20 mm. Use area = π x r² and give your answer to 3 significant figures.(2)

(c)Explain why the plates were incubated at a maximum of 25 degC rather than at 37 degC, which would produce faster bacterial growth.(2)

(d)Identify the independent variable in this investigation.(1)

(Total for Question 11 is 7 marks)

A student dissected a fish head to examine the structure of the gills, as part of a required practical investigating the gas exchange system of a named organism.

(a)State two safety precautions that should be taken when dissecting a fish head.(2)

(b)Explain how the structure of the gills, as observed in your dissection, is adapted for efficient gas exchange.(6)

(Total for Question 12 is 8 marks)

A student investigated the effect of caffeine on the heart rate of Daphnia (water fleas), using a control group (0% caffeine solution) and a treatment group (0.5% caffeine solution), with six replicates in each group.
Heart rate, control 0% caffeine (beats per minute): 148, 152, 145, 150, 149, 151
Heart rate, 0.5% caffeine (beats per minute): 168, 172, 165, 170, 175, 169

(a)Calculate the mean heart rate for the control group (0% caffeine), to 1 decimal place.(2)

(b)The mean heart rate for the 0.5% caffeine group is 169.8 bpm. Calculate the standard deviation of heart rate for this group, using SD = √ sum(x - mean)² / (n - 1) . Give your answer to 3 significant figures.(3)

(c)A t-test comparing the two groups gives a calculated t-value of 11.6. The critical value at p = 0.05 for 10 degrees of freedom is 2.23. State, with reference to both values, whether caffeine has a statistically significant effect on Daphnia heart rate, and explain why a t-test, rather than a chi-squared test, is the appropriate statistical test to use here.(2)

(d)Suggest and explain one ethical issue that should be considered when using Daphnia in this type of investigation, and one modification to the experimental design that would help address it.(3)

(Total for Question 13 is 10 marks)

Mark scheme · ARP-Bio A-Level Biology: Required Practicals and Data Analysis

Question 1

(a) M1 25 x 10 = 250 um spanned by 100 egu (or equivalent correct method)
(a) A1 2.5 um cao
(a) Answer: 2.5 um per egu
(b) B1 45 um (ft from (a): 18 x their calibration value)
(b) Answer: 45 um
(c) M1 converts all five readings using the calibration factor: 40, 45, 42.5, 47.5, 37.5 um
(c) A1 mean = 42.5 um cao
(c) M1 sum of squared deviations from the mean = 62.5, divided by (n - 1) = 4, giving variance = 15.625
(c) A1 SD = 3.95 um (awrt 3.95) cao
(c) Answer: Mean = 42.5 um; SD = 3.95 um

Question 2

(a) B1 systematic sampling along a belt transect
(a) B1 allows the change in a variable (here, % cover / community) to be recorded at set intervals along a gradient, showing how the two factors are related, rather than sampling randomly (oe)
(a) Answer: Systematic sampling using a belt transect; it records data at set intervals along the gradient so the relationship between distance/light and clover cover can be seen
(b) B1 there is no correlation / no significant relationship (oe)
(b) B1 between light intensity and percentage cover of clover (both variables correctly named)
(b) Answer: There is no correlation between light intensity and the percentage cover of clover.
(c) B1 a strong, positive correlation (close to +1)
(c) B1 as light intensity increases, percentage cover of clover increases (oe)
(c) B1 rs (0.94) exceeds the critical value (0.886), so the correlation is statistically significant at p = 0.05 / unlikely to be due to chance
(c) Answer: Strong positive correlation; statistically significant since 0.94 > 0.886
(d) M1 √6 = 2.45 (or equivalent correct substitution)
(d) A1 2.98 klux (awrt 2.98) cao
(d) Answer: 2.98 klux
(e) L3 (5-6): A detailed evaluation identifying at least two clear limitations of the method with linked, appropriate improvements, using scientific reasoning throughout. Coherent and well structured.
(e) L2 (3-4): Some relevant limitations and/or improvements identified, with some scientific reasoning, though development is limited or the response is only partly linked.
(e) L1 (1-2): Basic or isolated statements about the method with little scientific reasoning or linkage between limitation and improvement.
(e) L0 (0): No relevant content.
(e) Answer: Level 3 response linking at least two limitations (e.g. subjectivity of % cover, single transect line) to specific improvements (e.g. point quadrats, repeated random transects)

Question 3

(a) B1 concentration of hydrogen peroxide
(a) Answer: Concentration of hydrogen peroxide
(b) B1 any one of: temperature; volume of H2O2 used; mass/surface area of potato tissue; time allowed for reaction (any one, ora)
(b) Answer: Temperature (or volume of H2O2, or mass/surface area of potato)
(c) M1 19 / 60
(c) A1 0.317 cm³ s^-1 (awrt 0.317) cao
(c) Answer: 0.317 cm³ s^-1
(d) B1 the rate increases only slightly / levels off (plateaus)
(d) B1 because enzyme (catalase) concentration is now the limiting factor
(d) B1 all active sites are occupied/saturated at any one time, so increasing substrate concentration further has little/no effect on rate
(d) Answer: Rate levels off because enzyme concentration becomes limiting once all active sites are saturated
(e) B1 to keep temperature constant across all repeats/concentrations
(e) B1 because temperature affects rate of reaction (kinetic energy of molecules/enzyme activity), so an uncontrolled temperature would be a confounding variable affecting the validity of the comparison
(e) Answer: Keeps temperature constant so it does not act as a confounding variable, since temperature affects reaction rate

Question 4

(a) M1 (4.50 - 4.85) / 4.85 x 100 (correct substitution)
(a) A1 -7.22% / a 7.22% decrease (awrt 7.22) cao
(a) Answer: -7.22% (a 7.22% decrease in mass)
(b) M1 correct interpolation between 0.2 mol dm^-3 (+3.1%) and 0.4 mol dm^-3 (-1.5%) to give a concentration of approximately 0.33-0.34 mol dm^-3
(b) A1 water potential of tissue approximately -930 kPa (accept -900 to -960 kPa), ft from their concentration
(b) Answer: Approximately 0.335 mol dm^-3; water potential approximately -930 kPa
(c) B1 distilled water has a higher (less negative) water potential than the water potential of the cells inside the potato tissue
(c) B1 water moves by osmosis, from a region of higher (less negative) water potential to a region of lower (more negative) water potential, across the partially permeable cell-surface membrane
(c) Answer: Water moves into the cells by osmosis, down the water potential gradient, because distilled water has a higher water potential than the potato cells
(d) B1 any one of: temperature; time left in solution; dimensions/surface area of potato cylinders; volume of solution used
(d) B1 any second, different variable from the list above
(d) Answer: Any two of: temperature, time in solution, size of potato cylinders, volume of solution

Question 5

(a) B1 mean at 25 degC = 2.2 cm³ CO2 min^-1
(a) B1 mean at 35 degC = 3.5 cm³ CO2 min^-1
(a) Answer: 2.2 cm³ CO2 min^-1 at 25 degC; 3.5 cm³ CO2 min^-1 at 35 degC
(b) M1 correct deviations from the mean (-0.1, 0.2, -0.2, 0.1, 0.0) and their squares
(b) M1 sum of squared deviations (0.10) divided by (n - 1) = 4, giving variance = 0.025
(b) A1 SD = 0.158 cm³ CO2 min^-1 (awrt 0.158) cao
(b) Answer: 0.158 cm³ CO2 min^-1
(c) B1 non-overlapping error bars suggest the difference between the means is likely to be real/genuine
(c) B1 rather than due to chance/random variation, although a statistical test (e.g. a t-test) is needed to confirm this
(c) Answer: Suggests the difference is likely genuine rather than due to chance, though a statistical test is needed to confirm this
(d) B1 degrees of freedom = (5-1) + (5-1) = 8
(d) B1 the calculated t-value (9.83) is greater than the critical value (2.31)
(d) B1 the null hypothesis (of no significant difference between the means) is rejected
(d) B1 the difference in respiration rate between 25 degC and 35 degC is statistically significant (at p = 0.05), unlikely to be due to chance
(d) Answer: 8 degrees of freedom; 9.83 > 2.31, so the difference is statistically significant

Question 6

(a) M1 1/20² : 1/40² = 1/400 : 1/1600 (correct substitution)
(a) A1 ratio = 4 : 1 cao
(a) Answer: 4 : 1
(b) B1 actual ratio = 22 : 7, approximately 3.1 : 1
(b) B1 this is lower than the predicted 4:1 ratio, e.g. because light intensity may not be the only limiting factor at these distances (for example CO2 concentration or temperature may limit the rate near the lamp), or because counting bubbles is not an accurate measure of the volume of oxygen produced (any suitable evaluative point)
(b) Answer: Actual ratio approximately 3.1:1, lower than the predicted 4:1, suggesting another factor may be limiting or that bubble counting is imprecise
(c) B1 any one of: temperature (e.g. using a water bath); CO2/hydrogencarbonate concentration; length/species of pondweed; time period over which bubbles are counted
(c) B1 any second, different variable from the list above
(c) Answer: Any two of: temperature, CO2 concentration, length of pondweed, counting time
(d) B1 bubbles may vary in size/volume, so counting the number of bubbles does not accurately measure the volume of oxygen produced
(d) B1 improvement: collect the gas (e.g. in a syringe or capillary tube) and measure its volume directly instead of counting bubbles
(d) Answer: Bubble size varies, so counting bubbles is inaccurate; collecting and measuring the volume of gas directly would improve this

Question 7

(a) B1 Rf = distance travelled by pigment / distance travelled by solvent (front)
(a) Answer: Rf = distance travelled by pigment / distance travelled by solvent
(b) M1 6.5 / 9.6
(b) A1 0.68 cao
(b) Answer: 0.68
(c) M1 9.1 / 9.6
(c) A1 0.95 cao
(c) B1 carotene has a higher Rf value than chlorophyll a, showing it is more soluble in the (non-polar) solvent and has less affinity for the stationary phase (paper), so it travels further
(c) Answer: 0.95; carotene is more soluble in the solvent than chlorophyll a
(d) B1 Rf value depends on the solubility of the pigment in the particular solvent used, which differs between solvents
(d) B1 Rf values can therefore only be reliably compared when the same solvent, temperature and chromatography paper are used
(d) Answer: Rf depends on the solvent used, so values can only be compared when the same solvent (and conditions) are used

Question 8

(a) B1 a colorimeter gives objective, quantitative/numerical readings
(a) B1 which are more precise and reliable, and reduce/remove observer bias, compared with judging colour by eye
(a) Answer: A colorimeter gives objective, quantitative readings, reducing bias compared to judging colour by eye
(b) M1 (0.42 - 0.15) / 0.15 x 100 (correct substitution)
(b) A1 180% cao
(b) Answer: 180%
(c) B1 a strong positive correlation between temperature and absorbance (pigment leakage)
(c) B1 as temperature increases, absorbance/pigment leakage increases
(c) B1 increasing temperature gives phospholipids and proteins in the cell-surface membrane more kinetic energy, disrupting/denaturing the membrane structure (e.g. increased fluidity of the phospholipid bilayer, denaturation of channel/carrier proteins), so the membrane becomes more permeable and more betalain pigment leaks out of the vacuole
(c) Answer: Strong positive correlation; increased temperature disrupts/denatures membrane structure, increasing permeability and pigment leakage
(d) B1 any one of: size/surface area of beetroot discs; time in the water bath; volume of water/solution used; time before colorimeter reading; same source/species of beetroot
(d) B1 any second, different variable from the list above
(d) Answer: Any two of: disc size, time in water bath, volume of solution, source of beetroot

Question 9

(a) B1 there is no significant difference between the observed and expected (9:3:3:1) ratio of offspring phenotypes (oe)
(a) Answer: There is no significant difference between the observed and expected 9:3:3:1 ratio
(b) M1 at least two correct (O-E)²/E values: 0.40, 0.13, 0.83, 0.10
(b) M1 all four (O-E)²/E values correct (allow rounding)
(b) A1 chi-squared = 1.47 (awrt 1.47) cao
(b) Answer: chi-squared = 1.47
(c) B1 the calculated value (1.47) is less than the critical value (7.82)
(c) B1 so the null hypothesis is accepted; there is no significant difference between the observed and expected ratios (any difference is due to chance)
(c) Answer: Accept the null hypothesis, since 1.47 < 7.82
(d) B1 chi-squared is used for categorical (nominal) data, i.e. frequencies/counts in discrete categories
(d) B1 whereas a t-test is used to compare the means of two sets of continuous (numerical) data
(d) Answer: Chi-squared is for categorical/frequency data; a t-test compares means of continuous data

Question 10

(a) B1 it allows the animal to move freely and choose between different conditions (e.g. damp/dry, light/dark) simultaneously
(a) B1 allowing the experimenter to observe which conditions the woodlice show a preference for/aggregate in
(a) Answer: Woodlice can move freely between conditions, allowing their preference to be observed
(b) B1 there is no significant difference between the observed and expected (even) distribution of woodlice among the four quadrants (woodlice show no preference)
(b) Answer: There is no significant difference between the observed and expected (even) distribution of woodlice
(c) M1 expected number per quadrant = 40/4 = 10, and at least three correct (O-E)²/E terms (3.6, 0.1, 0.4, 0.9)
(c) A1 chi-squared = 5.0 cao
(c) B1 degrees of freedom = 3
(c) Answer: Expected = 10 per quadrant; chi-squared = 5.0; degrees of freedom = 3
(d) B1 5.0 is less than the critical value (7.82), so the null hypothesis is accepted
(d) B1 there is no statistically significant difference between the observed and expected distribution; the woodlice show no significant preference for a particular combination of humidity and light (any apparent clustering is due to chance)
(d) Answer: Accept the null hypothesis, since 5.0 < 7.82; no significant preference shown

Question 11

(a) B1 two valid steps, e.g. flame the neck of the culture bottle/inoculating loop before and after use; work near a Bunsen burner flame; tape (do not fully seal) the lid of the petri dish and incubate upside down
(a) B1 a correct explanation of the purpose of one of the named steps (e.g. flaming kills/prevents contaminating microorganisms; the rising hot air current above a flame prevents microorganisms from the air settling on the plate; taping rather than sealing allows gas exchange while still restricting entry of unwanted microorganisms)
(a) Answer: e.g. Flame the inoculating loop before and after use, to kill contaminating microorganisms
(b) M1 r = 10 mm; area = π x 10² (correct substitution)
(b) A1 314 mm² (awrt 314) cao
(b) Answer: 314 mm²
(c) B1 37 degC is human body temperature, which favours the growth of microorganisms that could be human pathogens
(c) B1 incubating at a maximum of 25 degC reduces the likelihood of growth of pathogenic species, reducing the risk to students carrying out the practical
(c) Answer: Reduces the risk of growing human pathogens, which grow best near body temperature (37 degC)
(d) B1 concentration of tea tree oil
(d) Answer: Concentration of tea tree oil

Question 12

(a) B1 any one of: wear appropriate protective clothing (gloves, apron, eye protection); use a dissecting mat/tray; handle scalpels/sharp instruments carefully, cutting away from the body; dispose of biological waste appropriately and wash hands after
(a) B1 any second, different precaution from the list above
(a) Answer: Any two of: wear gloves/eye protection, use a dissecting tray, cut away from the body, dispose of waste and wash hands appropriately
(b) L3 (5-6): A detailed and coherent explanation linking at least four structural features of the gills to their function in gas exchange, using appropriate scientific terminology throughout.
(b) L2 (3-4): Some structural features are linked to function, with less detail or minor inaccuracies; reasonably clear overall.
(b) L1 (1-2): Basic, largely isolated statements about gill structure or function, with limited linkage between the two.
(b) L0 (0): No relevant content.
(b) Answer: Level 3 response linking at least four features (large surface area, thin epithelium, dense capillary network, counter-current flow, ventilation) to efficient gas exchange

Question 13

(a) M1 sum of values (895) / 6
(a) A1 149.2 bpm (awrt 149.2) cao
(a) Answer: 149.2 beats per minute
(b) M1 correct deviations from the mean for at least four of the six values
(b) M1 sum of squared deviations (approximately 58.8) divided by (n - 1) = 5, giving variance approximately 11.8
(b) A1 SD = 3.43 bpm (awrt 3.4-3.5) cao
(b) Answer: 3.43 beats per minute
(c) B1 the calculated value (11.6) exceeds the critical value (2.23), so the difference is statistically significant (the null hypothesis is rejected)
(c) B1 a t-test is appropriate because the data are continuous/numerical (comparing two means), whereas chi-squared is used for categorical/frequency data
(c) Answer: Significant, since 11.6 > 2.23; t-test is appropriate for continuous data (comparing means), not categorical data
(d) B1 Daphnia are living organisms and caffeine may cause them stress, harm or death, particularly at higher concentrations, raising an animal welfare concern
(d) B1 modification: use the lowest concentration and shortest exposure time needed to obtain valid results, and return/dispose of Daphnia appropriately and promptly after data collection, following the school's animal welfare/ethical guidelines
(d) B1 a control group (0% caffeine) should be used so that any effect of caffeine can be distinguished from effects of handling/environmental disturbance alone
(d) Answer: Welfare concern from possible harm/stress to Daphnia; minimise concentration/exposure time and use a control group to isolate the caffeine effect

A-Level Biology: Required Practicals and Data Analysis - Worksheets, Questions and Revision