A-Level Biology: Required Practicals and Data Analysis
A-level Biology required practicals and data analysis is the practical-skills strand AQA examines directly in the written papers (practical-related skills make up a substantial share of the overall marks), even though no single required practical is assessed as a written lab report. It covers what the twelve required practicals actually investigate: enzyme-controlled reaction rate, root-tip squashes and the mitotic index, a dilution-series calibration curve to find the water potential of plant tissue, membrane permeability, dissection of a gas-exchange or mass-transport organ, aseptic technique to investigate antimicrobial substances on microbial growth, chromatography of photosynthetic pigments, dehydrogenase activity in isolated chloroplasts using the redox indicator DCPIP, the rate of respiration in yeast, animal responses in a choice chamber, a colorimetric calibration curve for glucose concentration, and the effect of an environmental factor on the distribution of a species using quadrats and transects. On top of naming apparatus and technique, the exam questions test identifying independent, dependent and control variables; calculating and combining uncertainties; plotting and reading graphs; and evaluating a method to suggest a genuine improvement.
Method
- Read the described procedure and identify the independent variable (IV, the one factor deliberately changed), the dependent variable (DV, the one measured as a result), and at least two control variables (CV, factors kept constant so the comparison is fair) - state these explicitly, since 'identify the variables' questions are marked on precision, not a vague list.
- Check a described method against the core requirements of a fair test: a suitable range and number of values of the IV, repeats at each value (usually three) so a mean can be calculated, and a named control or control experiment where relevant (e.g. a boiled, denatured-enzyme control).
- Calculate the absolute uncertainty in a single reading as half the smallest scale division of the instrument used (a ruler, thermometer, syringe or measuring cylinder), unless a specific resolution is given in the question.
- Convert to a percentage uncertainty using (absolute uncertainty / measured value) x 100, so the precision of measurements taken with different instruments (or a calculated result) can be compared fairly.
- When plotting a graph, choose a scale that uses at least half of the grid in both directions, plot every point accurately, and draw a single best-fit line or curve (not dot-to-dot) unless the question specifically asks for a line through every point.
- Read gradients and calibration-curve values carefully: a gradient needs a large triangle spanning at least half the line, with the units of both axes carried through the calculation; a value read from a calibration curve (e.g. glucose concentration or water potential) must be checked against the correct axis.
- For an evaluation question, structure the answer as a specific weakness tied to its effect on the result (not 'human error'), followed by a specific, practicable improvement that fixes that weakness - vague answers like 'repeat the experiment' or 'use better equipment' score no marks unless tied to a named source of error.
Worked example
A student investigated the effect of pH on the rate of reaction of the enzyme catalase, using hydrogen peroxide solution and a fixed mass of celery extract (a source of catalase). The volume of oxygen gas produced in the first 30 seconds was measured by collecting gas in an inverted measuring cylinder over water, at five pH values (set using buffer solutions), with three repeats at each pH. At pH 7, the three repeat readings of oxygen volume were 24 cm^3, 28 cm^3 and 26 cm^3, using a measuring cylinder with a resolution of 1 cm^3. (a) Identify the independent, dependent and control variables in this investigation. (b) Calculate the mean volume of oxygen produced at pH 7, and the rate of reaction in cm^3/s. (c) Calculate the percentage uncertainty in the mean volume of oxygen at pH 7.
- Identify the variables: independent variable = pH of the buffer solution; dependent variable = volume of oxygen gas produced in 30 seconds; control variables = volume and concentration of hydrogen peroxide, mass (or volume) of celery extract, and temperature, all kept constant.
- Calculate the mean volume at pH 7: mean = (24 + 28 + 26) / 3 = 78 / 3 = 26 cm^3.
- Calculate the rate of reaction: rate = volume / time = 26 / 30 = 0.867 cm^3/s (3 s.f.).
- Find the absolute uncertainty from the instrument's resolution: half of 1 cm^3 = 0.5 cm^3.
- Compare this with the uncertainty from the spread of repeats: half the range = (28 - 24) / 2 = 2 cm^3, which is larger than the resolution-based uncertainty, so 2 cm^3 is used as the absolute uncertainty in the mean.
- Final answer (c): percentage uncertainty = (absolute uncertainty / mean value) x 100 = (2 / 26) x 100 = 7.7% (2 s.f.).
Practice questions
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Q1In an investigation into the effect of temperature on membrane permeability using beetroot discs, name the independent variable and the dependent variable.Show answer
Answer: Independent variable = temperature of the water bath; dependent variable = amount of pigment (betalain) leakage, usually measured as absorbance or percentage transmission using a colorimeter.
Q2Explain why a results table should include repeat readings and a mean, rather than a single reading at each value of the independent variable.Show answer
Answer: Repeats allow an anomalous result to be identified and excluded, and averaging repeats reduces the effect of random error, making the mean a more reliable estimate of the true value than any single reading.
Q3A student measures the length of a leaf as 8.4 cm using a ruler with a resolution of 0.1 cm. State the absolute uncertainty in this reading.Show answer
Answer: 0.05 cm (half the ruler's smallest scale division).
Q4A mean value of 15.0 cm^3 has an absolute uncertainty of 0.6 cm^3. Calculate the percentage uncertainty.Show answer
Answer: 4.0% ( (0.6 / 15.0) x 100 ).
Q5A student is asked to justify using a colorimeter, rather than judging colour change by eye, when measuring the rate of the DCPIP / isolated-chloroplast reaction. Give one reason.Show answer
Answer: A colorimeter gives a quantitative, numerical reading (percentage transmission or absorbance) rather than a subjective judgement, so results are more precise, more repeatable, and can be compared reliably between repeats or between students.
Q6State two variables that should be controlled in an investigation into the effect of substrate concentration on the rate of an enzyme-catalysed reaction.Show answer
Answer: Any two of: temperature, pH, volume or concentration of enzyme, total reaction volume.
Q7Name the technique used to separate the different photosynthetic pigments present in a leaf extract, and state the term for the ratio calculated from the resulting chromatogram.Show answer
Answer: Chromatography (paper or thin-layer); the ratio calculated is the Rf value (distance moved by the pigment divided by the distance moved by the solvent front).
Q8A student calculates a rate of reaction as (volume of gas produced) / (time taken). The volume has a percentage uncertainty of 4.0% and the time has a percentage uncertainty of 2.0%. Calculate the percentage uncertainty in the calculated rate.Show answer
Answer: 6.0% - when quantities are divided, their percentage uncertainties are added: 4.0% + 2.0% = 6.0%.
Exam-style questions
Written in the style of a A Level Science exam paper, with a full mark scheme.
A student examined a stained root-tip squash under an optical microscope and counted the number of cells in each stage of the cell cycle in one field of view. Results: interphase = 54 cells; prophase = 8 cells; metaphase = 5 cells; anaphase = 2 cells; telophase = 3 cells. (a) Calculate the mitotic index for this sample, giving your answer to 3 significant figures. (b) Give one precaution the student should take to make this count valid.
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A student investigated the effect of distance from a hedge on the percentage cover of clover in a field, using a 0.25 m^2 quadrat placed every 2 m along a single 20 m transect line running away from the hedge. Results (percentage cover of clover): 0 m = 62%; 2 m = 58%; 4 m = 45%; 6 m = 30%; 8 m = 12%; 10 m = 4%. (a) Suggest one abiotic factor, other than distance from the hedge itself, that could explain this pattern, and explain how it might cause the trend seen. (b) The student used only one transect line. Explain why this limits the conclusions that can be drawn, and suggest an improvement.
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A student investigated the antimicrobial effect of a plant extract using paper discs soaked in the extract, placed on an agar plate spread with a bacterial culture and incubated using aseptic technique. After 48 hours, the diameter of the clear zone of inhibition around one disc was measured as 18 mm, using a ruler with a resolution of 1 mm. (a) Calculate the area of the zone of inhibition, excluding the disc itself, which has a diameter of 6 mm, giving your answer to 3 significant figures. (b) Explain why the area of the disc itself must be excluded from this calculation.
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