The WBI16 paper: solving problems, applying knowledge, evaluating design and uncertainty
This unit is A Level (IA2) content, and it assumes everything from the AS units, including the practical skills of the Unit 3 notes (WBI13).
It is examined on its own paper, WBI16, Unit 6: Practical Skills in Biology II, a written paper of 1 hour 20 minutes and 50 marks.
You are never handed apparatus in the exam: instead you plan, analyse and judge experiments on paper, mostly the core practicals of Topics 5 to 8 (CP10 to CP18) and the recommended additional practical on habituation.
This first section explains how the paper works and teaches the first five skills listed for assessment.
The idea: a practical exam you sit at a desk
Unit 6 assesses your ability to apply your knowledge and understanding of experimental procedures and techniques, and your ability to plan whole investigations, analyse data and to evaluate your results and experimental methodology.
So the paper rewards three things: knowing what each core practical is for and how it is done, handling data with the right maths and statistics, and judging whether a method gives trustworthy evidence.
Here are the practical skills identified for assessment, in their exact wording.
Learn the list, because every question on the paper is built from one or more of them:
- Solve problems set in practical contexts.
- Apply scientific knowledge to practical contexts.
- Comment on experimental design and evaluate scientific methods.
- Present data in appropriate ways.
- Evaluate results and draw conclusions with reference to measurement uncertainties and errors.
- Identify variables, including those that must be controlled.
- Plot and interpret graphs.
- Process and analyse data using appropriate mathematical skills (see Appendix 6: Mathematical skills and exemplifications).
- Know and understand how to use a wide range of apparatus, materials and techniques safely, appropriate to the knowledge and understanding in this specification.
- Plan an investigation to test a hypothesis.
This section teaches the first five.
The next section teaches variables, graphs and the maths skills; the two sections after it build a bank of every Topic 5 to 8 core practical and the habituation practical; then the rest of the unit follows three headings: planning, implementation and measurements, and analysis.
What the paper looks like
Recent WBI16 papers have kept the same four-question shape.
Knowing it tells you where each skill will be tested.
Question
Marks
What it usually is
Q1
10
A core practical, opening on a photograph: one or two short recall items, a 5 or 6-mark "Describe an experiment", then a 3-mark theory explanation
Q2
10 to 13
Method, data and variables: a method or sampling part, a short calculation, two named variables, how to control one
Q3
10 to 14
Data presentation and analysis: a 3-mark graph on a printed grid and a full statistical test (null hypothesis, calculation with a printed formula, conclusion against printed critical values)
Q4
16 or 17
A whole plan for a student's hypothesis: (a) preliminary work, or an ethics and safety pair (2 or 3); (b) Devise a detailed method (8 or 9); (c) record, present and analyse (3); (d) limitations (2 or 3)
There is no multiple choice, no asterisked levels-marked question and no biological drawing on WBI16.
The commonest command words, by marks, are Describe, Devise, Calculate, Justify, State, Suggest and Draw (a graph).
No formula sheet is printed: the chi-squared, Spearman's rank and standard deviation formulae appear inside the question that uses them, with a table of critical values.
- Every marking point is worth (1). A long item is marked "n of m points": a 6-mark method might have eight creditable points, and any six score. There are no levels, no asterisks and no "max" lines: the header of the list sets the cap.
- A "/" separates alternatives for one point. Sentence-case notes widen or narrow a point: "Accept" adds wording that scores, "Ignore" means the words neither gain nor lose, "Reject" or "Do not accept" names wording that loses the point.
- Where printed lines ask for one answer each (abiotic variable, biotic variable), only the first answer on each line is marked, and a right answer next to a wrong one on a 1-mark item scores 0.
- In a calculation a correct answer alone gains full marks; working earns the method mark when the answer is wrong; a bare wrong answer earns nothing. No precision is stated, so round correctly to sensible figures: wrongly rounded values are refused.
- A conclusion can score on your own wrong value (error carried forward) if you compare it correctly with a stated critical value.
Skill 1: solve problems set in practical contexts
Every WBI16 question is set in a context you have not met: a different organism, a different independent variable, a field site with its own problems.
The examiners report the same thing every series: candidates knew the core practicals, but answers that ignored the context lost marks.
A stock answer is a starting point that you adapt.
The method is the same every time.
Read the whole question first, including the later parts, because they tell you what is coming (a correlation test later means your null hypothesis must be about correlation).
Then ask three questions of the context: what is being measured, what could make the comparison unfair here, and what in this setting rules out the usual answer?
A student compared the number of freshwater shrimps in kick samples from a fast-flowing riffle and a slow pool in the same stream.
Describe how the student could control one abiotic variable, other than water speed, that might affect the number of shrimps.
(2)
- choose the riffle and the pool close together on the same stretch of stream, with the same depth of water (1)
- measure the depth (or temperature, or dissolved oxygen) at each sampling point and only sample where the readings are similar (1)
A weaker answer: "Use a water bath to keep the temperature the same."
This scores 0.
Nothing in a stream can be put in a water bath; in a field study you control an abiotic factor by choosing sites where it is similar and measuring it to show that it is.
Skill 2: apply scientific knowledge to practical contexts
About one mark in ten on WBI16 is plain theory, asked inside a practical question: why the rate changes with temperature, how an ion inhibits an enzyme, how ATP is made in anaerobic respiration.
These are the topics you studied in the theory units, and these notes do not re-teach them.
The table shows where each practical's science is taught.
Practical
What it investigates
Theory taught in
CP10
light intensity, wavelength, temperature, carbon dioxide and photosynthesis
5a notes (photosynthesis, limiting factors)
CP11
ecology of a habitat: quadrats, transects, abiotic factors
5b notes (ecosystems, distribution and abundance)
CP12
temperature and the development of organisms
5b notes (temperature, enzymes and Q10)
CP13
growth of microorganisms in a liquid culture
6a notes (growth curves, measuring growth)
CP14
different antibiotics and bacteria
6a notes (antibiotics)
CP15
redox indicator and respiration in yeast
7a notes (respiration, hydrogen carriers)
CP16
respirometer, rate of respiration and RQ
7a notes (respiration, RQ)
CP17
exercise and spirometer traces
7b notes (ventilation and exercise)
CP18
amylase in germinating cereal grains
8b notes (gibberellin and germination)
habituation
response to a repeated stimulus
8a notes (habituation)
A theory explanation inside a practical question is marked like any Explain: one point per link in the chain, and the chain must reach the measured result.
A student found that the percentage of brine shrimp cysts that hatched in 3 days rose from 15 °C to 30 °C and fell sharply at 40 °C. Explain these results.
(3)
- hatching depends on enzyme-controlled reactions (respiration, breakdown of stored food) in the developing embryo (1)
- as temperature rises the molecules have more kinetic energy, so there are more successful collisions and more enzyme-substrate complexes form per second (1)
- above the optimum, bonds holding the tertiary structure break, the active site changes shape and the enzymes are denatured, so fewer cysts complete development (1)
A weaker answer: "Higher temperatures give more energy, so more hatch, until it gets too hot and the shrimps die."
This scores 0 or 1: it never names enzymes, active sites or enzyme-substrate complexes, and "die" is not an explanation.
Barley grains germinated on agar containing silver ions released much less amylase activity than grains on plain agar.
Describe how silver ions could reduce the activity of amylase.
(3)
- the silver ions bind to the amylase (to groups on its R groups) (1)
- this breaks bonds that hold the tertiary structure, so the shape of the active site changes (1)
- so starch no longer fits the active site and fewer enzyme-substrate complexes form (1)
Examiners report that most candidates know an ion "inhibits" the enzyme, but only a minority give the full chain: bonds in the tertiary structure, the shape of the active site, fewer enzyme-substrate complexes.
Skill 3: comment on experimental design and evaluate scientific methods
To comment on a design you test it against four questions.
1. Does it measure what it claims to? A method is valid only if the dependent variable really reflects the effect being studied, and only the independent variable changes. The time for methylene blue to decolourise is a valid measure of the rate of dehydrogenation in yeast; the colour "looking paler" is not.
2. Is it a fair test? Every other variable that could affect the dependent variable must be kept the same, and you must say how.
3. Is there a control? A control shows that the result is caused by the independent variable: boiled yeast in CP15, glass beads in the respirometer control tube in CP16, a disc soaked in sterile water in CP14.
4. Are there enough repeats and enough values? Repeats let you calculate a mean and a standard deviation and spot anomalies; five or more values of a continuous independent variable let you see a trend and use a correlation test.
A student investigated the effect of light intensity on the rate of photosynthesis of pondweed by moving a desk lamp to 10, 20, 30, 40 and 50 cm from a boiling tube and counting bubbles for 1 minute at each distance, once.
Describe three weaknesses in this design.
(3)
- counting bubbles is not a valid measure, as bubbles vary in size; collect the gas and measure its volume in a syringe or capillary tube (1)
- the lamp heats the water as it moves closer, so temperature changes as well as light intensity; place a heat shield (a tank of water) between lamp and tube (1)
- one reading at each distance gives no mean or standard deviation and no way to spot an anomalous result; take three readings at each distance (1)
(Accept also: no time for the plant to adjust to each new light intensity; carbon dioxide may run out over the run, so add sodium hydrogencarbonate.)
Skill 4: present data in appropriate ways
The right presentation depends on the data.
A table records raw data and processed values; a graph shows the pattern.
The later sections teach each in detail, but the decision is quick:
Data
Present as
a continuous independent variable (temperature, concentration, time) against a measured dependent variable
line graph, points joined with ruled lines or a line of best fit
two variables measured in the same samples, looking for a relationship
scatter graph, with a line of best fit if there is a trend
a categorical independent variable (two species, three sites, four antibiotics)
bar chart, bars separate, equal width, starting at zero
counts in categories compared with an expected number
table of observed and expected numbers, or a bar chart
a variable spread over several orders of magnitude (cell numbers in a culture)
a log scale (log₁₀ of the numbers on the y-axis)
Skill 5: evaluate results with reference to measurement uncertainties and errors
Every reading has an uncertainty: the range within which the true value probably lies.
For an instrument read from a scale, the uncertainty is usually taken as half the smallest division (a ruler marked in millimetres: ± 0.5 mm).
A digital stopwatch reads to 0.01 s, but your reaction time (about 0.2 s) is the real limit.
The bigger the uncertainty compared with the reading, the less you can trust a small difference between two readings.
This is a "learn it" formula: it is never printed.
It shows why a method gets better when the measured change is large: the same ± 0.5 mm on a 5 mm movement is 10%, on a 40 mm movement only 1.25%.
Two kinds of error matter in your evaluation:
- Random error scatters readings both ways: judging the end point of a colour change, timing by hand, natural variation between organisms. It is reduced by repeats and a mean, and its size is shown by the standard deviation.
- Systematic error shifts every reading the same way: a balance that does not read zero, a thermometer that reads 2 °C high, a capillary bore smaller than recorded. Repeats cannot remove it; calibration and correction can.
In a respirometer the coloured liquid moved 8.0 mm in 10 minutes.
The scale was marked in millimetres.
Calculate the percentage error in this distance, and suggest how the method could reduce it.
(3)
1. uncertainty = half a division = ± 0.5 mm (1)
2. percentage error = 0.5 ÷ 8.0 × 100 = 6.25 %, about 6 % (1)
3. run the respirometer for longer, or use a greater mass of organisms, so the liquid moves further and the same ± 0.5 mm is a smaller percentage of the reading (1)
What it means: a difference of 0.5 mm between two tubes would be within the uncertainty of one reading, so it could not count as evidence of a difference in rate.
- Valid: a method is valid when it measures what it claims to measure, with only the independent variable affecting the dependent variable.
- Control: a set-up identical to the test except for the factor being investigated (or with the organism killed or removed), to show that the result is caused by that factor.
- Uncertainty: the range either side of a reading within which the true value is likely to lie; for a scale, usually half the smallest division.
- Random error: error that scatters readings unpredictably above and below the true value; reduced by repeats and a mean.
- Systematic error: error that shifts every reading in the same direction by a similar amount; removed by calibration, not by repeats.
Before you write a word, read the whole question and underline its context: the organism, the setting and the bold words ("one", "two abiotic", "other than").
The reports on every recent series say the same two things: answers must fit the context, and candidates name what to do but not how to do it.
Every control needs a method; every risk needs a precaution.
- Writing the stock answer for the core practical when the context has changed it. A field study cannot use a water bath or a buffer; an invertebrate in a CP12-style study raises different ethical points from a vertebrate. Fix: name the setting in your answer and adapt each point to it.
- Describing when the command is Explain. "The rate went up then down" describes; an explanation needs enzymes, collisions, the active site and denaturation, linked to the result.
- Treating repeats as the answer to every error. Repeats reduce random error only. A balance that does not read zero needs calibrating.
- Giving more answers than asked. On printed answer lines only the first answer is marked, and a wrong answer next to a right one can cost the mark.
- Using the mark allocation badly. A 3-mark explanation needs three separate linked points; a 1-mark item needs one precise answer, not a paragraph.
Check your understanding
1. A student measured the time for 2 cm³ of yeast suspension with methylene blue to lose its blue colour at 25 °C. She timed by eye with a stopwatch reading to 0.01 s. She recorded times of 312.47 s, 298.12 s and 305.66 s.
(a) Suggest why recording the times to 0.01 s is not justified. (2)
(b) Suggest one way to make the end point less subjective. (1)
Total [3]
2. A student wanted to compare the rate of respiration of woodlice at 15 °C and 25 °C. She put woodlice into a respirometer at 15 °C, read it, then took the same woodlice out of the respirometer, warmed the tube in her hands until a thermometer read 25 °C, and read it again. Describe three weaknesses in this design. (3)
Total [3]
3. Mealworm larvae kept on bran mixed with copper sulfate grew more slowly than larvae kept on plain bran. Explain how copper ions could slow the growth of the larvae. (3)
Total [3]
4. A group measured the height of grass seedlings with a ruler marked in millimetres. One seedling was 12 mm tall and another 96 mm tall.
(a) Calculate the percentage error in each measurement. (2)
(b) State what your answers show about measuring small seedlings. (1)
Total [3]
5. A student says: "My balance always reads 0.12 g when nothing is on it, but that doesn't matter because I took five repeats and used the mean." Suggest why the student is wrong, and describe how to correct the problem. (2)
Total [2]
6. Seaweed was collected from a rocky shore to compare the number of small snails on it at the top and bottom of the shore. State the most appropriate way to present each data set.
(a) the mean number of snails per 100 g of seaweed at the two heights on the shore (1)
(b) the number of snails per 100 g of seaweed against the height above low water, measured at 12 heights (1)
Total [2]
7. Tomato plants grown in a hydroponic solution lacking nitrate ions produced fewer and smaller fruits than plants given a complete solution. Explain this result. (2)
Total [2]
Answers
1. (a) Two of (1 each): the end point is judged by eye, so the timing depends on deciding when the blue has gone; reaction time is about 0.2 s (or the end point is uncertain by several seconds), much larger than 0.01 s; so the figures after the decimal point are meaningless and the times should be recorded to the nearest second.
(b) One of (1): compare with a colour standard (a tube of yeast suspension with no indicator) under the same lighting; use a colorimeter and record the time to a set absorbance; have the same person judge every end point against a white background.
2. Three of (1 each): the same woodlice are used twice, so the second reading may be affected by stress or by time since feeding; warming in the hands does not hold the temperature at 25 °C, so it drifts during the reading; no time for the woodlice and the air to equilibrate at the new temperature; temperature change alone changes the gas volume, so a control tube (glass beads) is needed; no repeats with different woodlice; no mention of the same mass of woodlice, or of a carbon dioxide absorber.
3. Three of (1 each): copper ions bind to enzymes in the larvae (for example digestive enzymes or respiratory enzymes); this breaks bonds holding the tertiary structure; the active site changes shape; fewer enzyme-substrate complexes form, so less digestion or less respiration (less ATP) and less growth. (Twist: growth is the measured result, so the chain must reach growth, not stop at "inhibits the enzyme".)
4. (a) uncertainty ± 0.5 mm; 0.5 ÷ 12 × 100 = 4.2 % (1); 0.5 ÷ 96 × 100 = 0.52 % (1).
(b) the same uncertainty is a much larger percentage of a small reading, so measurements of small seedlings are much less precise (and differences between them are harder to trust) (1).
5. The reading of 0.12 g is a systematic error: every mass is 0.12 g too high, so the mean is also 0.12 g too high; repeats reduce only random error (1). Zero (tare) the balance before use, or subtract 0.12 g from every reading (1).
6. (a) a bar chart (two separate bars, equal width, starting at zero), ideally with standard-deviation error bars (1).
(b) a scatter graph (or line graph) with height on the x-axis and a line of best fit (1).
7. Nitrate ions are needed to make amino acids, and so proteins (including enzymes), and nucleic acids (1); without them the plant cannot make enough protein (or chlorophyll) for growth, so fewer and smaller fruits form (1). (The marks come in a pair: the use of nitrate, then why fruiting depends on it.)