The WBI13 paper: solving problems and applying knowledge in practical contexts
This unit is the whole of the AS practical paper, WBI13 (Unit 3: Practical Skills in Biology I).
It is a written paper of 1 hour 20 minutes and 50 marks, worth 20% of the International Advanced Subsidiary and 10% of the full International A Level.
There are exactly three questions, each worth between 12 and 24 marks, and each is built on one practical you met in Units 1 and 2.
There is no multiple choice, no asterisked extended-writing item and no levels marking.
You need a scientific calculator, a ruler and an HB pencil, and at least 5 marks on every paper are mathematics at Level 2 or above.
This first section explains how the paper works and teaches the first two skills listed for assessment: solve problems set in practical contexts and apply scientific knowledge to practical contexts.
- Each marking point is one fact, step or value, worth "(1)". There are no letter codes, no "max" and no levels: a header such as "four of the following points" sets the cap, or the part's mark total does.
- Long items (a 6-mark method, a 5-mark "justify each step", a 4-mark evaluation) list more points than marks: any n of m points score.
- "/" separates alternatives; words in round brackets are not needed; a bold word (often "and") must be there; a linking word such as "so" or "to" shows the reason that must come with a step. A reverse argument is accepted.
- Give exactly as many answers as asked, one per numbered line: where the scheme says so, only the first answers are marked.
- A stated format (2 s.f., 3 s.f., standard form, metres) is a mark of its own. Errors carry forward (ecf) where the scheme says so, for example your own gradient used in an equation.
- Graphs are marked on letters: A axes the right way round, L labels with units, then two of S a linear scale, P points plotted correctly, a best-fit line or the right chart type.
The idea: one familiar method, an unfamiliar context
Every WBI13 question takes a practical you have done (or should have done) and moves it somewhere new.
The colour-standards method arrives as starch in a root crop; the membrane-permeability practical arrives as pigment leaking from a flower petal in a detergent; the enzyme practical arrives as a meat tenderiser working on a protein food.
The biology and the method are familiar.
The context is not, and the context always contains one detail that makes the stock answer wrong.
That is what "solve problems set in practical contexts" means.
You are not being asked to recite a method.
You are being asked to work out what this experimenter must do, with this material, to answer this question, and why each step matters.
Examiners report the same failure in every series: answers that were learned in advance and do not fit the set-up in front of them.
The fourteen practicals behind every question
The three questions are drawn from fourteen practicals: the nine core practicals (CP1 to CP9) and the five recommended additional practicals (RAPs) printed in Topics 1 to 4.
The RAPs have no statement numbers of their own, but they are examined: across three recent papers they carried 51 of the 150 marks, including two whole questions on the heart dissection.
The bank later in this unit summarises each one; its science is taught in the theory unit shown.
Practical
What it does
Theory notes unit
CP1
estimate reducing sugar (Benedict's) and starch (iodine) with colour standards
1a
RAP after 1.8
dissect a mammalian heart
1a
CP2
measure the vitamin C content of food and drink with DCPIP
1b
CP3
effect of alcohol and temperature on membrane permeability (pigment leakage)
2a
RAP after 2.4
tissue water potential with graded solute concentrations
2a
RAP after 2.6
estimate protein with biuret reagent and colour standards
2a
CP4
effect of temperature, pH, enzyme and substrate concentration on initial rate
2a
CP5
observe and draw animal cells; measure with a graticule; scale
3a
RAP after 3.13
factors affecting the growth of pollen tubes
3b
CP6
root tip squash to observe the stages of mitosis
3b
CP7
plan diagrams of root, stem and leaf sections; identify sclerenchyma, xylem and phloem
4a
RAP after 4.8
plant mineral deficiencies
4a
CP8
tensile strength of plant fibres
4a
CP9
antimicrobial properties of plants, with aseptic technique
4a
Units 2b and 4b hold no practical, and no recent WBI13 question drew on them, but their data skills (percentages, sampling, tables) are the same skills this unit teaches.
How a WBI13 question runs
A typical question follows one material from start to finish:
1. Context. One or two sentences of fact about an organism, food, crop, fibre or chemical, often with a photograph.
2. Knowledge. A short recall part about the material: the structure of starch, the stages of mitosis, the role of the pollen tube, the cell types in a fibre.
3. Variables and controls. Name the independent variable, a control, or variables to keep the same.
4. Method. Describe all or part of a method, or justify each step of one.
5. Data. A calculation, a table to draw or a graph to plot.
6. Judgement. Conclusions, a comment on the results, a discussion or an evaluation.
Across three recent papers the marks split roughly like this: knowledge in context 25%, planning and method 43%, presenting data 14%, calculation and graph algebra 9%, analysis and evaluation 9%.
So about three quarters of the marks are won in words, not numbers.
Solving a problem in a practical context: a routine
1. Read the stem twice and mark the bold words. WBI13 prints in bold the number of answers wanted (two), restrictions (only, without) and the step or photograph meant (step 3).
2. Name the practical underneath. Pigment leaking from discs is CP3. Clear zones round discs is CP9. A colour compared with a row of tubes of known concentration is a colour-standards method (CP1 or the biuret RAP).
3. Find the twist. What is different here? A Petri dish holding seedlings, not bacteria. A drink that is already coloured. A temperature chosen for the enzymes of the plant, not to stop pathogens.
4. Picture yourself doing it. One report advises visualising each step in the laboratory before writing. If you cannot picture where your hands are and what the apparatus looks like, the method is not yet clear enough to earn marks.
5. Answer for this set-up. Use the organism, reagent and quantities in the stem.
Applying scientific knowledge to practical contexts
About a quarter of WBI13 marks are Topic 1 to 4 knowledge used inside a practical.
You need it in two ways.
First, as straight recall in context: describe the structure of a polysaccharide in a crop being tested, name the stages of mitosis in a squash, name the cell types in a fibre.
Second, as the reason behind a method step:
Method step
The knowledge that explains it
Taught in
incubate at a moderate temperature
enzymes have an optimum; high temperatures denature them
2a
a boiled-enzyme control
boiling denatures the enzyme, so any change is not enzyme-catalysed
2a
rinse cut beetroot discs
cut cells release pigment that is not from leakage through membranes
2a
blot potato cylinders the same way
surface water changes the mass but is not water taken in by osmosis
2a
warm acid on a root tip
breaks down the material holding cells together so they separate when squashed
3b
sucrose in pollen-tube medium
water potential of the medium and an energy source for tube growth
3b
lid taped, not sealed, on an agar plate
keeps conditions aerobic so anaerobic pathogens are not favoured
4a
incubate plates below 37 °C
reduces the chance of growing pathogens adapted to body temperature
4a
These notes do not re-teach that science: each line points to the theory unit that does.
What WBI13 adds is the link: you must say why the knowledge makes the step necessary, in the context given.
The WBI13 command words
Command
Share of marks
What earns marks on WBI13
Describe
26%
a method or part of one, as specific steps with named apparatus and quantities; or a structure or difference
Explain
9%
a reason for each point; often a short description first, then why
Calculate
8%
the key step and the answer in the form asked
Plot
8%
the four graph marks A, L, S, P
Draw
7%
a table, a plan, or a line or arrow on an image
Complete
7%
a table, key or labels
State, Name, Give
10%
one fact per line, exactly as many as asked
Compare and contrast
5%
at least one similarity and one difference
Suggest
4%
reasons fitted to the unfamiliar context
Devise, Justify
7%
a whole method; each step with its reason
Evaluate, Comment on, Discuss
7%
specific points tied to the data, strengths and limitations
Write
1%
the equation of a line
Words such as Determine, Deduce, Assess, Predict and Sketch did not appear in these three papers, but the reports warn that any command word on the list can be used on any unit, so learn them all.
- Describe: Give an account of something. Statements in the response need to be developed as they are often linked but do not need to include a justification or reason.
- Explain: An explanation requires a justification/exemplification of a point. The answer must contain some element of reasoning/justification, this can include mathematical explanations.
- Suggest: Use your knowledge and understanding in an unfamiliar context. May include material or ideas that have not been learnt directly from the specification.
The command-word definitions above are worth learning word for word: they tell you exactly what the examiner is looking for.
A stem reads: "Hibiscus petals contain a red pigment in their vacuoles.
A student cut equal discs from the petals and placed them in solutions of a household detergent at different concentrations.
After 20 minutes the student measured the colour of each solution."
Name the practical underneath and the twist.
1. Pigment leaving cells into the surrounding liquid, measured by colour, is CP3 (membrane permeability).
2. The twist is the independent variable: a detergent concentration, not alcohol or temperature. Detergents disrupt the phospholipid bilayer, so the biology still comes from 2a.
3. Everything else transfers: equal discs, rinsing, a fixed volume, a fixed time, a water control, a colorimeter zeroed with the solvent.
Answer: CP3 with a new independent variable.
Write the CP3 method, but every variable and control must be about detergent, not alcohol.
A student investigated the effect of a fungicide on the germination of mung bean seeds. All the dishes were kept at 25 °C. Explain why the dishes were kept at 25 °C. (2)
1. germination depends on enzymes (for example amylase breaking down stored starch) (1)
2. 25 °C is close to their optimum / not high enough to denature them, so germination is fast enough to see an effect, and temperature does not affect the results (1)
Answer: two linked points, both about the seeds' enzymes.
A weaker answer says "to kill microorganisms" or "because it is room temperature": the first misreads the stem (there is no culture here), and the second gives no biological reason.
Neither scores.
A sports scientist measured the glycogen content of muscle samples before and after exercise. Describe the structure of a glycogen molecule.
(3)
1. made of α-glucose monomers (1)
2. joined by 1,4 glycosidic bonds, with 1,6 glycosidic bonds at branch points (1)
3. highly branched (more branched than amylopectin), so the molecule is compact (1)
Answer: three structural points.
A weaker answer ("glycogen stores energy in muscles and is broken down quickly in exercise") drifts into function and earns nothing: the command is about structure.
Examiners note this drift every time a polysaccharide is asked.
A student planned to compare the vitamin C content of orange juice and blackcurrant juice by titrating each juice into 1.0 cm³ of blue DCPIP until the blue colour disappeared. Suggest why this method may not give a valid result for the blackcurrant juice, and how the student could overcome the problem.
(3)
1. blackcurrant juice is deeply coloured (purple-red) (1)
2. so the end point, when the blue of DCPIP disappears, is hard to see / is masked by the juice colour (1)
3. dilute the juice by a known factor (and multiply back) / compare each tube with a tube of juice and water against a white tile / use a colorimeter to detect the end point (1)
Answer: the twist is the juice's own colour.
A stock answer ("repeat it three times and take a mean") does not touch the problem and scores nothing.
Invented data: the time for trypsin to clear a milk suspension was 410 s at 15 °C, 205 s at 25 °C, 98 s at 35 °C, 85 s at 45 °C and no clearing in 600 s at 65 °C.
Describe the effect of temperature on the time taken.
(2)
1. time falls as temperature rises from 15 to 45 °C, from 410 s to 85 s (1)
2. at 65 °C there was no clearing within 600 s (1)
Explain the results.
(3)
1. as temperature rises, enzyme and substrate molecules have more kinetic energy, so there are more successful collisions / enzyme-substrate complexes per second (1)
2. at 65 °C the trypsin is denatured: bonds holding its tertiary structure break, so the active site changes shape (1)
3. so the substrate no longer fits the active site and no protein is hydrolysed (1)
Answer: Describe gives the pattern with values and no reasons; Explain gives the biology behind each feature.
Writing the explanation where a description is asked wastes time; writing the description where an explanation is asked scores almost nothing.
Before you write a word, find the practical underneath and the one detail that makes this set-up different.
Then answer in the stem's own terms: its organism, its reagent, its quantities.
Examiners in every recent series reported that answers learned in advance, which ignore the context, score poorly.
- A stock answer for the wrong set-up. "Keep the temperature, pH and volume the same" earns nothing when one of those is the thing being measured. Choose variables that matter in this context.
- Misreading unfamiliar apparatus. A Petri dish of seedlings is not a microbiology plate; an answer about aseptic technique there scores nothing.
- Function where structure is asked. "Describe the structure" wants monomers, bonds and shape; uses and roles score nothing.
- Describing when asked to explain. An Explain or Justify item needs the reason for each point, often linked with "so" or "because".
- More answers than asked. Where only the first answers are marked, a wrong first answer cannot be rescued by a right third one. One answer per numbered line.
- Answers the examiner cannot read. Write legibly, one point per line; a recent report noted marks lost to handwriting (and, for candidates who type, to unchecked typing errors).
Check your understanding
1. Give the reagent that should be used in each of these investigations.
(a) estimating the protein concentration of soya milk using colour standards (1)
(b) estimating the starch concentration of the water left after boiling cassava (1)
Total [2]
2. A food scientist boiled a protein drink for 10 minutes, cooled it and tested it with biuret reagent. The mixture turned purple, just as it did before boiling. A student concluded that "boiling does not damage the proteins in the drink". Explain why this conclusion is not valid. (3) Total [3]
3. A student extracted oil from sunflower seeds and tested it with the emulsion test. Describe the structure of a triglyceride molecule. (3) Total [3]
4. A plant breeder germinated pollen from tomato flowers on agar containing different concentrations of calcium ions. The slides were kept in the dark at 22 °C.
(a) Describe what the pollen tube does to bring about fertilisation. (2)
(b) State the independent variable in this investigation. (1)
(c) Explain why all the slides were kept in the dark at the same temperature. (2)
Total [5]
5. A student placed 20 mung bean seeds on damp filter paper in each of six Petri dishes, to test the effect of a herbicide on germination. A classmate said the lids should be taped and the dishes incubated below 25 °C, "as in the antimicrobial practical". Suggest why this advice does not apply to this investigation. (3) Total [3]
6. A student investigated the effect of pH on the time taken for an amylase solution to break down starch. The table shows the invented results.
pH
Mean time for starch to disappear / s
4
540
5
260
6
120
7
150
8
390
(a) Describe the effect of pH on the time taken. (2)
(b) Explain the effect of pH on the time taken. (3)
Total [5]
Answers
1. (a) biuret reagent (1). (b) iodine solution / iodine in potassium iodide (1). Benedict's reagent for either scores 0.
2. Any three from: biuret reagent detects peptide bonds (1); boiling denatures proteins by breaking hydrogen / ionic bonds in the tertiary structure (1); but peptide bonds are not broken by boiling, so the primary structure remains and the test is still positive (1); so a positive biuret result cannot show whether the proteins kept their shape / function (1).
3. Glycerol (1); joined to three fatty acids (1); by ester bonds, formed by condensation (1).
4. (a) The tube grows down through the style, secreting enzymes that digest a path (1); it carries the male nuclei to the ovule / embryo sac (1). Both are needed: a role in growth alone is not the full role. (b) Concentration of calcium ions (in the agar) (1). (c) Light and temperature could affect tube growth / temperature affects the enzymes involved in growth (1); so keeping them the same means any difference is due to the calcium ion concentration only (1).
5. Any three from: no microorganisms are being cultured, so aseptic sealing is not the point (1); seeds need oxygen for aerobic respiration during germination, so the lid need not be taped (1); the temperature should suit the seeds' germination enzymes / be the same for all dishes, not be chosen to avoid human pathogens (1); taping is to stop people being exposed to cultured bacteria, which are absent here (1).
6. (a) Time is shortest at pH 6, 120 s (1); time increases on either side of pH 6, to 540 s at pH 4 and 390 s at pH 8 (1). (b) pH 6 is (close to) the optimum pH of this amylase (1); at other pH values the H⁺ concentration alters ionic / hydrogen bonds in the enzyme, changing the shape of the active site (1); so fewer enzyme-substrate complexes form per second and starch is hydrolysed more slowly (1).