Biology › Practicals
Practical capabilities
Twelve things you have to be able to do at the bench and in the field, with the maths each one carries and the places marks go missing.
Every board assesses practical work and no two organise it the same way: some run a separate endorsement over a named list of activities, some examine it in a written paper, one sits an external practical exam. Writing to a single list first would have baked one board's numbering into the library and left every other route reading like a translation. So these capabilities are ours, described by the technique. Which route requires which of them has not been worked out yet, and until it has, this page names no board rather than guessing at one.
One note on the numbers you will meet in the lessons. Worked practical results here are idealised. Real readings scatter, and manufacturing a plausible-looking scatter would teach you to read noise that was never measured, so where a lesson needs a table it says the figures are idealised and spends its effort on the analysis instead.
P01Optical microscopy and calibrated measurement
Set up a light microscope, focus safely at each magnification, and turn what is on the slide into a number with units.
What you have to be able to do
- Focusing with the coarse then fine adjustment, low power first
- Calibrating an eyepiece graticule against a stage micrometer
- Measuring a structure in graticule divisions and converting to micrometres
- Recognising an artefact introduced by preparation rather than present in the living cell
The maths it carries
- Magnification as image size over actual size, rearranged either way
- Converting between millimetres, micrometres and nanometres
- Standard form for anything below a micrometre
Where marks are lost
- Calibration is specific to the objective in use. Change objective and the division is worth a different distance.
- An answer in the wrong prefix loses the mark even when the arithmetic is right, so carry the unit through every line.
- Resolution and magnification are separate quantities. A microscope can magnify past its resolution and produce a larger blur.
P02Preparing biological material: squashes, sections and mounts
Prepare a temporary slide from living tissue, stain it so the structure of interest is visible, and interpret what the stain shows.
What you have to be able to do
- Making a root tip squash and spreading it to a single cell layer
- Choosing a stain for what it binds to rather than for its colour
- Cutting a section thin enough to transmit light
- Lowering a coverslip to exclude air bubbles
The maths it carries
- Counting a sample and expressing part of it as a proportion
- Mitotic index from a count of cells in division against the total
- Deciding how large a count has to be before the proportion means anything
Where marks are lost
- A mitotic index without the number of cells counted is not a measurement. State the denominator.
- Cells fixed at one instant are a snapshot, so the proportion in a stage reflects how long that stage lasts, not how important it is.
- Squashing too hard destroys the chromosomes you came to look at, and not enough leaves overlapping layers that cannot be scored.
P03Qualitative biochemical tests, and what they do not tell you
Run the standard tests for the major biological molecules and state precisely what a positive result licenses you to conclude.
What you have to be able to do
- Benedict's test for reducing sugars, and the acid hydrolysis step that brings a non-reducing sugar into range
- Iodine in potassium iodide for starch
- Biuret for peptide bonds
- The emulsion test for lipids, and why the water goes in second
The maths it carries
- Reading a colour change as an ordered scale rather than a number
- Judging when a qualitative result needs a quantitative follow-up
Where marks are lost
- Biuret detects the peptide bond, so it responds to any polypeptide and says nothing about which protein is present.
- A brick-red Benedict's result is not a concentration. If a question asks how much, colorimetry is the answer, not a colour word.
- A negative test can mean absent, or below the threshold of the test. Those are different claims and only one is safe to write.
P04Colorimetry, dilution series and calibration curves
Build a series of known concentrations, measure them, and read an unknown off the line rather than guessing at it.
What you have to be able to do
- Making a serial dilution and stating the concentration at each step
- Selecting a filter of the colour the solution absorbs
- Zeroing the instrument against the right blank
- Plotting the calibration curve and interpolating the unknown
The maths it carries
- Serial dilution arithmetic, including a factor applied repeatedly
- Plotting a curve of best fit and reading a value back from it
- Recognising that interpolation is safe and extrapolation is not
Where marks are lost
- The blank is the solvent and everything else except the substance being measured. A blank of pure water when the samples carry buffer measures the buffer as well.
- A calibration curve is valid only across the range you calibrated. An unknown darker than your strongest standard needs diluting and re-reading, not extending the line.
- The filter is the complementary colour to the solution: a blue solution is measured through a red filter.
P05Measuring the rate of an enzyme-controlled reaction
Follow a reaction over time, extract an initial rate from it, and control the variables that would otherwise explain the result.
What you have to be able to do
- Choosing what to measure: product formed, substrate lost, or gas collected
- Holding temperature with a water bath and pH with a buffer
- Taking readings often enough early on to see the initial slope
- Recognising when the substrate has run down rather than the enzyme having stopped working
The maths it carries
- Rate as a gradient, and the initial rate as the gradient at t = 0
- Drawing a tangent to a curve and taking its gradient
- Rate as the reciprocal of a time to a fixed end point, with units
Where marks are lost
- The initial rate is the only rate that reflects the conditions you set. Any later gradient also reflects how much substrate is left.
- A buffer is a control, not a detail. Without one, an enzyme that releases an acidic product changes its own pH as it works.
- Using time-to-endpoint as a proxy for rate inverts the axis: a longer time is a slower reaction, and graphs get drawn upside down every year because of it.
P06Investigating membrane permeability
Damage a membrane in a controlled way and quantify how much has leaked out.
What you have to be able to do
- Cutting tissue to a standard size so surface area is not a confounding variable
- Washing cut tissue to remove pigment released by the cutting itself
- Holding each sample in its treatment for the same length of time
- Measuring the leaked pigment by colorimetry rather than by eye
The maths it carries
- Plotting a response against a treatment level
- Identifying a threshold from a curve that is flat and then steep
- Percentage transmission and absorbance as inverse descriptions
Where marks are lost
- The pigment on the outside of a freshly cut piece came from the knife, not from the treatment. Washing is the control that makes the reading mean something.
- At high temperatures, typically well above the physiological range, the damage is to the protein and the lipid together, so an answer that mentions only denatured protein is half the story; the exact temperature at which leakage climbs varies with the tissue.
- Deeper colour means more leakage means a more permeable membrane. State that chain explicitly; a lone absorbance value is not an explanation.
P07Water relations of plant tissue: osmosis and transpiration
Measure the water potential of a tissue by finding the solution that leaves it unchanged, and measure water movement through a whole shoot.
What you have to be able to do
- Preparing a graded series of solutions of known water potential
- Blotting and weighing tissue consistently before and after
- Setting up a potometer without trapping air, and cutting the shoot under water
- Holding light, temperature and air movement steady while changing one of them deliberately
The maths it carries
- Percentage change in mass, and why percentage rather than raw grams
- Reading the intercept where the change is zero
- Rate of water uptake from distance moved and the bore of the tube
Where marks are lost
- The intercept is the answer. The point where mass change crosses zero is the solution whose water potential matches the tissue.
- A potometer measures uptake, not transpiration. Most water taken up is transpired, but some is used and retained, so uptake is the honest word for what was measured.
- Water potential is zero for pure water at atmospheric pressure and negative for every solution at atmospheric pressure, so a more concentrated solution has a lower value. The sign catches people out more often than the concept does.
P08Dissection and biological drawing
Open an organ or system safely, identify its structures, and record them in a drawing that could be used by somebody who was not there.
What you have to be able to do
- Handling instruments and material safely, and disposing of both correctly
- Working from the outside inwards, cutting away from yourself
- Drawing in clean single lines with no shading and no sketching
- Labelling with straight ruled lines that do not cross
The maths it carries
- Drawing to a stated scale and calculating the magnification of the drawing
- Measuring a real structure and expressing it in sensible units
Where marks are lost
- A biological drawing carries a scale bar or a magnification. Without one it records shape and nothing else.
- Draw what is in front of you, not what the textbook diagram shows. Examiners can tell, and the differences are often the point.
- Naming a structure is not describing it. Questions usually want the feature and the function it serves in the same sentence.
P09Aseptic technique and the effect of antimicrobials
Work without contaminating a culture or yourself, and measure how effectively a substance stops microbial growth.
What you have to be able to do
- Flaming a loop and a bottle neck, and working close to the flame
- Pouring and spreading a plate without lifting the lid clear
- Placing discs at a spacing that keeps their zones separate
- Sealing, inverting and incubating below human body temperature, and never opening a plate once grown
The maths it carries
- Measuring the diameter of a zone of inhibition, across the disc and in two directions, and halving it to get the radius
- Calculating the area of a zone from that radius, and reading the question to see which of the two it wants
- Population growth as a doubling process, and mean generation time
Where marks are lost
- Diameter and area are both in use, and the question decides which. Some published methods measure the diameter and then calculate the area from it; others compare diameters directly and are marked that way. Measure the diameter either way, since the area cannot be found without it, and then give whichever the question asked for.
- The two do not rank drugs the same way. Area goes as the square of the radius, so doubling a diameter quadruples an area: a zone twice as wide is four times the area, and a table of areas will spread the same discs further apart than a table of diameters. Say which measure a comparison rests on before drawing a conclusion from it.
- The plate is incubated below body temperature deliberately, to avoid selecting for organisms that grow well in a human.
- A clear zone means growth was inhibited. Whether the organisms were killed or merely stopped is a separate question the plate does not answer.
P10Chromatography and the separation of pigments
Separate a mixture of pigments and identify each one by how far it travelled relative to the solvent.
What you have to be able to do
- Concentrating the spot by applying and drying it repeatedly
- Keeping the origin above the solvent surface
- Running the tank sealed so the atmosphere stays saturated
- Marking the solvent front the moment the run is stopped
The maths it carries
- Rf as distance moved by the pigment over distance moved by the solvent
- Recognising Rf as a ratio, so it has no units and never exceeds one
- Comparing an unknown Rf against reference values run in the same solvent
Where marks are lost
- If the origin sits below the solvent, the pigment dissolves into the tank instead of running up the paper.
- Rf values depend on the solvent, so a value compared against a table from a different solvent proves nothing.
- Measure to the centre of each spot, consistently, and say that you did. Spots spread, and the edge is not a defined point.
P11Measuring respiration and photosynthesis rates
Measure a metabolic rate by following a gas or an indicator, with controls that rule out the explanations you are not testing.
What you have to be able to do
- Setting up a respirometer with an alkali to absorb carbon dioxide
- Running a thermobarometer alongside, so a pressure change from the room is subtracted rather than measured
- Using a redox indicator such as DCPIP to follow light-driven electron transfer in chloroplast extract (the Hill reaction); in respiring cells the same indicator class reports dehydrogenase activity
- Equilibrating apparatus before the first reading is taken
The maths it carries
- Volume of gas from the distance a meniscus moves and the tube's bore
- Rate per unit mass of tissue per unit time, and why all three parts are needed
- Reading a rate from the steepest straight part of a curve
Where marks are lost
- Without a control tube, a respirometer measures the weather. Room temperature and atmospheric pressure move the meniscus as readily as respiration does.
- In a respirometer with alkali present, the movement measures oxygen uptake. Remove the alkali and it measures the difference between two gases, which is a different quantity.
- A boiled or killed sample is the control that shows the change was biological. Stating that it was included earns the mark; assuming it does not.
P12Field sampling, distribution and behavioural response
Estimate what lives where, and how much of it, without walking to the interesting-looking patch and calling that a sample.
What you have to be able to do
- Generating random coordinates and placing quadrats on them
- Running a belt or line transect where the point is a gradient
- Mark, release and recapture for a mobile population, with a mark that neither harms nor advertises the animal
- Using a choice chamber or maze with the stimulus as the only variable that differs
The maths it carries
- Mean, and a measure of spread, from a set of quadrat counts
- Percentage cover against frequency, and when each is appropriate
- The Lincoln index for population estimate from a recapture
- A statistical test chosen for the question asked, with the null hypothesis written before the data are seen
Where marks are lost
- Random means generated coordinates, not a walk that felt unplanned. Questions ask how randomness was achieved and reject answers that only assert it.
- The recapture estimate assumes marked animals mix back in, the mark stays on, and nothing joins or leaves in between. Quote the assumption the question is probing rather than all three by rote.
- A transect answers a question about a gradient. Using one where there is no gradient produces a tidy graph of nothing.