IB Biology SLSkill Set 1 — Experimental TechniquePaper 1 & 2Practical skill~12 min read
Applying Lab Techniques
There is a fixed list of techniques the IB expects you to have met, and for each one you need two things: what it is for, and how you actually do it. This page walks through them in the order they turn up in the course, with the calculations that come attached — because that is where the marks usually sit.
📘 What you need to know
Chromatography separates a mixture of pigments; the Rf value identifies each one.
Colorimetry measures how much light a coloured solution absorbs, so colour becomes a number.
Serial dilution makes a set of known concentrations from one stock, each step diluting by the same factor.
Eyepiece graticule + stage micrometer lets you measure real sizes down a microscope.
Temporary mounts get a fresh specimen under the lens with no trapped air bubbles.
Dichotomous keys identify organisms by a chain of either/or questions.
Random sampling avoids bias; systematic sampling follows a gradient on purpose.
Karyograms show chromosomes sorted by size and shape; cladograms show how closely species are related.
Paper and thin layer chromatography
What it is for: splitting a mixture into the substances it contains — most often the pigments in a leaf.
A drop of the mixture goes on a pencil start line near the bottom of the paper. The paper stands in solvent, and the solvent creeps up. Every pigment gets pulled along by the solvent and held back by the paper, and the balance between those two is different for each one. So they end up at different heights, and the mixture separates.
🧩 How to run one
Draw the start line in pencil, about 1 cm up. Pencil does not dissolve; ink does and would run with your sample.
Add a small spot of extract, let it dry, and spot again on the same place. A small concentrated spot beats one big smudge.
Stand the paper in solvent so the solvent level is below the start line. If the solvent covers the spot it just washes away.
Cover the container so the solvent does not evaporate.
Take it out before the solvent reaches the top, and mark the solvent front straight away — it disappears as it dries.
Measure from the start line to the centre of each spot, and from the start line to the front.
Retention factorRf = distance moved by the spot ÷ distance moved by the solvent
An R f value is always between 0 and 1, and it has no units, because it is one distance divided by another.
WORKED EXAMPLE
A chlorophyll spot moves 3.6 cm from the start line while the solvent front moves 8.0 cm. Calculate the Rf value.
Step 1: write the formula
Rf = distance moved by spot ÷ distance moved by solvent
Step 2: put the numbers inRf = 3.6 ÷ 8.0Rf = 0.45No units. If your answer comes out above 1 you have divided the wrong way round.
Colorimetry and serial dilutions
What colorimetry is for: turning a colour into a measurement. A colorimeter shines light through a sample and reports how much is absorbed. Darker solution, more absorbance. It is far more reliable than a person deciding which tube looks darker.
You use it for anything where colour changes with concentration — the intensity of a Benedict’s result, pigment leaking out of beetroot cells, or a starch and iodine colour fading as amylase works.
🧩 Using a colorimeter properly
Pick the filter of the opposite colour to the solution. A blue solution absorbs red light, so use a red filter.
Zero the machine with a blank — a cuvette of distilled water or the solvent on its own.
Measure a set of solutions of known concentration and plot absorbance against concentration. That is your calibration curve.
Measure the unknown and read its concentration off the curve.
Keep the clear sides of the cuvette clean, and always face them the same way in the holder.
Where serial dilution comes in: you need those known concentrations for the calibration curve, and a serial dilution is the neat way to make them. Each step dilutes by the same factor, so one stock solution gives you a whole range that spans several orders of magnitude.
Serial dilution covers a huge range of concentrations quickly. The cost is that an error early in the row carries through every tube after it.
WORKED EXAMPLE
You have a 1.0 mol dm−3 glucose stock. Make 10 cm3 of 0.2 mol dm−3 solution.
Step 1: use the dilution relationshipc1V1 = c2V2Step 2: rearrange for the volume of stock you needV1 = (0.2 × 10) ÷ 1.0 = 2 cm3Step 3: make it up to the final volume
2 cm3 of stock, then add water up to 10 cm3 — so 8 cm3 of water.
2 cm3 stock + 8 cm3 water“Make up to 10 cm3” is not the same as “add 10 cm3“. Getting that wrong changes your concentration.
Physical and digital molecular modelling
What it is for: seeing shape. A structural formula on paper is flat, and biological molecules are not — the shape is often the whole point.
Physical kits (balls and sticks) are quick and show bond angles and how groups sit in space. Great for comparing an alpha and beta glucose ring, or seeing why a saturated fatty acid is straight and an unsaturated one kinks.
Digital models handle molecules far too big for a kit — a whole enzyme, a section of DNA — and you can rotate, zoom and colour by property.
Both are models, so both simplify. A ball-and-stick model shows no electron clouds and no movement.
The light microscope and the eyepiece graticule
What it is for: measuring things that are far too small for a ruler.
An eyepiece graticule is a tiny ruler in the eyepiece. Its divisions are just numbers though — they mean nothing until you calibrate them, because their real size changes with every objective lens. A stage micrometer is a slide with a scale of known size printed on it, and you use it to find out what one eyepiece division is worth.
The graticule never changes, but what each division is worth does. Calibrate for the lens you are actually using.
WORKED EXAMPLE
Using the calibration above, a cell spans 12 eyepiece divisions. Find its real width, then the magnification of a drawing of it that is 60 mm wide.
Step 1: real width from the calibration12 × 4 = 48 µmStep 2: put both lengths in the same unit48 µm = 0.048 mmStep 3: magnification = image size ÷ actual size60 ÷ 0.048 = 1250Width 48 µm, drawing magnified ×1250Magnification has no units, so if you end up with millimetres in your answer, something went wrong.
Preparing a temporary mount
What it is for: getting fresh material under the lens quickly, without the fuss of a permanent slide.
🧩 Making a temporary mount
Put one drop of water or stain in the middle of a clean slide.
Add a thin specimen — a single onion epidermis layer, a thin section, a smear. Light has to pass through it.
Lower the coverslip at an angle, using a mounted needle, and let it fall slowly. This pushes the air out ahead of it instead of trapping bubbles.
Soak up spare liquid at the edge with filter paper.
Focus on low power first, then move up. Never rack down towards the slide while looking through the eyepiece.
Bubble or cell? An air bubble has a thick, very dark outline and a bright middle, and it is perfectly round. New students draw them as cells all the time.
Stains
Most cell parts are transparent, so a stain is what makes them visible.
Iodine colours starch grains and plant cell walls; methylene blue shows nuclei in cheek cells.
Stains are chemicals with hazards of their own — check the bottle, and keep them off skin and clothes.
Identifying and classifying organisms
What it is for: naming what you have found, reliably, without being an expert on everything.
A dichotomous key is a chain of paired statements. Each pair splits the possibilities in two, and each answer sends you to the next pair until you reach a name.
Good key questions use features that do not change: number of legs, leaf edge shape, presence of a shell. Bad ones use size or colour, which vary with age, season and light.
Classification groups organisms by shared features, and modern classification tries to reflect evolutionary relationships rather than surface appearance.
Sampling: random and systematic
What it is for: describing a whole habitat without counting every organism in it.
The two methods answer different questions, and picking the wrong one wastes a whole day of fieldwork.
Random placement removes your own bias. Systematic placement follows a gradient on purpose — up a shore, out from a path, into the shade of a wood.
Doing it properly
Random: lay two tape measures at right angles as axes, generate pairs of random numbers, and use them as coordinates. Choosing spots that “look typical” is bias, not sampling.
Systematic: run a transect line and place a quadrat at a fixed interval — every 2 m, say. Use it when you expect a gradient in light, moisture, salinity or trampling.
Sample size: more quadrats gives a mean you can trust. One quadrat is a story, thirty is data.
Record either percentage cover (good for plants and things you cannot count) or abundance (a count of individuals).
Karyotyping and karyograms
What it is for: checking the number and appearance of a person’s chromosomes.
Cells are stopped in mitosis, stained and photographed, so the chromosomes are condensed and visible.
The images are then cut out and sorted into pairs by size, centromere position and banding pattern. That sorted arrangement is the karyogram.
It shows the number of chromosomes, the sex chromosomes present, and whole-chromosome faults such as an extra copy of chromosome 21.
It cannot show a change in a single base. A karyogram is a wide-angle view, not a close-up.
Cladogram analysis
What it is for: showing how closely species are related, based mainly on molecular evidence such as DNA and protein sequences.
A node is a point where a lineage splits — a shared ancestor of everything above it.
Relatedness is read from the nodes, not from how near the labels sit on the page. Two names printed next to each other may join at a very old node.
A clade is an ancestor plus every descendant it has.
Branch length can represent the number of sequence differences, which acts as a rough clock for how long ago the split happened.
For every technique on this list, be ready for two questions: what is it for, and how do you do it? The IB asks both, and the “how” answers are the ones students leave vague.
💡 Exam tip
Chromatography: pencil start line, solvent below the spot, mark the front immediately. All three are standard marks.
Colorimetry: mention the blank and the calibration curve — without those the readings mean nothing.
Show the working in dilution questions. c1V1 = c2V2 written out earns method marks even if the arithmetic slips.
Convert units before you divide in magnification questions, and give the answer as × something with no units.
Say why your sampling method suits the habitat. “Random to avoid bias” and “systematic to follow the gradient” are the phrases.
Read cladograms from the nodes downwards, never from left to right across the tips.
⚠ Common mix-up
Drawing the chromatography start line in pen. The ink separates too and ruins the run.
Solvent above the start line, which dissolves the sample straight off the paper.
Dividing the wrong way for Rf. Solvent distance is always the bigger number, so it goes on the bottom.
Using graticule divisions as if they were micrometres. They mean nothing until you calibrate.
Forgetting to recalibrate after switching objective lens.
“Random” sampling done by eye. If you chose the spot, it was not random.
Reading a cladogram across the tips. Neighbouring labels are not necessarily close relatives.
Up next: Using Tech to Collect Data — sensors, data loggers, databases and mesocosms, and how to use them without losing track of the biology.
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