The big idea: Paper 1B and the IA reward you for collecting valid data and handling it correctly. For this micro you need to know what each standard technique is for and how to read its results — not to memorise long protocols.
Split them into two groups:
Field sampling — quadrats, transects and kite diagrams estimate how many organisms are present and how they change across a habitat.
Lab techniques — chromatography (separates pigments, measured by Rf), gel electrophoresis (separates DNA by size), PCR (copies/amplifies DNA), and a respirometer (measures respiration rate).
| Technique | What it is for | Key idea / what you measure |
|---|---|---|
| Quadrat | Estimating how many organisms (or % cover) are in an area | Count inside a known frame, take a MEAN per quadrat, then scale up to the whole area |
| Transect | How organisms / abiotic factors change ALONG a set distance (e.g. up a shore) | Lay a tape line and sample at intervals — shows a gradient, not just an average |
| Kite diagram | Showing how the abundance of several species changes along a transect | A symmetrical band per species; its WIDTH = abundance at that point on the line |
| Chromatography (Rf) | Separating a mixture (e.g. leaf pigments) by how far each component travels | Each component has a fixed Rf = its distance ÷ the solvent's distance |
| Gel electrophoresis | Separating DNA (or protein) fragments by SIZE | An electric field pulls negatively-charged DNA through the gel; SMALL fragments travel furthest |
| PCR | Making millions of copies of a DNA sample (amplifying it) | Repeated heating/cooling cycles double the DNA each round |
| Respirometer | Measuring the RATE of respiration of small organisms | Measures O₂ used (or CO₂ given off) per unit time |
| Mesocosm | A small sealed model ecosystem for ecological experiments | Lets you control variables and study a community without disturbing the wild |
- Quadrat
- A square frame of known area placed (usually randomly) so you can count organisms or estimate % cover inside it; repeat and take a mean.
- Transect
- A line (tape) laid across a habitat; you sample at set intervals to show how organisms or abiotic factors CHANGE along the distance.
- Kite diagram
- A chart that shows each species as a symmetrical band along a transect — the band's width shows how abundant that species is at each point.
- Chromatography
- A technique that separates a mixture (e.g. leaf pigments) according to how far each component travels in a solvent.
- Rf value
- The distance a component moves divided by the distance the solvent moves; a fixed number (0–1) that identifies the component.
- Gel electrophoresis
- Separates charged molecules (DNA fragments) by size: an electric field pulls them through a gel, and the smallest move furthest.
- PCR (polymerase chain reaction)
- A technique that amplifies DNA — it makes millions of copies of a sample by repeated heating and cooling cycles.
- Respirometer
- An instrument that measures the rate of respiration of small organisms, usually from the volume of oxygen used per unit time.
Match the tool to the job: If the question says 'along a distance' or 'gradient' → transect (and a kite diagram to display it).
If it says 'how many of each species in an area' → quadrats + a mean.
If it says 'amplify / copy DNA' → PCR. If it says 'separate DNA by size' → electrophoresis. If it says 'separate pigments' → chromatography (Rf).
The Rf equation (learn this one): Chromatography spreads a pigment mixture into separate spots. Each pigment has a fixed Rf value you can calculate:
Both distances are measured from the same start line (the pencil origin) to the centre of each spot, in the same units (mm). Because it is a ratio of two lengths, Rf has no units and is always between 0 and 1 — the pigment can never move further than the solvent.
IB-style question — calculate an Rf value
On a chromatogram the solvent front rose 80 mm from the start line. A blue-green chlorophyll spot moved 52 mm from the same line. Calculate the Rf value of this pigment. [2]
Worked solution
- Write the formula first.
- Put in the numbers (same units, mm).
- Divide — the mm cancel, so Rf has no units. (2 s.f.). It is between 0 and 1, as it must be. (Mark 1: correct working/formula with both distances. Mark 2: Rf = 0.65.)
Final answer
(no units).
Reading a whole chromatogram: Run the same calculation for every spot and you can identify each pigment by its Rf. The pigment that travels furthest (highest Rf) is the most soluble in that solvent; the one nearest the start line has the lowest Rf.
| Pigment band | Distance moved by pigment (mm) | Distance moved by solvent (mm) | Rf = pigment ÷ solvent |
|---|---|---|---|
| Carotene (top, yellow-orange) | 76 | 80 | 76 ÷ 80 = 0.95 |
| Chlorophyll a (blue-green) | 52 | 80 | 52 ÷ 80 = 0.65 |
| Chlorophyll b (yellow-green) | 36 | 80 | 36 ÷ 80 = 0.45 |
| Xanthophyll (yellow, lowest) | 16 | 80 | 16 ÷ 80 = 0.20 |
Now the field-sampling calculation. A single quadrat is just a sample — to estimate the total number in a habitat you take the mean per quadrat, work out the density (number per m²), then scale up to the whole area.
Estimating a population from quadrats:
then .
Using a mean of many quadrats (not one) makes the estimate more reliable and reduces the effect of a freak count.
IB-style question — estimate a population from quadrat data
A student placed ten 0.25 m² quadrats randomly on a 400 m² lawn and counted daisy plants: 6, 4, 7, 5, 3, 8, 5, 6, 4, 2. Estimate the total number of daisies on the lawn. [3]
Worked solution
- Total counted = 6+4+7+5+3+8+5+6+4+2 = 50 daisies in 10 quadrats.
- Density .
- Scale up to the whole lawn. daisies. (Mark 1: total or mean. Mark 2: density per m². Mark 3: ×400 = 8000.)
Final answer
Density = 50 ÷ (10 × 0.25) = 20 per m²; estimate = 20 × 400 = 8000 daisies.
Carry the units: Mark schemes want the units carried through: an Rf is a bare number (no units), but a density is per m² and a population estimate is a count. Drop the unit and you can lose the final mark.
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How this is tested: On Paper 1B you typically (a) read or compute a value from a result — most often state the Rf equation and calculate an Rf, or read off the pigment with a given Rf; (b) name the technique for a stated job (amplify DNA → PCR; separate DNA by size → electrophoresis; change along a distance → transect); and (c) interpret a kite diagram or chromatogram (most/least abundant species, most soluble pigment).
Answer the calculation by writing the formula, substituting with units, then dividing — and quote Rf to about 2 significant figures.
IB-style question — analyse a pigment chromatogram
A student separated the pigments from a fern leaf using paper chromatography. The solvent front rose 90 mm from the start line. The lowest spot (a yellow xanthophyll) moved 18 mm; the highest spot (carotene) moved 81 mm.
(a) State the equation for an Rf value. [1]
(b) Calculate the Rf value of the xanthophyll. [2]
(c) Identify which pigment is the most soluble in the solvent, giving a reason. [2]
How to score all five marks
- (a) The equation. . (1 mark.)
- (b) Substitute with units. — both measured from the start line.
- (b) Divide. (no units; lies between 0 and 1). (Mark 1: correct working. Mark 2: 0.20.)
- (c) Most soluble = travels furthest. Carotene is most soluble: it moved 81 mm, the greatest distance / it has the highest Rf (). The more soluble a pigment is in the solvent, the further it is carried. (Mark 1: carotene. Mark 2: because it travelled furthest / highest Rf.)
Final answer
(a) Rf = distance moved by pigment ÷ distance moved by solvent. (b) Rf = 18 ÷ 90 = 0.20. (c) Carotene — it moved furthest (81 mm, Rf = 0.90), so it is the most soluble in the solvent.
✓ Why this scores full marks: It writes the formula before substituting, keeps the units in the substitution, gives Rf as a bare number between 0 and 1, and in (c) links 'most soluble' to 'travelled furthest / highest Rf' — the examiner wants both the pigment and the reasoning, not just a name.
| Paper chromatography | Gel electrophoresis | |
|---|---|---|
| Separates | A mixture of pigments / small molecules | DNA (or protein) fragments |
| By | How soluble each component is / how far it travels in the solvent | SIZE — driven by an electric field |
| What moves furthest | The MOST soluble component (highest Rf) | The SMALLEST fragment |
| Driving force | Solvent rising up the paper (capillary action) | An electric field (DNA is negatively charged → moves to the + electrode) |
| Measured value | Rf = pigment distance ÷ solvent distance | Fragment size (read against a known size 'ladder') |