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5.1.3Tools

Applying Techniques in Biology — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 4 of 10

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 6Separating and Measuring

5.1.3 Applying Techniques in Biology
Chromatography
  • Mixtures separate on paper or a thin layer as a solvent moves up;
    • each substance travels a different distance depending on solubility and attraction to the paper.
Rf value
  • Rf equals the distance moved by the substance divided by the distance moved by the solvent front;
    • it is always less than 1.
Colorimetry
  • A colorimeter measures absorbance of light by a coloured solution, which relates to its concentration.
Serial dilution
  • Repeatedly diluting a solution by a fixed factor, such as 1 in 10, gives a range of known concentrations.

Part 2 of 6Microscopy and Observation

1
Make a temporary mount by placing a thin specimen in a drop of water or stain on a slide and lowering a coverslip at an angle.
2
Start with the lowest-power objective, and focus using the coarse then the fine adjustment.
3
Calibrate the eyepiece graticule against a stage micrometer for each objective lens.
4
Draw what is seen with clear continuous lines, no shading, and label the structures.
5
Calculate magnification from the size of the drawing divided by the actual size.

Part 3 of 6Sampling Ecosystems

Sampling Ecosystems
Random sampling
  • Quadrats are placed at coordinates chosen by random numbers, which avoids bias when an area is uniform.
Systematic sampling
  • Samples are taken at regular intervals along a transect, which suits a gradient such as moving away from the shore.
Identification
  • Dichotomous keys classify organisms using clear observable features.
Reliability
  • More samples reduce the effect of chance on estimates of abundance.

Part 4 of 6Genetic and Evolutionary Tools

Karyotyping
  • Chromosomes from a dividing cell are photographed and arranged in pairs by size and banding to show the full chromosome set.
Uses of karyograms
  • They can reveal chromosome number errors such as Down syndrome, and show the sex of an individual.
Cladograms
  • Branching diagrams show evolutionary relationships based on shared derived characteristics or DNA sequences.
Molecular models
  • Physical or digital models show the shapes of molecules such as proteins and DNA.

Part 5 of 6Worked Example

Question

  • In a chromatography experiment the solvent front moved 8.0 cm and a pigment spot moved 3.6 cm from the origin.
  • Calculate the Rf value.

  1. Rf = distance moved by pigment ÷ distance moved by solvent front.
  2. Rf = 3.6 ÷ 8.0.
  3. The distances use the same unit, so Rf has no unit.

Answer Rf = 0.45.

Part 6 of 6Exam Checklist & Tips

5.1.3 – Key Points for the Exam

  • Chromatography A technique for separating the components of a mixture.
  • Rf value Distance moved by a substance divided by the distance moved by the solvent front.
  • Serial dilution A stepwise dilution by a constant factor.
  • Karyogram An image of the chromosomes of a cell arranged in pairs.
  • Cladogram A tree diagram showing evolutionary relationships.

Common Mistakes

  • Drawing the origin line in ink or below the solvent level, so that pigments dissolve into the solvent.
  • Giving an Rf value greater than 1.
  • Using only the high-power lens to find a specimen.
  • Shading or sketching with overlapping lines in a biological drawing.

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