5.1.3Tools
Applying Techniques in Biology — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 4 of 10
Part 1 of 5Observing Structure
Light microscopy
- Thin sections are placed on a slide, often stained,
- and viewed at increasing magnification; focus with the coarse and then the fine adjustment.
Scale and magnification
- Magnification = image size ÷ actual size, and a scale bar lets the real size of a structure be found.
Graticule
- An eyepiece graticule is calibrated against a stage micrometer at each objective lens before it is used for measuring cells.
Dissection
- Organs are cut with a scalpel after pinning the specimen, and annotated drawings with a title, scale and labels record what is seen.
Electron microscopy
- Electron micrographs show ultrastructure at much higher resolution, but only dead, prepared specimens can be examined.
Part 2 of 5Separating and Analysing Molecules
Separating and Analysing Molecules
Chromatography
- Pigments or other molecules are carried by a solvent up a stationary phase, and each substance travels at a different speed.
Rf value
- Rf = distance moved by the spot ÷ distance moved by the solvent front, which is always between 0 and 1.
Gel electrophoresis
- Charged molecules such as DNA fragments are separated by size as an electric current moves them through a gel.
Colorimetry
- The absorbance of a coloured solution is measured and compared with standard solutions to find concentration.
Serial dilution
- A solution is diluted step by step by the same factor, for example 1 : 10, to produce a range of concentrations.
Part 3 of 5Sampling and Culturing
Quadrats
- Frames placed at random positions estimate the number or percentage cover of plants or slow-moving animals in an area.
Transects
- A line across a habitat shows how species change along an environmental gradient such as a shore.
Mark-release-recapture
- Animals are caught, marked, released and recaptured,
- so that population size can be estimated from the proportion of marked animals.
Aseptic culture
- Using sterile equipment and flame sterilisation helps microbes to be grown on agar without contamination.
| Technique | Used to |
|---|---|
| Chromatography | Separate pigments |
| Gel electrophoresis | Separate DNA fragments by size |
| Transect | Study change along a gradient |
Part 4 of 5Worked Example
Question
- An image of a cell measures 45 mm across, and the actual cell is 0.15 mm across.
- Calculate the magnification.
- In a chromatogram a pigment moved 3.2 cm while the solvent front moved 8.0 cm.
- Find its Rf value.
- Magnification = image size ÷ actual size = 45 ÷ 0.15 = 300, so ×300.
- Rf = distance moved by pigment ÷ distance moved by solvent front.
- Rf = 3.2 ÷ 8.0 = 0.40.
Answer The magnification is ×300 and the Rf value is 0.40 (no units).
Part 5 of 5Exam Checklist & Tips
5.1.3 – Key Points for the Exam
- Magnification How many times larger an image is than the object.
- Rf value Ratio of the distance moved by a substance to the distance moved by the solvent.
- Quadrat A square frame used to sample organisms.
- Stage micrometer A slide with a precise scale used to calibrate an eyepiece graticule.
Common Mistakes
- Measuring the image and actual sizes in different units when calculating magnification.
- Measuring the Rf distance from the top of the paper instead of from the starting line.
- Placing quadrats where plants look most interesting instead of at random.
- Forgetting to include a scale bar or magnification on a drawing.
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