Knowledge Organiser: Practical Investigations in Cell Biology
Key Terms
- Magnification: Image size ÷ Real size
- Resolution: Ability to distinguish two nearby points
- Iodine: Stains starch/cell walls dark blue-black
- Methylene blue: Stains nuclei blue
- Eyepiece graticule: Scale in eyepiece for measuring cells
- Stage micrometer: Calibration slide for graticule
Must-Know Formulas
- MIR Triangle: M = I ÷ R (Magnification)
- I = M × R (Image size)
- R = I ÷ M (Real size)
- Unit conversion: 1 mm = 1000 µm
- % change in mass: ((Final - Initial) ÷ Initial) × 100
Three Required Practicals
| Practical | Key Skill | Independent Variable |
|---|---|---|
| RPA1: Microscopy | Preparing slides, focusing, biological drawing, magnification calculation | Objective lens / magnification used |
| RPA2: Osmosis in plant tissue | Cutting equal-sized potato cylinders, measuring % change in mass | Concentration of sucrose solution |
| RPA (Mitosis observation) | Preparing and staining root tip squash slides, identifying mitosis stages | Region of root tip observed |
Light vs Electron Microscope
| Feature | Light microscope | Electron microscope |
|---|---|---|
| Maximum magnification | ~×1,500 | ~×2,000,000 |
| Resolution | ~200 nm | ~0.2 nm (TEM) |
| Living specimens | Yes | No — specimen must be dead and in a vacuum |
| Colour images | Yes | No — false colour is added artificially |
| Cost and size | Low cost; portable | Very expensive; room-sized |
Common Mistakes
- Calculating magnification incorrectly: Always use magnification = image size ÷ actual size, with both measurements in the same unit. Convert mm to µm (× 1000) before dividing to avoid errors by a factor of 1000.
- Saying staining "magnifies" the cell: Stains (e.g. iodine, methylene blue) make cell structures more visible by adding colour contrast — they do not increase magnification or resolution.
- Confusing magnification with resolution: Magnification is how many times larger the image is; resolution is the ability to distinguish two separate points. An electron microscope has higher resolution AND magnification than a light microscope.
- Not describing a fair test in osmosis investigations: When investigating osmosis in potato cylinders, all variables except concentration (e.g. volume of solution, surface area, temperature, time) must be controlled and stated.
Memory Aid
- Cover I → Image size = M × R
- Cover M → Magnification = I ÷ R
- Cover R → Real size = I ÷ M
- Iodine = dark blue-Inky black (stains starch and cell walls)
- Methylene blue = blue nucleus (Makes the nucleus visible)