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.