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1.4.2Inquiry Process

Cell Theory: Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 5 of 160

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Section 1 of 5Using a Light Microscope

1.4.2 Cell Theory: Skills

A light microscope passes light through a thin specimen and focuses it through a series of lenses so that cells and some organelles can be seen.

Eyepiece and objectives
  • The eyepiece lens and the rotating objective lenses together determine the total magnification of the image.
Stage and light source
  • The specimen is held on the stage and lit from below, so it must be thin enough for light to pass through.
Focusing
  • The coarse knob brings the specimen roughly into focus, and the fine knob sharpens the image.
Starting position
  • Always begin with the lowest power objective, which gives a wide field of view and protects the lens and slide.
Staining
  • Many cells are almost transparent, so a stain is added to make structures visible.

Section 2 of 5Preparing and Observing a Slide

A temporary mount is quick to make and is suitable for observing living or freshly cut material.

1
Cut or peel a very thin layer of tissue, or place a few drops of a liquid sample, on the centre of a clean slide.
2
Add a drop of stain if needed, then lower a coverslip at an angle so that no air bubbles are trapped.
3
Clip the slide onto the stage and focus with the low power objective using the coarse focus.
4
Move to a higher power objective only after the image is sharp, and then use only the fine focus.
5
If the image is blurred, return to low power and check that the specimen is thin enough and the slide is clean.

Section 3 of 5Measuring Cells and Calculating Magnification

Sizes are measured with a graticule, which is calibrated against a stage micrometer.

Measuring Cells and Calculating Magnification
Eyepiece graticule
  • A scale in the eyepiece with arbitrary divisions; it must be calibrated for each objective lens.
Stage micrometer
  • A slide with an accurately engraved scale in known units, used to find the actual length of each graticule division.
Magnification formula
  • Magnification equals image size divided by actual size, so actual size equals image size divided by magnification.
Unit conversion
  • Convert image and actual sizes to the same unit first; 1 mm = 1000 µm and 1 µm = 1000 nm.
Quantity to findFormula
Magnificationimage size ÷ actual size
Actual sizeimage size ÷ magnification
Image sizeactual size × magnification

Section 4 of 5Worked Example

Question

  • With one objective lens, 100 µm on a stage micrometer lines up with 40 divisions of the eyepiece graticule.
  • A cell measures 18 graticule divisions.
  • A drawing of this cell is 18 mm long.
  • Find the actual length of the cell and the magnification of the drawing.

  1. One graticule division = 100 µm ÷ 40 = 2.5 µm.
  2. Actual cell length = 18 × 2.5 µm = 45 µm.
  3. Convert the drawing18 mm × 1000 = 18 000 µm.
  4. Magnification = 18 000 ÷ 45 = 400.

Answer Actual length 45 µm; magnification ×400

Section 5 of 5Exam Checklist & Tips

1.4.2 – Key Points for the Exam

  • Graticule A scale placed in the eyepiece of a microscope to measure objects in the field of view.
  • Stage micrometer A microscope slide carrying a precise scale used to calibrate the graticule.
  • Magnification How many times larger the image appears compared with the real object.
  • Field of view The area of the specimen visible through the microscope at one time.
  • Temporary mount A slide prepared quickly with a coverslip for immediate observation.

Common Mistakes

  • Using the high power objective first, which makes it hard to find the specimen and risks cracking the slide.
  • Using the same graticule calibration for every objective lens, when each lens needs its own.
  • Dividing sizes without converting to the same unit, producing an answer that is wrong by a factor of 1000.
  • Moving the coarse focus when using a high power objective instead of the fine focus.

Last updated:

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.