Skills applied: microscopy, graticules and mitotic index (CP5, CP6, CP7)

Calibrating an eyepiece graticule (CP5)

An eyepiece graticule has arbitrary divisions; their size depends on the objective lens. Calibrate with a stage micrometer (a slide with a scale, usually 1 mm in 100 divisions, so each division is 10 µm).

1. Line up the two scales.

2. Count how many eyepiece units (epu) match a whole number of stage divisions.

3. 1 epu = (stage divisions x 10 µm) ÷ eyepiece units.

4. Recalibrate for every objective.

Then actual size = number of epu x size of one epu.

Mitotic index (CP6)

Method recap (see Core practical 6, Unit 2): root tip (garlic or onion) warmed in hydrochloric acid (breaks down the middle lamella so cells separate), stained (toluidine blue or acetic orcein), squashed to a single layer of cells.

Mitotic index = number of cells in mitosis ÷ total number of cells x 100

Count cells in several fields of view near the tip; decide rules in advance for cells at the edge of the field; the index is highest just behind the tip, where cells divide.

Drawings (CP5, CP7)

Method recap (see Core practicals 5 and 7, Unit 2):

• sharp pencil, single continuous lines, no shading;
• draw what you see, in proportion; drawing should fill at least half the space;
• label lines drawn with a ruler, not crossing, no arrowheads;
• title and magnification or scale bar.

In CP7, plant tissue sections stained (for example with toluidine blue) show xylem vessels, phloem sieve tubes and sclerenchyma fibres. A tissue map (low power plan) shows tissue outlines only, no individual cells.

Worked example

25 eyepiece divisions line up with 10 stage micrometer divisions (each 10 µm). A cell is 18 eyepiece divisions long. Calculate its length.

1. 10 stage divisions = 100 µm
2. 1 epu = 100 ÷ 25 = 4.0 µm
3. Cell = 18 x 4.0 = 72.0

Answer: 72 µm

Check yourself

1. 40 eyepiece divisions line up with 20 stage divisions of 10 µm each. Calculate the size of one eyepiece division in µm.
   Show answer

   5 µm. 20 x 10 = 200 µm; 200 / 40 = 5 µm.

2. One eyepiece unit is 2.5 µm at high power. A nucleus measures 6 eyepiece units. Calculate its diameter.
   Show answer

   15 µm. 6 x 2.5 = 15 µm.

3. In a root tip squash, 24 of 160 cells are in mitosis. Calculate the mitotic index as a percentage.
   Show answer

   15 %. 24 / 160 x 100 = 15%.

4. Further back from the root tip, 9 of 225 cells are in mitosis. Calculate the mitotic index as a percentage.
   Show answer

   4 %. 9 / 225 x 100 = 4%.

5. Why must the eyepiece graticule be recalibrated when the objective lens is changed?
   • The magnification changes, so each eyepiece division covers a different real length
   • The graticule moves
   • The stage micrometer changes size
   • The light intensity changes
   Show answer

   The magnification changes, so each eyepiece division covers a different real length. The eyepiece scale stays the same size while the specimen image is enlarged more.

6. Why is the root tip warmed in hydrochloric acid in CP6?
   • To stain the chromosomes
   • To stop the cells dividing
   • To break down the middle lamella so cells separate and can be squashed into one layer
   • To make the cells larger
   Show answer

   To break down the middle lamella so cells separate and can be squashed into one layer. Separated cells spread into a single layer.

7. Which is a rule for a biological drawing?
   • Use clear single pencil lines with no shading
   • Shade dark areas to show depth
   • Use arrows on label lines
   • Draw in pen so it does not smudge
   Show answer

   Use clear single pencil lines with no shading. Clear lines, no shading, ruled labels.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018): Unit 3, Unit 6 and Appendix 6 mathematical skills

Last updated 8 November 2026. Spotted a mistake? Tell us.