1. Magnification and Size
Measuring biological specimens using millimetres as units

2. To accurately measure the size of cellular structures we need a suitable scale:

3. Ideally, we need a scale we can see directly alongside the cells we are observing:

4. Start by putting a ruler under the microscope:

5. Appearance of ruler at medium magnification

6. Appearance of tissue at medium magnification

7. Estimating cell size at medium magnification12
1 mm
1mm / 12 cells = mm 5
1mm / 5cells=
0.2 mm
1 mm

8. Diameter of field of view/mm
Other units
Diameter of field of view/mm
No. of cells lengthways
No. of cells widthways
Mean length (mm)
Mean width (mm)
1.00 5 12
0.2
0.083
1mm = 1000µm
Mean length of cells = 0.2 x 1000 = 200µm
Mean width of cells = x 1000 = 83µm

10. Calculating actual size:

11. Calculating magnification & actual size:

12. Calculating actual size:

13. Calculating magnification:

14. Calculating magnification & actual size:

15. Calculating actual size:

16. Cell Magnification ____ 0.005 mm
This diagram shows the general structure of an animal cell from an electron microscope.
Calculate the magnification factor.
Calculate the length of G - a mitochondrion.
Calculate the diameter of the nucleus.
Calculate the width of the cell at its widest part.
____ mm

17. Plant cell magnification
Diagram shows the generalised structure of a plant cell as seen with an electron microscope.
Calculate the magnification factor.
Calculate the width of the cell.
Calculate the length of the vacuole.
____ mm

18. Exam tip
Magnification of an Illustration = Size of Illustration / Actual Size
For example, a magnification of 1x means the illustration is the same size as the object, 100x means the illustration is 100 times bigger than the object, etc. It is important that you first calculate or measure the actual size of an object, and from that calculate the magnification of the illustration .
Always draw the magic triangle and fill in the information you have.
Always check your units.