Biological drawings and measuring specimens

Experimental skills · Practical skills · note 4 of 7

Spec S4Download all Practical skills notes (PDF)Download all notes (PDF)13 pages

Biological drawings and measuring specimensSpec S4

You may be asked to draw a specimen in front of you (a flower, seed or leaf, often seen through a hand lens) or a photograph of one, then add labels and calculate the magnification or the actual size.

Drawing rules

Use a sharp pencil to give fine lines that are clear and unbroken.
Make the drawing large, using most of the space provided.
Show all the features you can see in the specimen, in the correct proportions, and nothing you cannot see.
Use no shading and no colour.
Draw label lines with a ruler. Each line must touch the feature it labels. Do not use arrowheads, and do not let label lines cross.
Write labels outside the drawing, and add a title.

Common faults in biological drawings
Feature	Badly drawn	Well drawn
Outline	sketchy, broken or doubled lines	one clear, continuous line
Size	small, in a corner of the box	fills most of the space
Detail	shading, colour or features copied from a textbook	only the features seen, no shading
Labels	freehand lines with arrowheads that stop short	ruled lines that touch the feature

A poor drawing of a leaf (small, sketchy broken outline, shaded veins, freehand label lines with arrowheads that cross or stop short) beside a good drawing (large, clear outline, no shading, ruled label lines touching the vein, leaf blade, midrib and petiole, the title Drawing of a leaf and a line X to Y along the length of the leaf). — A good biological drawing is large, clear and unshaded, with ruled label lines that touch the features.

Measuring and magnification

Measure with a ruler in millimetres. To compare a drawing with a specimen or photograph, measure the same feature on both, for example the length along a line drawn between two marked points.

magnification = image size ÷ actual size

Here the image is your drawing or the photograph. Rearranged: actual size = image size ÷ magnification. Both sizes must be in the same unit. Magnification has no unit and is written with a × sign, for example ×3.

Magnification of a drawing

The actual length of a leaf, measured with a ruler, is 45 mm. In a student's drawing the leaf is 135 mm long. Calculate the magnification of the drawing.

magnification = image size ÷ actual size
= 135 mm ÷ 45 mm
= 3

Answer: ×3

Actual size from a photograph

A photograph of a seed has a magnification of ×4. The seed is 52 mm long on the photograph. Calculate the actual length of the seed.

actual size = image size ÷ magnification
= 52 mm ÷ 4
= 13 mm

Answer: 13 mm

Supplement

Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

To convert between millimetres and micrometres: 1 mm = 1000 µm. Multiply mm by 1000 to get µm, and divide µm by 1000 to get mm. For example, a cell that is 30 mm long in an image magnified ×600 is 30 ÷ 600 = 0.05 mm = 50 µm long.

Practical skill:

Show your measurement, the formula and the working. If the answer has many decimal places, round it sensibly, for example ×2.5 rather than ×2.4615.