Lesson 4 of 6 / Cell structure and organisation
Drawing cells and calculating magnification
How can a drawing show both the observed features and their size?
I can make a clear biological drawing and calculate magnification using consistent units.
I can identify major cell structures and their functions using diagrams and light microscopy.Syllabus
Syllabus K325, Topic 1(a). Identify major cell structures and their functions using diagrams and light microscopy.
About 5 min
Make a guess. You are not marked.
A photo of a cell is labelled x400. You zoom in on your phone until the cell looks twice as big. What is the magnification of the image on your screen now?
Show the answer
About x800, because the image doubled in size.
Magnification is image size divided by actual size. The cell did not change, so doubling the image doubles it.
The key idea
Draw the features you observe, preserve their proportions, and compare image size with actual size in the same units.
This is practical preparation using an original specimen model. In the laboratory, your specimen is the evidence: do not add a structure merely because a textbook usually shows it.
Explore the idea
Make the evidence clear
Build a useful biological drawing
Use clear single lines and preserve the proportions of the observed cell. Leave out decorative shading.
Does enlarging an image change the cell?
The cell remains four scale-bar lengths: 4 x 20 = 80 micrometres. Its displayed size is smaller; its actual size is unchanged.
Original teaching model with illustrative dimensions. Practise drawing a real specimen in your laboratory; do not measure the screen with a ruler.
Explanation
Choose a clear view and look carefully before drawing. Make the drawing large enough to show useful detail. Use single, continuous lines rather than repeated sketch strokes, and preserve the relative sizes and positions of the structures you can distinguish. A nucleus need not sit in the centre.
A biological line drawing records visible features. Avoid decorative shading, colour effects and invented organelles. For a stained onion epidermis, cell walls and a nucleus may be distinguishable, while the membrane may not be resolved separately from the wall. Do not draw two clearly separated boundaries and claim you saw them if you did not.
Give a descriptive title and place labels where they are easy to read. Use ruled label lines that end precisely on the structure; avoid crossing them. Follow any instructions about the specimen, view and required labels. A clear drawing is not the same as a realistic shaded illustration.
Magnification compares the size of an image or drawing with the actual size of the specimen: magnification = image size / actual size. Rearranging gives actual size = image size / magnification. Convert both lengths to the same unit before dividing. One millimetre is 1000 micrometres.
For example, a drawing 40 mm wide represents a cell 80 micrometres wide. The actual width is 0.080 mm, so the drawing magnification is 40 / 0.080 = 500 times. This is a ratio, so write x500, without mm or micrometres after it.
A scale bar states the actual length represented by the bar. Compare a structure with that bar in the same image; both enlarge together when the image is resized. If a cell is four times as long as a bar labelled 20 micrometres, its actual length is 80 micrometres. Do not assume that a digital image still has its original displayed magnification after resizing.
Step by step
- 1
Observe before drawing
Select a clear region and distinguish visible evidence from an inferred structure.
- 2
Record the features clearly
Use a large line drawing, accurate proportions, a title and precise label lines.
- 3
Use a size reference
Read the scale bar or given dimensions, convert to matching units, then calculate.
Worked example
Read a scale bar after resizing
A printed cell image is 36 mm long. Its 20 micrometre scale bar is 9 mm long on the same print. Calculate the actual cell length.
One way to explain it
The cell is 36 / 9 = 4 times the length of the scale bar. Its actual length is therefore 4 x 20 = 80 micrometres. The ratio remains valid if the image and scale bar are enlarged together.
Why this answer works
- Compare the cell and bar on the same version of the image.
- Use the actual length represented by the bar.
- Do not treat 36 mm on the print as the actual size of the cell.
Watch out for this
A student says: "A cell image labelled x400 always remains x400 on any screen or printout." What is wrong with this?
Show the answer
Resizing changes image magnification. Use a scale bar that resizes with the image, or the dimensions and magnification explicitly given for the version in the question.
Check your understanding
A 30 mm drawing represents a cell 60 micrometres long. What is its magnification?
Show the answer
x500
60 micrometres = 0.060 mm, and 30 / 0.060 = 500. The magnification is a ratio with no unit.
Quick recall
How can a drawing show both the observed features and their size?