Understanding scale and size of cells
Joe Wolfensohn
Teacher
Contents
Recall Questions
This topic requires prior knowledge of standard form and metric units from mathematics to interpret cell sizes and scale effectively.
What is the basic unit of life?
The cell
Which type of cell has no nucleus?
Prokaryotic
What 2 sub-cellular structures do all cells have?
Cell membrane and cytoplasm
Topic Explainer Video
Check out this @JoeDoesBiology video that explains understanding scale and size of cells, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Understanding Scale and Size of Cells
Why Scale Matters
- Cells and organelles are microscopic, so we use scientific notation and units to describe them accurately.
- Understanding size and scale is essential for comparing structures like cells, bacteria, and viruses.
Units of Measurement
|
Unit |
Symbol |
Size Compared to 1 Metre |
|
Millimetre |
mm |
1 × 10⁻³ m (1/1,000 m) |
|
Micrometre |
µm |
1 × 10⁻⁶ m (1/1,000,000 m) |
|
Nanometre |
nm |
1 × 10⁻⁹ m (1/1,000,000,000 m) |
Unit Conversions:
-
1 m = 1000 mm
-
1 mm = 1000 µm
-
1 µm = 1000 nm
Make sure to check out the study note ‘Unit conversions and orders of magnitude’ in the exam skills section for more help with this.
No answer provided.
Approximate Sizes of Biological Structures
|
Structure |
Typical Size |
|
Human cell |
~10–20 µm |
|
Bacterial cell |
~1–5 µm |
|
Virus |
~20–300 nm |
|
Cell membrane thickness |
~7–10 nm |
|
Nucleus |
~5–10 µm |
Using Standard Form
Used to simplify very large or very small numbers:
- 0.000001 m = 1 × 10⁻⁶ m = 1 µm
- 1,000,000 µm = 1 × 10⁶ µm
Exam Tips
- Always convert to matching units before calculating magnification or comparing sizes.
- Read carefully: questions may give sizes in different units.
No answer provided.
Practice Question
A bacterial cell is 2 µm long. A virus is 100 nm long. How many times bigger is the bacterial cell than the virus? (3 marks)
- Convert 100 nm to µm: 100 nm = 0.1 µm
- Divide: 2 µm ÷ 0.1 µm = 20
- The bacterial cell is 20 times bigger than the virus.
More Practice
Try to answer the practice question from the TikTok video on your own, then watch the video to see how well you did!