Estimating sizes of sub cellular structures
Scientists often work with very small structures they can’t measure directly, so estimating and comparing sizes helps them understand how cells function, how diseases work, and how drugs interact at a microscopic level.
You will need to be able to make and explain rough size comparisons between sub-cellular structures — even if they can’t measure them exactly. It’s about estimating scale and size using standard form, comparisons, or orders of magnitude.
Estimate sizes using standard form
You should know approximate sizes of cell structures in metres (m) using standard form:
| Structure | Approximate size in standard form (m) | Notes |
|---|---|---|
| Cell | 1 x 10-5 | Animal cells |
| Nucleus | 1 x 10-6 | Visible under a light microscope |
| Mitochondrion | 2 x 10-6 | Mitochondrion is x100 larger than a ribosome, or 2 orders of magnitude larger |
| Ribosome | 2 x 10-8 | Cannot be seen with light microscope |
| Bacterium | 1 x 10-6 | same order of magnitude as nucleus |
| Plasmid | 1 x 10-8 | Tiny loop of DNA |
Judge relative sizes
For example:
A mitochondrion is about 10× smaller than a typical human cell or 1 order of magnitude smaller. Each order of magnitude is a change of x10
A ribosome is about 100× smaller than a mitochondrion or 2 orders of magnitude smaller.
You don’t need exact measurements — just a good idea of which structures are bigger, smaller, or similar in scale.
Use estimations when interpreting cell images
When you’re shown a diagram or microscope image:
Explain how big parts are relative to each other.
e.g. “The nucleus is about 1/4 the width of the cell.”
Practice Question
1. If a plant cell has a size of 1 x 10-5 and a ribosome has a size of 1 x 10-8. How many orders of magnitude different are they?
The plant cell is 1000 times larger than the ribosome. So, it is 3 orders of magnitude larger.
2. Why do scientists estimate and compare cell structure sizes?
Scientists often work with very small structures they can’t measure directly, so estimating and comparing sizes helps them understand how cells function, how diseases work, and how drugs interact at a microscopic level.
3. Using the table above, how big is a ribosome compared to a plasmid?
Size of a ribosome = 2 x 10-8
Size of a plasmid = 1 x 10-8
So the ribosome is twice the size of the plasmid.