(5) Microscopy - The scale of things in biology and orders of magnitude

Comparison of cells with other objects with approximate dimensions

The practical limit of magnification for an optical light microscope is x1000 to resolve features e.g. structural parts of cells.

In terms of dimensions we are talking about ~0.5 µm, the size of a larger virus.

Electron microscopes can resolve images down to ~0.2 nm, the size of a small atom.

This is a resolving power 25 000 times greater than that of an optical light microscope.

When comparing the size of small objects like cells, scientists refer to differences in sizes as an order of magnitude.

This means that an object is described as being greater of smaller by a factor of 10.

For example, the leaf cell size is one order (10 x or 10¹) greater in size than a red blood cell.

The common ladybird is two orders greater (100 x or 10 x 10, or 10²) than the size of a plant leaf cell.

The thickness of the cell membrane is three orders smaller {1/1000 or 1/(10 x 10 x10) or 10^-3} than the diameter of a red blood cell.

 

Order of magnitude question

What is the difference in order of magnitude when comparing the diameter of human hair with a HIV virus?

in terms of µm, the ratio is ~100/0.1 = ~1000 or ~10³

So the diameter of human hair is a magnitude of ~3 greater than the size of the HIV virus.

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Key points and more ideas

Source of information is based on textbooks and syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10)

The Scale of Things in Biology - orders of magnitude

1. Why Scale Matters in Biology

Biology deals with structures of vastly different sizes -from entire organisms to organelles inside cells. Understanding the scale helps you to appreciate orders of magnitude:

• Visualise microscopic and macroscopic structures.

• Compare sizes using standard form and orders of magnitude.

• Accurately interpret diagrams and microscope images.

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2. What Are Orders of Magnitude?

An order of magnitude refers to a tenfold (×10) increase or decrease in size.

• Each step up or down means the object is 10 times bigger or smaller.

• It’s a quick way to compare very large and very small numbers.

Examples  orders of magnitude:

• A mitochondrion (≈1 µm) is 3 orders of magnitude smaller than a frog egg (≈1 mm)

• because: 1 mm = 1000 µm and 1000 / 1 = 1000 which is 10³

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3. Approximate Sizes – Biological Structures & Everyday Objects and orders of magnitude

Note:

• 1 mm = 1,000 µm

• 1 µm = 1,000 nm

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4. Visualising Size Differences

Think of it like zooming in with a super microscope:

• Eye sees → person → hair → cell (may be!)

• Microscope sees → cell → nucleus → mitochondrion → virus → DNA

Each step gets 10× smaller than the one before!

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5. How to Compare Sizes in Exams

When comparing objects and calculating orders of magnitude:

• Convert all measurements to the same unit (usually µm or nm).

• Work out how many times larger or smaller one object is.

• Express answers using standard form or orders of magnitude.

Example Question:

A red blood cell is about 10 µm in diameter.

A virus is about 100 nm.

How many times smaller is the virus than the red blood cell?

→ Convert 100 nm = 0.1 µm
→ 10 / 0.1 = 100
→ So, the virus is 100 times smaller (2 orders of magnitude smaller, 10-²)

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6. Why It's Important in Human Biology

• Helps understand why viruses and organelles can't be seen with light microscopes.

• Supports medical applications like measuring pathogens and designing drugs.

• Sharpens your ability to interpret microscope images and scale bars.

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Keywords, phrases and learning objectives for this part on relative size and orders of magnitude

Appreciate from investigative techniques, including microscopy, you can measure the scale of objects in biology.

This can be expressed as orders of magnitude in powers of 10.

In terms of  orders of magnitude, appreciate the comparison of size of cells with other objects and sub-structures including insects, human hair, leaf cell, blood cell, virus, cell membrane, DNA molecules and carbon atom in terms of their approximate size and dimensions.

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