B. Life Systems · Cells

Seeing Smaller: How Technology Opened Up the World of Cells

B1.1 A3.2 A3.3 A3.1About 50 minutes

Learning goals

I can compare the sizes of cells and other small things using micrometres.
I can explain the limits of light microscopes and how electron microscopes and other technologies go further.
I can assess how different technologies have improved our understanding of cells.
I can describe contributions to cell biology made by scientists from different places, including Canada.

Words to know

micrometre (µm)
One thousandth of a millimetre. 1 mm = 1000 µm. Most cells are measured in micrometres.

nanometre (nm)
One thousandth of a micrometre. 1 µm = 1000 nm. Viruses and parts of cells are measured in nanometres.

electron microscope
A microscope that uses a beam of electrons instead of light. It shows much finer detail than a light microscope.

stain
A dye that colours certain cell parts so they are easier to see.

fluorescence
Glowing: some substances give off light of one colour when another kind of light shines on them. Scientists use glowing markers to light up parts of cells.

How small is small?

Your thumbnail is about 1 cm wide. Split 1 mm into 1000 equal parts and each part is a micrometre (µm). Most cells are a few micrometres to about 100 micrometres across. Your eyes alone can usually see things down to about 0.1 mm (100 µm), so most cells are just too small to see.

Thing	Approximate size	Can you see it without a microscope?
Width of a human hair	about 50 to 100 µm	Yes, just barely
Human egg cell	about 100 µm (0.1 mm)	Just barely, as a tiny dot
Typical plant cell	about 10 to 100 µm	No
Typical human body cell	about 10 to 30 µm	No
Red blood cell	about 7 to 8 µm	No
Typical bacterium	about 1 to 2 µm	No
Mitochondrion	about 1 µm long	No
Flu virus	about 0.1 µm (100 nm)	No; too small even for a school light microscope

Sizes are approximate; real cells vary. 1 mm = 1000 µm, and 1 µm = 1000 nm.

A size ladder: each rung is 10 times bigger than the one below. Things near the top dashed line (about 0.1 mm) are just barely visible to your eyes; smaller things need a microscope, and only the flu virus is below the light-microscope limit.

The limit of light

A light microscope can make a cell look bigger, but at some point the picture just gets blurrier, not clearer. Light microscopes cannot show details smaller than about 0.2 µm. That is their limit of resolution. The best light microscopes magnify about 1000× to 2000×; school microscopes usually go to 400×.

That is why scientists in the 1800s could see the nucleus but not the tiny structures inside a mitochondrion, and could not see viruses at all.

Technologies that went further

Stains (1800s and on): dyes such as iodine and methylene blue colour certain parts, so the nucleus and other parts stand out.
Electron microscopes (1930s and on): use a beam of electrons instead of light. They can show details far smaller than 0.2 µm, including viruses and the folded inner membranes of mitochondria. The first electron microscope was built in Germany in 1931 by Ernst Ruska and Max Knoll. In 1938, James Hillier and Albert Prebus, at the University of Toronto, built one of the first high-resolution electron microscopes in North America. Trade-off: the specimen must be in a vacuum and specially prepared, so living cells cannot be viewed. Images are black and white; colours you see in photos are added by computer.
Fluorescence microscopes: glowing markers attach to one kind of molecule, so scientists can watch where it goes in a living cell. A glowing protein first found in a jellyfish (green fluorescent protein) is widely used; the scientists who developed it won the 2008 Nobel Prize in Chemistry.
Cell culture: growing cells in dishes with nutrients, so they can be studied or tested with medicines outside the body.
Digital cameras and computers: record videos of cells dividing, measure cells automatically and build 3-D models.

Onion skin cells stained with iodine, seen through a light microscope as rows of long brick-shaped cells, each with a dark dot. — Iodine stain makes the cell walls and the small round nucleus in each onion cell stand out. Without the stain these cells are almost see-through. AI-generated image

The same mitochondrion and flu virus through two kinds of microscope. Higher resolution turns a blurry blob into a structure with a folded inner membrane, and makes the virus visible at all.

Technology	What it lets us learn	Limitation
Light microscope	Shape of cells, nucleus, cell wall, chloroplasts; watch living cells move	Can't show details smaller than about 0.2 µm
Stains	Makes clear parts visible	Many stains kill the cells
Electron microscope	Tiny structures inside organelles; viruses	No living cells; expensive; specimens need special preparation
Fluorescence microscope	Where specific molecules are in living cells	Only shows what is marked
Cell culture	How cells grow, divide and respond to medicines	Cells in a dish may act differently than in a body

Worked example

Let's work out actual size from a photo

An electron microscope photo shows a bacterium 30 mm long. The magnification is 15 000×. How long is the real bacterium?

Use: actual size = image size ÷ magnification.
30 mm ÷ 15 000 = 0.002 mm.
Change to micrometres: 0.002 mm × 1000 = 2 µm.
Check: typical bacteria are 1 to 2 µm long, so this makes sense.

The bacterium is about 2 µm long.

Worked example

Let's assess a technology

Assess this claim: "The electron microscope is better than the light microscope."

Criteria for "better": detail shown, whether living cells can be seen, cost, ease of use.
Detail: the electron microscope wins by far; it shows structures and viruses a light microscope can't.
Living cells: the light microscope wins; electron microscopes need specimens in a vacuum.
Cost and ease: light microscopes are cheaper and easier, which is why schools use them.
Conclusion: neither is simply better. Scientists choose the right tool for the question.

The claim is only partly true: electron microscopes show much more detail, but light microscopes can show living cells and are cheaper and easier to use.

Math meets science

Cell biology uses math constantly: converting units (mm, µm, nm), dividing to find actual size, and working with very large and very small numbers. Scientists often write them in powers of 10: 1 µm = 0.001 mm = 10⁻³ mm. You will use these powers of 10 more in Grade 9 science and math.

Just for fun

What if you were magnified 15 000×, like the bacterium in the electron microscope photo? A 1.6 m tall student would be 24 km tall, almost three times the height of Mount Everest. Good luck finding a hoodie.

Hands-on activity

Make a size ladder

You need: paper, a ruler, a pencil, a calculator.

Draw a tall "ladder" with 7 rungs. Label the rungs from the bottom: 1 nm, 10 nm, 100 nm, 1 µm, 10 µm, 100 µm, 1 mm. Each rung is 10 times bigger than the one below it.
Place each thing from the size table on the closest rung: hair, egg cell, plant cell, body cell, red blood cell, bacterium, mitochondrion, flu virus.
Draw a line across the ladder at about 0.1 mm (100 µm): "limit of the human eye". Draw another at 0.2 µm: "limit of light microscopes".
Colour or mark which things need an electron microscope.
Write two sentences: Which technology would you need to study a flu virus? Why?

What to look for: each step is ×10; the virus is the only thing below the light-microscope line; hair and egg cell are at or just above the eye line.

Your turn

How many micrometres are in 0.5 mm?

Why can't a school light microscope show a flu virus?

Give one advantage and one disadvantage of an electron microscope.

What does a stain do?

Which Canadian university was home to an early high-resolution electron microscope in 1938?

A photo shows a cell 50 mm wide at 1000× magnification. How big is the real cell?

More practice: warm-up

1 mm = ___ µm

1 µm = ___ nm

Which is bigger: a red blood cell or a bacterium?

What kind of microscope uses a beam of electrons?

About what is the smallest thing your eyes can see?

More practice: on your own

Change 25 µm to mm.

About how many red blood cells (8 µm each) would fit in a row across 1 mm?

Why might a researcher choose a fluorescence microscope instead of an electron microscope to study how a medicine moves into cells?

A cell in a dish behaves differently than the same kind of cell in the body. What limitation of cell culture does this show?

A bacterium is 2 µm long. At 400× magnification, how long would its image be?

Challenge

A mitochondrion is 1 µm long. How many times larger would an electron microscope need to magnify it so its image is 5 cm long?

Assess how electron microscopes changed our understanding of mitochondria and viruses. Use the idea of resolution.

A computer program measures cells in thousands of microscope photos automatically. Give one benefit and one risk.

Check your understanding

Quiz

1 mm equals…

Light microscopes can't show details smaller than about…

Which can show a living cell moving?

A photo shows a cell 20 mm wide at 400× magnification. The real cell is…

Which is smallest?

Why are stains used?

For parents and teachers

Big idea: each new technology (better lenses, stains, electron microscopes, fluorescent markers, cell culture, computers) let scientists see smaller or more clearly, and changed what we know about cells. Each also has limits.

How to help at home:

Practise unit conversions together (mm ↔ µm) with a ruler: point out that a millimetre mark holds 1000 micrometres.
Search for images labelled "electron micrograph" with your child and talk about the false colours added by computer.

Common difficulties:

Dividing when they should multiply (and the reverse). Actual size = image size ÷ magnification; image size = actual size × magnification.
Thinking an electron microscope is simply "better". Push for criteria: detail, living cells, cost.