Sizes of Cells (HSC SSCE Biology): Revision Notes

Sizes of Cells

Introduction to the microscopic world

Modern biology has expanded our ability to observe increasingly smaller structures in living organisms. We've moved beyond measuring objects in centimetres and millimetres. Today, scientists routinely work with measurements in micrometres (also called microns) and even nanometres. We now live in what biologists call the "nano age" - an era where we can study life at the molecular and atomic level.

Understanding metric units for cell measurement

In biology, we use the metric system to measure objects of vastly different sizes. The table below shows how metres relate to smaller units of measurement:

Here are the key relationships you need to know:

Unit	Symbol	Relationship to metre	Reciprocal
Metre	m	$1$ m	-
Centimetre	cm	$1$ m $= 10^2$ cm	$1$ cm $= \frac{1}{100}$ m
Millimetre	mm	$1$ m $= 10^3$ mm	$1$ mm $= \frac{1}{1000}$ m
Micrometre (micron)	µm	$1$ m $= 10^6$ µm	$1$ µm $= \frac{1}{1000000}$ m
Nanometre	nm	$1$ m $= 10^9$ nm	$1$ nm $= \frac{1}{1000000000}$ m

Converting between units

When converting between units, remember these useful relationships:

• $1$ cm $= 10$ mm
• $1$ mm $= 1000$ µm
• $1$ µm $= 1000$ nm

Each step down the scale represents a 1000-fold decrease in size (except cm to mm, which is only 10 times smaller).

What can we see at different scales?

Different tools allow us to observe structures at different size ranges. The diagram below illustrates what can be seen by the naked eye, light microscopes, and electron microscopes:

Naked eye viewing range

The human eye can see objects down to approximately $0.1$ mm (or $100$ µm). This includes:

• Large organisms and body parts (metres to centimetres)
• The thickness of a human hair (approximately $100$ µm)

Light microscope range

Light microscopes use visible light to magnify specimens. They can resolve objects down to approximately $200$ nm (or $0.2$ µm). This allows us to see:

• Most eukaryotic cells ($10-100$ µm)
• Red blood cells ($7-8$ µm diameter)
• Bacteria ($1-5$ µm)
• Large cellular organelles

Electron microscope range

Electron microscopes use beams of electrons instead of light, achieving much higher resolution. They can visualize structures as small as $0.1$ nm. This includes:

• Viruses ($30-300$ nm)
• Large molecules like DNA
• Individual proteins
• Even atoms ($0.1$ nm)

Comparing sizes of biological structures

Biological structures span an enormous range of sizes - from whole organisms that we can see easily, down to individual atoms that are invisible even under the most powerful microscopes.

The scale below shows the relative sizes of different objects, from macroscopic to nanoscopic:

Macroscopic level (visible to naked eye):

• Height of a 5-year-old child: approximately $1$ m
• Width of a hand: approximately $1$ dm ($10^{-1}$ m)
• Width of a finger: approximately $1$ cm ($10^{-2}$ m)
• Thickness of human hair: approximately $100$ µm ($10^{-4}$ m)

Microscopic level (requires light microscope):

• Red blood cell: $7-8$ µm diameter ($10^{-5}$ m)
• Bacterium: $1-5$ µm ($10^{-6}$ m)

Nanoscopic level (requires electron microscope):

• Virus particle: $30-300$ nm ($10^{-7}$ to $10^{-8}$ m)
• DNA molecule: approximately $2$ nm diameter ($10^{-9}$ m)
• Glucose molecule: approximately $1$ nm ($10^{-9}$ m)
• Atom: approximately $0.1$ nm ($10^{-10}$ m)

Drawing scaled diagrams of cells

When studying cells under a microscope, it's important to create accurate drawings that represent their true proportions. Scaled diagrams help you maintain this accuracy.

What is a scale bar?

A scale bar is a reference line included in a diagram that shows the relationship between the size of your drawing and the actual size of the specimen. Without a scale bar, there's no way to know how large the real object is.

Creating a scale bar: step-by-step process

To draw a cell to scale with an appropriate scale bar, follow these steps:

Step 1: Determine the actual size of the object you want to draw (for example, from measurements taken using a microscope).

Step 2: Decide on a suitable size for your drawing (typically $4-6$ cm for a cell diagram).

Step 3: Calculate the scale using this formula:

Step 4: The result should be a simple whole number. This number tells you what distance your scale bar represents.

Step 5: Draw your diagram to the chosen size, then add a scale bar at the bottom showing the calculated measurement.

Size ranges of different cells and organisms

Different types of cells and organisms vary enormously in size. Understanding these size differences helps you identify what type of structure you're looking at and which microscopy technique would be needed to observe it.

Eukaryotic cells

Plant cells are typically the largest cells:

• Size range: $10-100$ µm
• Visible under light microscope
• Contain chloroplasts, cell wall, and large vacuoles

Animal cells are generally smaller than plant cells:

• Human red blood cells: $7-8$ µm diameter
• Human cheek cells: approximately $60$ µm diameter
• Most animal cells: $10-30$ µm
• Visible under light microscope

Prokaryotic cells

Bacteria are much smaller than eukaryotic cells:

• Escherichia coli (E. coli): $1-5$ µm long
• Most bacteria: $1-10$ µm
• Visible under light microscope, but only just
• No membrane-bound nucleus or organelles

Protists and algae

Protozoans vary in size:

• Trypanosoma (causes sleeping sickness): approximately $25$ µm long
• Can be similar in size to eukaryotic cells

Green algae:

• Chlamydomonas: $5-6$ µm
• Visible under light microscope

Viruses

Viruses are much smaller than cells and require electron microscopy:

• Tobacco mosaic virus: $300$ nm long
• T4 bacteriophage: $225$ nm long
• HIV (AIDS virus): $100$ nm diameter
• Polio virus: $30$ nm diameter
• All measured in nanometres, not micrometres
• Cannot be seen with a light microscope

Comparing prokaryotes and eukaryotes