GCSE Biology

Cell Biology
Transport Systems
Bioenergetics
Homeostasis and Response
Ecosystems

Sizes of Cells

In science, objects are typically measured in metres. However, many times in biology the objects are very small, so the metre is not the best measurement. For example:

If we divide a metre (m) into 100 equal parts, each part is one centimetre (cm). The word ‘centi’ means hundredth and in science, this word is used in different measurements (e.g. centimetre, centilitre). So, 1 cm is one-hundredth of a metre.

Scientists often use standard form to represent the measurements of very small objects. 1 cm is 1 × 10⁻² m.

If we divide 1 cm into ten equal parts, each part is one millimetre (mm). So, 1 cm = 10 mm. The word ‘milli’ means thousandth, so 1 mm is equal to one-thousandth of a metre. However, we can also write it in standard form, as 1 × 10⁻³ m.

In biology, 1 mm is still quite big, so we use even smaller measurements. If we divide one metre into a million equal parts, each part is a micrometre, which has the symbol μm. The word ‘micro’ means millionth, so a micrometre is a millionth of a metre. In standard form, we write a micrometre as 1 × 10⁻⁶ m.

There are times when scientists work with even smaller objects, so even smaller measurements are required. If we divide a metre into one billion equal parts, each part is one nanometre (nm). The word ‘nano’ means billionth, so 1 nm is one billionth of a metre. In standard form, we write a nanometre as 1 × 10⁻⁹ m.

• Nanometres are used when measuring a protein structure. For example, the protein in the diagram above is 10 nm.

Order of Magnitude

Differences in size can often be described as differences in the order of magnitude. The difference in size is typically calculated in factors of 10.

• To increase a number by one order of magnitude, you are multiplying the number by 10.
• To decrease a number by one order of magnitude, you are dividing the number by 10.

So, there is one order of magnitude between a cell that measures 1 × 10⁻⁶ m and a cell that measures 1 × 10⁻⁵ m.

Quizzes