Comparing sizes
The diagram shows the size of a selection of organisms, different cells and other structures.
We can compare sizes using a straightforward calculation.
For instance, the length of the leaf cell above is ten times the diameter of a red blood cell.
The calculation would be:
\( \frac{70~μm}{7~μm} = 10 \)
When comparing the size of a bacterium with the Human Immunodeficiency Virus (HIV) different units have to be converted to be consistent.
The length of the bacterium = 70 μm = 70 000 nm
The diameter of the HIV = 100 μm = 100 000 nm
Therefore the length of the HIV is \( \frac{100~000}{70~000} = 1.4 \) times that of the bacterium. This means they differ by one order of magnitude.
Question
What is the width of a cheek cell compared with a Salmonella bacterium?
140 times the size.
Calculation:
\( \frac {70~μm}{0.5~μm}\)
Order of magnitude
Differences in size are often described as differences in A number, to the base ten, often used to make comparisons. For each order of magnitude, a number is ten times the previous one.. That's the difference calculated in factors of 10.
