Cell size
1.1
The smallest thing we can see is about 0.04 mm, so you can see some of the largest cells with the naked eye. For all cells, however, we need a microscope to see them in any detail. Most animal and plant cells are 0.01–0.10 mm in size. The unit we use to measure most cells is the micrometre, symbol μm. For some sub-cellular structures, or organisms such as viruses, it is best to use a smaller unit: the nanometre, symbol nm. 1 m or 10 −3 m 1 millimetre (mm) = 1000 1 mm or 10 −3 mm or 10 −6 m 1 micrometre (μm) = 1000 1 μm or 10 −3 μm or 10 −9 m 1 nanometre (nm) = 1000 4
What size is the smallest thing our eye can see, in m?
5
What is the range in size of most animal and plant cells, in μm?
Order of magnitude Figure 1.3 shows the size of plant and animal cells compared with some other structures.
ant length 3 mm
hair diameter 100 µm
leaf cell red blood cell length 70 µm diameter 7 µm
bacterium length 1 µm
virus 100 nm
DNA diameter 2.5 nm
carbon atom 0.34 nm
Figure 1.3 Size and scale
When comparing the sizes of cells, scientists often refer to differences in order of magnitude. That’s the difference calculated in factors of 10. So, the difference in order of magnitude for the HIV and the plant cell: The plant cell in Figure 1.1b is 100 μm = 10 -4 m. The human immunodeficiency virus (HIV) is 100 nm = 10 -7 m. The difference in order of magnitude is 103, expressed as 3. 6 7
A cell membrane measures 7 nm across. Convert this to micrometres. A white blood cell measures 1.2 × 10 −5 m. An egg cell measures 1.2 × 10 −4 m. Calculate the difference in order of magnitude.
REMEMBER! You’ll notice that this system of units uses, and gives names to, multiples and sub-multiples of units at intervals of thousands (103) or thousandths (10 −3). A common exception is the centimetre, 1 or 100
10 −2 of a metre. But it is often convenient to use centimetres, particularly in everyday life.
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