1. Living organisms THE SIZE OF CELLS
UNIT 1
Living organisms
THE SIZE OF CELLS
Natural Science 2. Secondary Education 1

2. The size of cells How can we measure a cell?
UNIT 1
The size of cells
How can we measure a cell?
Cells cannot be seen by the naked eye, so it is difficult to get an idea of their size or measure them. Because they are so small, cells must be measured in tiny units called micrometres (µm).
To get an idea of how big a micrometre is, think of one of the centimetres marked on a ruler: each centimetre contains 10 mm.
Now think of one of those millimetres. A single millimetre contains 1000 µ.
1cm
1mm
1000 µm
Natural Science 2. Secondary Education 2

3. The size of cells So, how big are cells?
UNIT 1
The size of cells
So, how big are cells?
In order to see and measure cells, we need to use microscopes.
Using these instruments, the measurements below have been obtained:
Actual lengths of some cells
Lengths when magnified 1000 times
Bacteria = 1 µm = 0,001 mm
1000 µm = 1 mm
Human red blood cell = 10 µm
10000 µm = 1 cm
Protozoan = 100 µm
µm = 10 cm
This is how we see a protozoan when it is magnified 1000 times by a microscope. It is invisible to the naked eye.
Natural Science 2. Secondary Education 3

4. The size of cells Comparing sizes
UNIT 1
The size of cells
Comparing sizes
Imagine we measure a eukaryotic cell and see that it has a diameter of about 15.5 µm. This is an average-sized cell.
In a single millimetre on a ruler, we could line up 64 of these cells in a row.
Therefore, 640 cells would fit in 1 centimetre.
In other words, 128 cells could fit on the head of a pin with a 2 mm diameter.
15,55 µm
1cm
= 640 cells
Natural Science 2. Secondary Education 4

5. The size of cells Magnifying images in order to see cells
UNIT 1
The size of cells
Magnifying images in order to see cells
Microscope lenses magnify what we see through them many times. Normally a microscope has several lenses of increasing magnification so that we can see cells or tissue specimens in great detail.
10x magnification
40x magnification
Natural Science 2. Secondary Education 5

6. The size of cells Magnifying images in order to see cells
UNIT 1
The size of cells
Magnifying images in order to see cells
Microscope lenses magnify what we see through them many times. Normally a microscope has several lenses of increasing magnification so that we can see cells or tissue specimens in great detail.
10x magnification
40x magnification
Two different-sized cells
100x magnification
Natural Science 2. Secondary Education 6