UNIT

1Living organisms

Natural Science 2. Secondary Education

THE SIZE OF CELLS

UNIT

1The size of cells

Natural Science 2. Secondary Education

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).

1cm 2 3 4 5 6 7 8 9 10

1mm 2 3 4 5 6 7 8 9 10

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.

1000 µm

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:

This is how we see a protozoan when it is

magnified 1000 times by a microscope.

It is invisible to the naked eye.

UNIT

1The size of cells

Natural Science 2. Secondary Education

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 100000 µm = 10 cm

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.

1cm 2 3 4 5 6 7 8 9 10

= 640 cells

15,55 µm

• 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.

UNIT

1The size of cells

Natural Science 2. Secondary Education

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

UNIT

1The size of cells

Natural Science 2. Secondary Education

100x magnification

Two different-sized 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.

UNIT

1The size of cells

Natural Science 2. Secondary Education

Magnifying images in order to see cells

10x magnification

40x magnification