Investigating cells

The size of cells

• - A typical plant cell is about 0.1mm in diameter and an animal cell is 0.2mm in diameter.
• - Prokaryotic cells are smaller - often about 0.002mm long.
• -To describe the size of cells and sub-cellular structures, scientists use units that have different prefixes.
• Centimetre (cm) = 100 = 1 x 10²
• Millimetre (mm) = 1 000 = 1 x 10³
• Micrometer (μm) = 1 000 000 = 1 x 10⁶
• nanometer (nm) = 1 000 000 000 = 1 x 10⁹

Using microscopes to look at cells

• - It is not possible to see cells as separate objects using the naked eye.
• - The ability to see two or more objects as separate objects is called resolution.
• - The light microscope was developed in the late 16th century and gave a greater resolution than the human eye.
• - It allowed scientists to see plant, animal and bacterial cells.
• - Some sub-cellular structures are even snaller than the resolution achieved by a light microscope and cannot be seen using this method.
• - In 1933, scientists first used an electron microscope.
• - An electron microscope passes electrons, rather than light, through the specimen and can give a much better resolution.

Calculating magnification

• - When a microscope is used to look at cells, scientists will often take photographs or produce photos.
• - These images are many times larger than the real cell ot structure.
• - The magnification is how many times larger the image is than the real object.
• Equation:
• Magnification = size of image ÷ size of real object

Example of calculating magnification

• A student looks at a plant cell under a microscope using a magnification of x120.
• The image of cell is 1.5cm in height.
• Calculate the height of the actual cell.
• 1.5 x 10 = 15mm
• 120 = 15 ÷ actual height
• actual height = 15 ÷ 120
• = 0.125mm

Key points

Key points

• The size of cells is often given in micrometers, e.g. a plant cell may be 100μm, an animal cell 20μm and a bacterium 2μm in size.
• Magnification is not the same asresolution. Light microscopes can be built to magnify more and more, but after a certain point the images do not get any clearer.

Key words

• - Resolution
• - Electron microscope
• - Magnification