# Cell size and magnification

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• Created by: racheon
• Created on: 14-04-14 10:03
Define micrometre.
Equal to 1 millionth of a metre, and 1000th of a millimetre, μm. It's the standard unit for measuring cell dimensions.
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What is the international unit for length?
The metre.
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How many millimetres are in a metre?
1000.
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Why can't we use metres and millimetres to measure cells and organelles?
They are so small that these units would be too big.
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How long are most animal cells?
20-40 millionths of a metre long.
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How big is one nanometre?
1000th of a micrometre, 1 thousand millionths of a metre.
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How wide are cell surface membranes, ribosomes and viruses?
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What is the resolution for the human eye, light microscope and electron microscope?
100μm, 200nm and 0.2nm.
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What do we use to measure cells etc down a microscope?
A graticule, which is transparent with a ruler etched on it. The scale is superimposed on the specimen and the dimensions can be measured in eyepiece units.
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Why is the scale of a graticule arbitrary?
It represent different lengths at different magnifications, and has to be calibrated for each lens.
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What happens to the specimen image and actual size as you view it on higher magnifications?
The image looks bigger, but the actual size has not increased.
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How do you put a graticule in a microscope?
By placing it on a slide called a stage micrometer and placing that on the stage.
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How long is a usual graticule, and what is it divided into?
It's 1mm long and is divided into 100 divisions of 0.01mm or 10μm.
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With a x4 objective lens and a x10 eyepiece, what will 40 and 1epu equal?
40epu=1mm, 1epu=1000/40=25μm.
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With a x10 objective lens and a x10 eyepiece, what will 100 and 1epu equal?
100epu=1000μm, 1epu=10μm.
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With a x40 objective lens and a x10 eyepiece, what will 1epu equal?
2.5μm.
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With a x100 objective lens and a x10 eyepiece, what will 1epu equal?
1μm.
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The length of a nucleus is 3.2epu. At x100 magnification, how long is the actual nucleus? If we zoomed in to x400 magnification, the length of the nucleus is now 13epu. What is it's actual length?
At x100 magnification 1epu=10μm, so the actual length is 3.2x10=32μm. At x400 magnification 1epu=2.5μm, so the actual length is 13x2.5=32.5μm.
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What is the relationship between actual size, magnification and image size, and what does this mean you can work out?
Actual size=image size÷magnification, so you can work out the magnification of a micrograph or drawing, or the actual size of a cell or part of it.
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Define nanometre.
One thousandth of a micrometre. It's a useful unit for measuring the sizes of organelles and large molecules.
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## Other cards in this set

### Card 2

#### Front

What is the international unit for length?

The metre.

### Card 3

#### Front

How many millimetres are in a metre?

### Card 4

#### Front

Why can't we use metres and millimetres to measure cells and organelles?

### Card 5

#### Front

How long are most animal cells?