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# Units of Measurement in Biology

## Measurements that are based on the International System of Units (SI), which is form of the metric system.

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How do you measure something really really small?

If we are talking about a cell, then not with a ruler. Units must exist that can describe sizes many times smaller than the smallest marking on a ruler.

### Units of Measurement

The measurements that scientists use are based on the International System of Units (SI), which is a form of the metric system. The term SI is shortened from the French term Le Système international d'unités . It is the world's most widely used system of units, both in science and business. It is useful to scientists because it is based on multiples of 10. The SI was developed in 1960 from an older metric system and is used in almost every country.

The SI is not static, as the technology of measurement progresses, units are created and definitions are changed through international agreement among many nations. The international system of units is made up of a seven base units, shown in the SI Base Units Table below . From these seven base units several other units are derived.

SI Base Units
Name Symbol Quantity
meter m length
kilogram kg mass
second s time
ampere A electric current
kelvin K thermal energy (temperature)
mole mol amount of substance
candela cd uminous intensity

A prefix may be added to SI units to make a multiple of the original unit. An SI prefix is a name or symbol that is put before a unit of measure (or its symbol) to form a decimal or a multiple of the unit. For example, kilo - is a multiple of a thousand and milli - is a multiple of a thousandth, so there are one thousand millimeters in a meter, and one thousand meters in a kilometer . All prefixes are multiples of 10, as you can see from the SI Prefixes Table below . The prefixes are never combined; a millionth of a kilogram is a milligram not a microkilogram .

SI Prefixes
Name Symbol Factor of 10
tera- T 1,000,000,000,000 (10 12 ) trillion (thousand billion)
giga- G 1,000,000,000 (10 9 ) billion (thousand million)
mega- M 1,000,000 (10 6 ) million
kilo- k 1000 (10 3 ) thousand
hecto- h 100 (10 2 ) hundred
deca- da 10 (10 1 ) ten
deci- d 1 (10 -1 ) tenth
centi- c 0.1 (10 -2 ) hundredth
milli- m 0.01 (10 -3 ) thousandth
micro- µ 0.00001 (10 -6 ) millionth
nano- n 0.00000001 (10 -9 ) billionth
pico- p 0.00000000001 (10 -12 ) trillionth

### Vocabulary

• International System of Units (SI) : The modern form of the metric system; a system of units of measurement devised around seven base units and the convenience of the number ten.
• SI prefixes : A name or symbol that is put before a unit of measure (or its symbol) to form a decimal or a multiple of the unit.

### Summary

• The measurements that scientists use are based on the International System of Units (SI), which is form of the metric system. Based on multiples of ten, It is the world's most widely used system of units, both in science and business.

### Practice

Use this resource to answer the questions that follow.

1. When was the SI system established?
2. What is the SI system based on?

### Review

1. What is SI?
2. Why is it important that scientists use common units of measurement?
3. Which one of the following units of measurement would be the most appropriate in determining the mass of a banana? Kilograms, micrograms, or grams.
4. What is the standard SI unit for measuring volume, weight, time, and length?
5. What is the shorthand unit for .000056 grams, 5600000000 seconds, .56 liter, and 560 meter.