Measurements in Biology
Measurements provide quantitative information that is critical in studying and
practicing science. Each measurement has an amount and a unit (standard of
comparison for measurements). Scientists primarily use the SI units or metric
systems. The metric system is a decimal based system of measurement. The current
international standard for the metric system is the International System of Units. SI
units are standards fixed by international agreement in the International System of
Units (Le Système International d’Unités). All units in this system can be expressed
in terms of seven base units: the meter (length), kilogram (mass), second (time),
ampere (electric current), kelvin (temperature), mole (amount of substance) and
candela. The candela (cd) is the unit of luminous intensity. There are other units,
some which can be derived from these base units. For example
Celsius (°C): unit of temperature; water freezes at 0 °C and boils at 100 °C
kelvin (K): SI unit of temperature; 273.15 K = 0º C
cubic meter (m3): SI unit of volume
liter (L): unit of volume; 1 L = 1,000 cm3
cubic centimeter (cm3): volume of a cube with an edge length of exactly 1 cm
kilogram (kg): standard SI unit of mass; 1 kg = approximately 2.2 pounds
milliliter (mL): 1/1,000 of a liter; equal to 1 cm3
SI units such as meters, seconds, and kilograms, as well as derived units, such as
liters (for volume) and g/cm3 (for density), are standards of comparison for
measurements. When we buy a 2-liter bottle of a soft drink, we expect that the
volume of the drink was measured, so it is two times larger than the volume that
everyone agrees to be 1 liter. Without units, a number can be meaningless, confusing,
or possibly life threatening.
Suppose a doctor prescribes phenobarbital to control a patient’s seizures and states
a dosage of “100” without specifying units. Not only will this be confusing to the
medical professional giving the dose, but the consequences can be dire: 100 mg
, given three times per day can be effective as an anticonvulsant, but a single dose of
100 g is more than 10 times the lethal amount.
In many cases, fractions or multiples of units in the SI system are used. Fractional
or multiple SI units are named using a prefix and the name of the base unit. For
example, a length of 1000 meters is also called a kilometer because the
prefix kilo means “one thousand,” which in scientific notation is 103 (1 kilometer =
1000 m = 103 m). Microseconds would be 10−6 seconds and megahertz 106 hertz.
The prefixes used and the powers to which 10 are raised are listed in the Table below.
Common Unit Prefixes
Prefix Symbol Factor Example
femto F 10−15 1 femtosecond (fs) = 1 × 10−15 m (0.000000000001 m
Pico P 10−12 1 picometer (pm) = 1 × 10−12 m (0.000000000001 m)
Nano N 10−9 4 nanograms (ng) = 4 × 10−9 g (0.000000004 g)
micro µ 10−6 1 microliter (μL) = 1 × 10−6 L (0.000001 L)
Milli M 10−3 2 millimoles (mmol) = 2 × 10−3 mol (0.002 mol)
centi C 10−2 7 centimeters (cm) = 7 × 10−2 m (0.07 m)
Deci D 10−1 1 deciliter (dL) = 1 × 10−1 L (0.1 L )
Kilo K 103 1 kilometer (km) = 1 × 103 m (1000 m)
mega M 106 3 megahertz (MHz) = 3 × 106 Hz (3,000,000 Hz)
Giga G 109 8 gigayears (Gyr) = 8 × 109 yr (8,000,000,000 Gyr)
Tera T 1012 5 terawatts (TW) = 5 × 1012 W (5,000,000,000,000 W
Measurements provide quantitative information that is critical in studying and
practicing science. Each measurement has an amount and a unit (standard of
comparison for measurements). Scientists primarily use the SI units or metric
systems. The metric system is a decimal based system of measurement. The current
international standard for the metric system is the International System of Units. SI
units are standards fixed by international agreement in the International System of
Units (Le Système International d’Unités). All units in this system can be expressed
in terms of seven base units: the meter (length), kilogram (mass), second (time),
ampere (electric current), kelvin (temperature), mole (amount of substance) and
candela. The candela (cd) is the unit of luminous intensity. There are other units,
some which can be derived from these base units. For example
Celsius (°C): unit of temperature; water freezes at 0 °C and boils at 100 °C
kelvin (K): SI unit of temperature; 273.15 K = 0º C
cubic meter (m3): SI unit of volume
liter (L): unit of volume; 1 L = 1,000 cm3
cubic centimeter (cm3): volume of a cube with an edge length of exactly 1 cm
kilogram (kg): standard SI unit of mass; 1 kg = approximately 2.2 pounds
milliliter (mL): 1/1,000 of a liter; equal to 1 cm3
SI units such as meters, seconds, and kilograms, as well as derived units, such as
liters (for volume) and g/cm3 (for density), are standards of comparison for
measurements. When we buy a 2-liter bottle of a soft drink, we expect that the
volume of the drink was measured, so it is two times larger than the volume that
everyone agrees to be 1 liter. Without units, a number can be meaningless, confusing,
or possibly life threatening.
Suppose a doctor prescribes phenobarbital to control a patient’s seizures and states
a dosage of “100” without specifying units. Not only will this be confusing to the
medical professional giving the dose, but the consequences can be dire: 100 mg
, given three times per day can be effective as an anticonvulsant, but a single dose of
100 g is more than 10 times the lethal amount.
In many cases, fractions or multiples of units in the SI system are used. Fractional
or multiple SI units are named using a prefix and the name of the base unit. For
example, a length of 1000 meters is also called a kilometer because the
prefix kilo means “one thousand,” which in scientific notation is 103 (1 kilometer =
1000 m = 103 m). Microseconds would be 10−6 seconds and megahertz 106 hertz.
The prefixes used and the powers to which 10 are raised are listed in the Table below.
Common Unit Prefixes
Prefix Symbol Factor Example
femto F 10−15 1 femtosecond (fs) = 1 × 10−15 m (0.000000000001 m
Pico P 10−12 1 picometer (pm) = 1 × 10−12 m (0.000000000001 m)
Nano N 10−9 4 nanograms (ng) = 4 × 10−9 g (0.000000004 g)
micro µ 10−6 1 microliter (μL) = 1 × 10−6 L (0.000001 L)
Milli M 10−3 2 millimoles (mmol) = 2 × 10−3 mol (0.002 mol)
centi C 10−2 7 centimeters (cm) = 7 × 10−2 m (0.07 m)
Deci D 10−1 1 deciliter (dL) = 1 × 10−1 L (0.1 L )
Kilo K 103 1 kilometer (km) = 1 × 103 m (1000 m)
mega M 106 3 megahertz (MHz) = 3 × 106 Hz (3,000,000 Hz)
Giga G 109 8 gigayears (Gyr) = 8 × 109 yr (8,000,000,000 Gyr)
Tera T 1012 5 terawatts (TW) = 5 × 1012 W (5,000,000,000,000 W
