397

Other units employed in biological systems include the following:

Unit Abbreviation Comment

Mole mol

Becquerela Bq Units of radioactivity

Curie Ci

Calorie cal

Kilocalorie kcal or Cal

Svedburg S (1 S = 10−13 sec)

a1 becquerel = 1 dps or 60 dpm, while 1 Ci = 3.7 × 1010 Bq.

The following prefixes and symbols are used to indi-cate multiples or decimal fractions of the SI units.

Prefix Factor Symbol

exa 1018 E

peta 1015 P

tera 1012 T

giga 109 G

mega 106 M

kilo 103 k

hector 102 h

deca 101 da

deci 10−1 d

centi 10−2 c

milli 10−3 m

micro 10−6 μ

nano 10−9 n

pico 10−12 p

femto 10−15 f

atto 10−18 a

The Systemé International d’Unités (SI units), which is now generally accepted as the universal scientific sys-tem of measure, is based on the meter and kilogram (m, kg) rather than the centimeter and gram of the older cgs system. It has seven basic units of measurement:

Quantity SI unit Symbol

Amount of substance mole mol

Mass kilogram kg

Time second s

Length meter m

Electric current ampere A

Thermodynamic temperature kelvin K

Light intensity candela cd

Several important units employed in biological systems which are derived from SI units include the following:

Quantity to be measured

Definition in terms of basic units SI unit Symbol

Volume 10−3 m3 liter L

Frequency s−1 hertz Hz

Pressure kg m−1 s−2 pascal Pa

Energya kg m2 s−1 joule J

aOne calorie is the energy required to heat 1 cm3 of water from 14.5 to 15.5°C. One calorie, which is equivalent to 1000 calories or 1.0 kilocalorie, is the standard unit employed nutritionally. To convert calories into joules, multiply by the conversion factor of 4.185.

For a complete description of SI units see the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (1970). Pure Appl. Chem. 21, 3–44.

Appendix F

Units of Measurement in Biological Systems