Scientific Units and Abbreviations
SCIENTIFIC UNITS AND ABBREVIATIONS
1.
INTRODUCTION - BASIC AND SUPPLEMENTARY UNITS
• 1960 the International General Conference on weights and measures adopted the International System of
Units (SI units).
• Under this system the following fundamental units are used:
mass
kilogram
kg
length
metre (note spelling)
m
time
second
s
• Other basic units are:
Electric current
ampere
A
Luminous intensity
candela
cd
Tempereature
kelvin
K
Amount of substance
mole
mol
• Supplementary units are:
Plane angle
radian
rad
Solid angle
steradian
sr
3
• All other units should be defined in these terms (e.g. volume in m ).
• However, not all these SI units are appropriate for use in biological systems, and in biological sciences (as in
other sciences) not all the units currently in common use are in SI units.
2.
PREFIXES AND SUFFIXES
3
-3
• The following prefixes and their symbols (in brackets) are used to denote multiplication factors of 10 and 10
intervals which are used for multiples and fractions of units:
12
10
tera (T)
9
10
giga (g)
6
10
mega (M)
3
10
kilo (k)
-3
10
milli (m)
-6
10
micro (µ)
-9
10
nano (n)
-12
10
pico (p)
-15
10
femto (f)
-18
10
atto (a)
3
• For example, the prefix “kilo” means that the basic unit is to be multiplied by 10 (i.e. by 1000).
-3
• Similarly the prefix “milli” means that the basic unit is to be multiplied by 10 (i.e. by 0.001); or effectively is to
3
be divided by 10 (i.e. by 1000).
• Use the prefixes in conjunction with the basic units so that numbers are kept between 0.1 and 1000.
• The following prefixes are not recommended for general use, but may be used in specific cases, e.g.
centimetres (cm) and hectares (ha).
2
10
hecta (h)*
1
10
deka
(da)*
-1
10
deci
(d)*
-2
10
centi
(c)*
• In biological sciences we are primarily concerned with the units for mass, length, volume and concentration.
Note that:
• units are never written as plural: kg not kgs.
• there should be a space between the value and the unit: 1 ml not 1ml.
• the multiplier precedes the unit without a space: mm not m m.
-2
• exponents follow the unit without a space: cm .
-2 -1
• some variables have several dimensions, in these instances the units are separated by a space: cm s .
Scientific Units and Abbreviations
3.
MEASUREMENTS IN SI UNITS
Mass:
• Kilograms (kg), grams (g), milligrams (mg) and micrograms (µg) are all used as appropriate.
• One mole of a compound is its relative molecular mass [R.M.M.] in grams (g). The abbreviation for moles is
mol (not m or M).
• Daltons are numerically equal to molecular mass. The mass of larger macromolecules and particles may be
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given in kilodaltons (kd) (1 dalton = 1.67 x 10 g).
Length:
Preferred units
metres
millimetres (mm)
micrometres (µm)
nanometres (nm)
Discouraged or non-SI units
yards, feet, inches, etc.
-6
microns (µ), m x 10 : use µm
-9
millimicrons (mµ), m x 10 : use nm
-10
Angstrom units (Å) m x 10 :
use nm, 1 nm = 10 Å
centimetres (cm not c. or cms) may also be used.
• An idea of scale in biological systems can be gained from Figure 1.
Volume:
3
• The SI unit of volume, the cubic metre (m ) is too large for most purposes in biological sciences laboratory
work.
• Consequently two systems of quasi-SI units are widely used; both systems are to be found in the practical
schedules.
3
dm
equal to
1 (litres)
3
cm
equal to
ml (millilitres)
3
mm (rarely used)
equal to
µl (microlitres)
Concentration:
Concentration in solutions:
• Concentrations are usually given as mass of solute per volume of solution. The preferred forms are
-3
-1
g dm (= g 1 )
-3
mg dm , or
-3
µg dm
• In some cases you may see solution concentrations described as percentages (%).
• Percentage (%) implies percentage on a weight per volume basis (i.e. % w/v) and refers to g solute dissolved
3
-1)
in 100 cm solution (= g 100 ml .
3
3
-1
• A value given as x % (v/v) refers to cm solute in 100 cm solution (= x ml 100 ml ).
• Where several solvents are mixed (e.g. for chromatography), you may see ratios expressed a:b:c:d. These
are nearly always ratios by volume.
3
• Solutions may also be described in parts per million (p.p.m.). This refers to mg solute dissolved in 1 dm (=
-1
3
-1
mg l ) or µg in 1 cm (= µg ml ).
-3
-3
• The SI unit of concentration, mol m , is equal to the non-SI term ‘millimolar’ (mM) whilst kmol m is equal to
3
the non-SI term ‘molar’ (M). A molar solution is more usually described as 1 mole dissolved in 1 dm or 1 litre
• You will frequently encounter millimolar (mM) and micromolar (µM) solutions in biology.
Concentrations in biological systems:
Most chemicals are present in biological systems in minute amounts. The highest concentrations usually found
in biology are mM solutions. These are the normal concentrations of substrates for oxidation (so-called fuels) in
the extracellular fluids bathing cells. The concentration of compounds within cells is usually much lower (µM or
nM), and may vary depending on the metabolic state of the cell. The concentration of hormones is usually
extremely low (nM or pM) although the concentration will vary depending on the state of the organism. Many
substances are extremely toxic to organisms i.e. they may be harmful at extremely low (pM) concentrations.
Scientific Units and Abbreviations
Examples
• Plasma glucose (humans)
• Plasma free fatty acid (humans)
++
• Extracellular Ca
++
• Intracellular Ca (total)
• Intracellular glucose-6-phosphate
• Plasma cortisol
• Plasma 2 5-hydroxycholecalciferol (Vit D)
• Levels of TBT (tributyl tin) with
toxic effect in dogwhelks
≅ 5 mM
≅0.5 mM
2.5 mM
1 µM
≅ 100 µM
100-700 nM
15-100 nM
6 pM
-1
≅0.9 gl
-1
≅ 0.13 gl
-1
≅25 mg 1
-1
36-250 µg 1
-1
6-40 nm 1
-1
2 ng 1
Heat (energy):
The calorie and kilocalorie (= Calorie) have been replaced by the SI unit for energy, the Joule (J).
2 -1
1J
= 1 kg m s
= 1 Nm (Newton metre)
= 1 Ws (Watt second)
1 calorie
1 kilocalorie
= 4.184 J
= 1 Calorie = 4.184 kJ.
Light:
• Light intensity is a property of the light source and is rarely needed by biologists. Illumination measurements
-2
(units: lux, lumens m ) are related to the spectral sensitivity of the human eye and are rarely useful. It is
-2
-2 -1
preferable to use units based on energy (W m ) or photon content (mol photons m s ).
Irradiance:
• Irradiance = irradiant flux density = energy fluence rate.
-2 -1
-2
• The SI units are J m s or W m ;
-2 -1
• photon irradiance is given in mol m s
Absorbance:
Absorbance (A ) which is log Io/I where Io is the intensity of the incident light and I is the intensity of the
transmitted light is preferred to the older terms 'extinction' and 'optical density (OD)'. It is a quotient and has no
units [see Appendix 2]. Molar absorption coefficient “e” is the absorbance of a 1 molar solution in a 1 cm
3
-1
-1
-1
-1
lightpath (dm mol cm or litre mol cm ).
The term “Optical Density” (OD) is still used when referring to suspensions of bacteria since sensu stricto light
passing into a suspension of particles is both absorbed and scattered. As with absorbance optical density is a
quotient and has not units. Nevertheless, A is used to denote `apparent absorbance' in plant tissue where the
value is affected by light scattering.
Time:
• The following units are all used where appropriate in biology.
Unit
Abbreviation
Seconds
s
(not sec. secs)
minutes
min
(not min. mins)
hours
h
(not h. hr. hrs)
days
d
weeks
months
years
y, yr
or a (annum)
Note that the last abbreviation, ‘a’, can be very confusing if you don't recognise it for what it is.
Scientific Units and Abbreviations
Pressure:
-2
• The S.I. unit of pressure is the pascal (Pa). In terms of force per unit area this is equal to 1 N m .
•
The non-SI unit of pressure, the bar, is still acceptable;
5
1 bar = 10 Pa or 0.1 MPa
• The pressure unit “atmosphere” (at) is no longer allowed as a scientific unit; 1 at = 1.013 bar.
• Pounds per square inch (psi) is a non-SI unit. It has been used in Appendix 9 only because the autoclave
pressure gauge is calibrated in psi.
Osmotic potential:
• The osmotic potential of a solution is usually expressed in pressure units although on thermodynamic
grounds energy per unit volume might appear more appropriate. For example 0.2 M sucrose solution has an
-3
osmotic potential of 0.54 MPa or 5.4 bar or 540 J dm .
Scientific Units and Abbreviations
Distance in metres (m)
(log scale)
Electromagnetic
radiation
(wavelength)
-12
10
hard X rays
atoms: C-H bond 0.109 nm
small molecules - glucose
globular proteins
macromolecules - haemoglobin 6.5 nm
- membranes 8 nm
assemblies - ribosomes 30 nm
- viruses 20 nm - 200 nm
-9
soft X rays
10
electron
microscope
UV/visible spectrum/photosynthetically
400-700 nm
active radiation
resolution of light microscope 0.2 µm
-6
10
near IR (or far-red)
light
microscope
bacteria, mitochondria
chloroplasts
animal cells
far IR
-3
plant cells
10
microwave
hens eggs
radiowaves
1
man’s height
10
3
length of blue whale
height of redwood tree
Radio 4
Capital Radio
depth of Atlantic trench
10
6
10
9
11
distance to sun 1.5 x 10 m
10
12
Figure 4.1 Orders of magnitude in biological systems