Plant Physiology - Musiyenko, M. M. 2001

Appendices
Units of Measurement and Dimensions

I. SI Units of Measurement

Measurements of physical quantities are expressed as a number followed by a unit of measurement. The system of units currently recommended for worldwide use is called the SI system (Système International d'Unités).

Base units in the SI system:

·meter, standard of length;

·kilogram, standard of mass;

·second, standard of time;

·ampere, standard of electric current;

·kelvin, standard of Temperature;

·candela, standard of luminous intensity;

·mole, standard of amount of substance.

II. Commonly Used Units of Length from Legacy Measurement Systems

The terms micron (μ or μm) and angstrom (Å) have been used as units of length when measuring intracellular Organelles, chemical bonds, etc.:

1 micron = 1 μ = 1 μm = 1 μm = 1×10^-6 m = 1×10³ nm = 1×10⁴ Å

1 Angstrom = 0.1 nm = 1×10 ^-4 μm = 1×10^-10 m

1 nanometer = 10 Å = 1×10^-3 μm = 1×10^-9 m

III. SI Units for Pressure

1 Pascal (Pa) = 9.8692×10^-6 standard atmospheres = 7.5006×10^-3 mmHg (torr) 1 standard atmosphere = 760 mmHg = 1.0133×10⁵ Pa

1 mmHg = 3.3864×10³ Pa = 3.3421×10^-2 std. atm.

IV. Solution Concentration

Molarity (M) is the number of moles of solute per 1 L of solution (the term "molar" corresponds to mol×L^-1).

Molality (m) is the number of moles of solute per 1 kg of solvent.

% (weight / volume) is the weight in grams of solute per 100 mL of solution.

% (weight / weight) is the weight in grams of solute per 100 g of solution.

% saturation is the salt concentration in solution as a percentage of the maximum possible concentration at a given temperature.

V. Units of Enzymatic Activity

Most often, enzyme activity is characterized by The rate of the catalyzed reaction, for example, in micromoles of converted substrate per 1 minute or in micromoles of formed product per 1 minute.

The official SI unit of enzymatic activity is the katal (kat). It is used primarily in chemical kinetics. A katal is The amount of enzyme that catalyzes The conversion of 1 mole of substrate per 1 second.

The former international unit (U or IU) is defined as the amount of enzyme that can convert 1 micromole of substrate per 1 minute.

Enzymatic activity is most commonly expressed in microkatals (µkat = 1 × 10^-6), nanokatals (nkat = 1 × 10^-9 kat), and picokatals (pkat = 1 × 10^-12 kat), which correspond to the amount of enzyme that converts 1 micromole (µmol), 1 nanomole (nmol), or 1 picomole (pmol) of substrate per second, respectively.

Specific enzyme activity is defined as the number of katals per 1 kg of protein, or more frequently in practice, as the number of µkat per 1 mg of protein under specific conditions of pH, temperature, Substrate Concentration, etc.

Molar enzyme activity is defined as the number of katals per 1 mole of enzyme under specific conditions of pH, temperature, substrate concentration, etc.

VI. Units of Temperature Measurement

Base SI unit

Kelvin (K) — the thermodynamic unit of temperature:

1 K = 1°C;

OTHER UNITS

Degree Celsius (°C) — based on a centigrade temperature scale that uses the melting point of ice as zero and the boiling point of Water at standard atmospheric pressure as 100°. The Celsius temperature scale is defined as a thermodynamic scale with its zero shifted to the melting point of ice, which is 273.15 K.

VII. Luminous Units in the SI System

Candela (cd) — the unit of luminous intensity, defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10¹² hertz and that has a radiant intensity in that direction of 1/ 683 watt per steradian (Note: standard definition adapted for clarity/context).

Derived SI units

Lumen (lm) — the unit of luminous flux, defined as the light energy emitted per 1 second within a solid angle of 1 steradian (sr) by a uniform point source having a luminous intensity of 1 candela.

1 lm = cd × sr

Lux (lx) — the unit of illuminance, defined as the illuminance produced by a luminous flux of 1 lumen falling on a 1 m² perpendicularly oriented surface.

1 lx = lm × m ^-2

Most commonly used units in plant physiology:

Volume   — liter, 1 L = 10^-3 m³

Mass   — kilogram, 1 kg = 1000 g

Time    — minute, hour, day, year

Temperature   — degree Celsius (°C); 0°C = 273.15 K;

Energy   — joule, 1 J = 10⁷ erg = 0.2388 cal = 6.25 × 10¹⁸ eV; 1 calorie = 4.1868 J;

Pressure    — pascal, 1 Pa = 10^-5 bar; 1 atm = 760 mmHg = 1.013 bar = 1.013×10⁵ bar;

Electric potential — volt, 1 V = 1 J × A^-1 × s

Radioactivity — curie, 1 Ci = 37×10¹⁰ Bq; 1 mCi (millicurie) = 10^-3 Ci; 1 µCi (microcurie) = 10^-6 Ci;

Radiation dose  — gray, 1 Gy = 100 rad = 1 J/kg; 1 rad = 0.01 J/kg;

Molecular weight

(molar mass) — gram per mole (10^-3 kg × mol^-1); Dalton (Da), 1 Da = 1/12 of the mass of ¹²C = 1.6605 × 10^-27 kg