Dictionary of Chemistry [6th Ed.]

haviour can be very complex even
for simple dynamical systems (see
attractor). Systems, such as spin
glasses, in which ergodicity is
thought not to hold are described as
having broken ergodicity.

ergosterol A *sterol occurring in
fungi, bacteria, algae, and plants. It is
converted into vitamin D 2 by the ac-
tion of ultraviolet light.

ESCASee photoelectron spec-
troscopy.

eserine(physostigmine)An alka-
loid, derived from the calabar bean
plant, that inhibits cholinesterase by
covalently binding with it (see inhibi-
tion). Eserine is used to treat the eye
condition glaucoma.

211 esterification

e

N

N

O NH

CH 3

3 CH O

CH 3

3 CH

Eserine

ESI See electrospray ionization.

ESRSee electron paramagnetic
resonance.

essential amino acid An *amino
acid that an organism is unable to
synthesize in sufÜcient quantities. It
must therefore be present in the diet.
In man the essential amino acids are
arginine, histidine, lysine, threonine,
methionine, isoleucine, leucine, va-
line, phenylalanine, and tryptophan.
These are required for protein syn-
thesis and deÜciency leads to re-
tarded growth and other symptoms.
Most of the amino acids required by
man are also essential for all other
multicellular animals and for most
protozoans.

essential element Any of a num-

ber of elements required by living or-

ganisms to ensure normal growth,

development, and maintenance.

Apart from the elements found in or-

ganic compounds (i.e. carbon, hydro-

gen, oxygen, and nitrogen), plants,

animals, and microorganisms all re-

quire a range of elements in inor-

ganic forms in varying amounts,

depending on the type of organism.

The major elements, present in tis-

sues in relatively large amounts

(greater than 0.005%), are calcium,

phosphorus, potassium, sodium,

chlorine, sulphur, and magnesium.

The trace elements occur at much

lower concentrations and thus re-

quirements are much less. The most

important are iron, manganese, zinc,

copper, iodine, cobalt, selenium,

molybdenum, chromium, and sili-

con. Each element may fulÜl one or

more of a variety of metabolic roles.

essential fatty acids*Fatty acids

that must normally be present in the

diet of certain animals, including

man. Essential fatty acids, which in-

clude *linoleic and *linolenic acids,

all possess double bonds at the same

two positions along their hydrocar-

bon chain and so can act as precur-

sors of *prostaglandins. DeÜciency of

essential fatty acids can cause der-

matosis, weight loss, irregular

oestrus, etc. An adult human re-

quires 2–10 g linoleic acid or its

equivalent per day.

essential oilA natural oil with a

distinctive scent secreted by the

glands of certain aromatic plants.

*Terpenes are the main constituents.

Essential oils are extracted from

plants by steam distillation, extrac-

tion with cold neutral fats or solvents

(e.g. alcohol), or pressing and used in

perfumes,Ûavourings, and medicine.

Examples are citrus oils,Ûower oils

(e.g. rose, jasmine), and oil of cloves.

esteriÜcation A reaction of an al-