tant in metabolism. Long-chain fatty
acids (more than 8–10 carbon atoms)
most commonly occur as con-
stituents of certain lipids, notably
glycerides, phospholipids, sterols,
and waxes, in which they are
esteriÜed with alcohols. These long-
chain fatty acids generally have an
even number of carbon atoms; un-
branched chains predominate over
branched chains. They may be satu-
rated(e.g. palmitic (hexadecanoic)
acid and stearic (octadecanoic) acid)
or unsaturated, with one double
bond (e.g. oleic (cis-octodec-9-enoic)
acid) or two or more double bonds,
in which case they are called polyun-
saturated fatty acids (e.g. linoleic
acid and *linolenic acid). See also es-
sential fatty acids.
The physical properties of fatty
acids are determined by chain
length, degree of unsaturation, and
chain branching. Short-chain acids
are pungent liquids, soluble in water.
As chain length increases, melting
points are raised and water-solubility
decreases. Unsaturation and chain
branching tend to lower melting
points.
fatty-acid oxidation (β-oxidation)
The metabolic pathway in which fats
are metabolized to release energy.
Fatty-acid oxidation occurs continu-
ally but does not become a major
source of energy until the animal’s
carbohydrate resources are ex-
hausted, for example during starva-
tion. Fatty-acid oxidation occurs
chieÛy in the mitochondria. A series
of reactions cleave off two carbon
atoms at a time from the hydrocar-
bon chain of the fatty acid. These
two-carbon fragments are combined
with coenzyme A to form acetyl
coenzyme A (acetyl CoA), which then
enters the Krebs cycle. The forma-
tion of acetyl CoA occurs repeatedly
until all the hydrocarbon chain has
been used up.
f-block elementsThe block of el-
ements in the *periodic table consist-
ing of the lanthanoid series (from
cerium to lutetium) and the actinoid
series (from thorium to lawrencium).
They are characterized by having two
s-electrons in their outer shell (n) and
f-electrons in their inner (n–1) shell.f.c.c.Face-centred cubic. See cubic
crystal.F-centre See colour centre.
Fehling’s test A chemical test to
detect reducing sugars and aldehydes
in solution, devised by the German
chemist H. C. von Fehling (1812–85).
Fehling’s solution consists of Fehlings
A (copper(II) sulphate solution) and
Fehling’s B (alkaline 2,3-dihydroxybu-
tanedioate (sodium tartrate) solu-
tion), equal amounts of which are
added to the test solution. After boil-
ing, a positive result is indicated by
the formation of a brick-red precipi-
tate of copper(I) oxide. Methanal,
being a strong reducing agent, also
produces copper metal; ketones do
not react. The test is now little used,
having been replaced by *Benedict’s
test.
feldsparsA group of silicate miner-
als, the most abundant minerals in
the earth’s crust. They have a struc-
ture in which (Si,Al)O 4 tetrahedra are
linked together with potassium,
sodium, and calcium and very occa-
sionally barium ions occupying the
large spaces in the framework. The
chemical composition of feldspars
may be expressed as combinations of
the four components: anorthite(An),
CaAl 2 Si 2 O 8 ; albite(Ab), NaAlSi 3 O 8 ; or-
thoclase(Or), KAlSi 3 O 8 ; celsian(Ce),
BaAl 2 Si 2 O 8. The feldspars are subdi-
vided into two groups: the alkali
feldspars(including microcline, or-
thoclase, and sanidine), in which221 feldspars
f