tant in metabolism. Long-chain fatty
acids (more than 8–10 carbon atoms)
most commonly occur as constituents of certain lipids, notably
glycerides, phospholipids, sterols,
and waxes, in which they are
esteriÜed with alcohols. These longchain fatty acids generally have an
even number of carbon atoms; unbranched chains predominate over
branched chains. They may be saturated (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 polyunsaturated fatty acids (e.g. *linoleic
acid and *linolenic acid). See also essential 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 continually but does not become a major
source of energy until the animal’s
carbohydrate resources are exhausted, for example during starvation. Fatty-acid oxidation occurs
chieÛy in the mitochondria. A series
of reactions cleave off two carbon
atoms at a time from the hydrocarbon 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 formation of acetyl CoA occurs repeatedly
until all the hydrocarbon chain has
been used up.
f-block elements The block of elements in the *periodic table consisting 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-dihydroxybutanedioate (sodium tartrate) solution), equal amounts of which are
added to the test solution. After boiling, a positive result is indicated by
the formation of a brick-red precipitate 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.
feldspars A group of silicate minerals, the most abundant minerals in
the earth’s crust. They have a structure in which (Si,Al)O 4 tetrahedra are
linked together with potassium,
sodium, and calcium and very occasionally 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 ; orthoclase (Or), KAlSi 3 O 8 ; celsian (Ce),
BaAl 2 Si 2 O 8 . The feldspars are subdivided into two groups: the alkali
feldspars (including microcline, orthoclase, and sanidine), in which
221
feldspars
f
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acids (more than 8–10 carbon atoms)
most commonly occur as constituents of certain lipids, notably
glycerides, phospholipids, sterols,
and waxes, in which they are
esteriÜed with alcohols. These longchain fatty acids generally have an
even number of carbon atoms; unbranched chains predominate over
branched chains. They may be saturated (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 polyunsaturated fatty acids (e.g. *linoleic
acid and *linolenic acid). See also essential 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 continually but does not become a major
source of energy until the animal’s
carbohydrate resources are exhausted, for example during starvation. Fatty-acid oxidation occurs
chieÛy in the mitochondria. A series
of reactions cleave off two carbon
atoms at a time from the hydrocarbon 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 formation of acetyl CoA occurs repeatedly
until all the hydrocarbon chain has
been used up.
f-block elements The block of elements in the *periodic table consisting 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-dihydroxybutanedioate (sodium tartrate) solution), equal amounts of which are
added to the test solution. After boiling, a positive result is indicated by
the formation of a brick-red precipitate 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.
feldspars A group of silicate minerals, the most abundant minerals in
the earth’s crust. They have a structure in which (Si,Al)O 4 tetrahedra are
linked together with potassium,
sodium, and calcium and very occasionally 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 ; orthoclase (Or), KAlSi 3 O 8 ; celsian (Ce),
BaAl 2 Si 2 O 8 . The feldspars are subdivided into two groups: the alkali
feldspars (including microcline, orthoclase, and sanidine), in which
221
feldspars
f
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