Geology Reference
In-Depth Information
consists of about 70% cellulose, the remainder being an
aromatic constituent called
lignin
, which gives wood
its toughness and structural utility.
end. No doubt this contributes to their high solubility
in polar solvents like water or ethanol, each end being
solvated by a sheath of solvent molecules (Box 4.1).
The capacity of the amino acids to react with acids or
bases according to circumstances means that molecules
can also react with each other. Reaction between the
amino group of one molecule and the carboxyl group
of another joins the two molecules together:
Acids, amino acids and proteins
Acidic behaviour in organic molecules is associated
with the presence of a
carboxyl
group, as illustrated by
ethanoic
(acetic) acid, familiar as the sharp-tasting con-
stituent of vinegar:
H
H
O
H
OH
N
C
C
H
C
C
H
O
OH
+
H
H
H
Carboxyl group
O
H
N
C
C
The acidity arises from dissociation of the OH present
in the carboxyl group, releasing an H
+
ion (proton).
Groups of atoms like the carboxyl group, which
involve the organic molecule to which they belong in
certain specific types of reaction, are called
functional
groups
. Another example is the
amino group
(NH
2
),
whose presence in the same molecule as the carboxyl
group is the characteristic of particularly important
class of organic acids, the
amino acids
. About 20 amino
acids occur in the living world. The simplest, glycine,
can be recognized as a derivative of ethanoic acid:
H
H
OH
H
H
O
H
N
C
C
O
+
H
2
O
N
C
C
H
H
H
OH
H
H
H
OH
Nature uses this
peptide linkage
reaction to assemble
amino acid units into huge
protein
molecules, whose
relative molecular masses run into thousands. Proteins
are the essential constituents of the living cell. Nitrogen
plays a fundamental role in other key molecules of
living things, such as chlorophyll (Box 9.5)
Proteins in decaying organic matter break down rap-
idly by
hydrolysis
(Box 9.3), the reverse of the reaction
above, leading to the production of simpler proteins
and amino acids.
N
C
C
Carboxyl group
O
Amino group
H
H
In solution, amino acids behave as weak acids: like
ethanoic acid, they dissociate by releasing a proton
from the carboxyl group:
+
−
(9.1)
NH CH COOH HNHCHCOO
2
→+
2
2
2
At the same time, however, the amino group has the
capacity to accept a proton, thereby behaving as a base:
Inorganic carbon
The element carbon exists in the Earth primarily in
two crystalline forms, graphite and diamond (whose
structures and properties are compared in Chapter 7).
Diamond crystallizes naturally only at very high pres-
sure, equivalent to depths within the Earth greater
+
−
+
−
(9.2)
HNHCHCOO
+
→
NH CH COO
2
2
3
2
Amino acids thus have the remarkable property of
forming dipolar ions or
zwitterions
(a German term
meaning 'hybrid ion') with opposite charges at each
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