Environmental Engineering Reference
In-Depth Information
intramuscular saturated solid fats). Humans can synthesize all the fatty acids needed
for normal development except linoleic and
α
-linolenic acid, which fortunately are
readily available in plant oils.
Carbohydrates
(saccharides) dominate the biosphere's phytomass and are
indispensable both because of their essential metabolic functions and because of
their structural roles, but they are virtually absent in muscle. Simple sugars—
monosaccharides
(glucose, fructose, galactose, and others) and
disaccharides
(such
as sucrose or milk's lactose, with 16.5 kJ/g)—store energy in forms that are readily
digestible by heterotrophs, and ribose (C
5
H
10
O
5
) is the backbone of RNA and, after
phosphorylation, a key subunit of adenosine triphosphate. The polymerization of
simple sugars produces long-chained
polysaccharides,
whose two key roles are to
store energy (in plants as starch, in heterotrophs as glycogen) and to form structures
such as the exoskeletons of arthropods with chitin and the stems, stalks, and trunks
of plants with cellulose.
Cellulose,
a polymer of glucose with the formula of (C
6
H
10
O
5
)
n
, is a linear poly-
saccharide with a molecular weight usually in the range of 300,000-500,000 daltons,
and is the biosphere's most ubiquitous structurally strong macromolecule (its energy
content is 17.5 kJ/g). In contrast, monomers making up
hemicelluloses
include most
pentoses (i ve-carbon sugars, including xylose, mannose, galactose, and arabinose)
and produce an amorphous and structurally weak biopolymer. The third major
constituent of phytomass,
lignin
, is a complex polymer of coniferyl, coumaryl, and
synapyl alcohols that links with hemicelluloses, i lls cell wall spaces, stiffens plant
structures, and, being relatively hydrophobic, helps conduct water inside plants (its
energy content is 26.4 kJ/g).
The proportion of lignin in phytomass ranges from as little as 15% in crop resi-
dues to just over 50% in nut shells, with softwoods having higher concentrations
(
25%) than hardwoods and bark containing more lignin than heartwood. Phyto-
mass containing 25% lignin and 75% cellulose would thus have almost exactly
50% carbon, and this generic value has been used for decades as the standard carbon
content of wood, without examining variations between and within common species.
Cornwell et al. (2009) analyzed a large set of wood property studies and found
median values for lignin, cellulose, and other carbohydrate polymers of, respectively,
26%, 46%, and 21% for leafy trees and 29%, 44%, and 23% for coniferous species,
with extractives averaging less than 5% for the former and more than 6% for
the latter.
Cellulose has a carbon content of 44.4%, but the value for lignin cannot be
reduced to a single i gure. Idealized lignin (composed solely of coniferyl units) con-
tains 65% C (Freudenberg and Nash 1968), and wood analyses showed that the
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