Page 51 - Color Atlas of Biochemistry

P. 51

42 Biomolecules

Plant polysaccharides B. Starch
Starch, a reserve polysaccharide widely dis-
Two glucose polymers of plant origin are of
special importance among the polysac- tributed in plants, is the most important car-
bohydrate in the human diet. In plants, starch
charides: β1 4-linked polymer cellulose
and starch,which is mostly α1 4-linked. is present in the chloroplasts in leaves, as well
as in fruits,seeds,and tubers. The starch con-
tent is especially high in cereal grains (up to
A. Cellulose 75% of the dry weight), potato tubers (ap-
proximately 65%), and in other plant storage
Cellulose, a linear homoglycan of β1 4-
linked glucose residues, is the most abundant organs.
In these plant organs, starch is present in
organic substance in nature. Almost half of the the form of microscopically small granules in
total biomass consists of cellulose. Some special organelles known as amyloplasts.
40–50% of plant cell walls are formed by cel- Starch granules are virtually insoluble in cold
lulose. The proportion of cellulose in cotton
fibers, an important raw material, is 98%. Cel- water, but swell dramatically when the water
is heated. Some 15–25% of the starch goes
lulose molecules can contain more than 10 4
6
glucose residues (mass 1–2 10 Da) and can into solution in colloidal form when the mix-
reach lengths of 6–8 µm. ture is subjected to prolonged boiling. This
(“soluble
amylose
proportion
is
called
Naturally occurring cellulose is extremely
mechanically stable and is highly resistant to starch”).
Amylose consists of unbranched α1 4-
chemical and enzymatic hydrolysis. These
properties are due to the conformation of linked chains of 200–300 glucose residues.
Due the α configuration at C-1, these chains
the molecules and their supramolecular or- form a helix with 6–8 residues per turn (1).
ganization. The unbranched β1 4linkage re-
sults in linear chains that are stabilized by Thebluecoloringthatsoluble starch takes on
when iodine is added (the “iodine–starch re-
hydrogen bonds within the chain and be-
tween neighboring chains (1). Already during action”) is caused by thepresenceofthese
helices—theiodineatoms form chains inside
biosynthesis, 50–100 cellulose molecules as-
sociate to form an elementary fibril with a the amylose helix, and in this largely non-
diameter of 4 nm. About 20 such elementary aqueous environment take on a deep blue
color. Highly branched polysaccharides turn
fibrils then form a microfibril (2), which is
readily visible with the electron microscope. brownor reddishbrowninthe presence of
iodine.
Cellulose microfibrils make up the basic
framework of the primary wall of young plant Unlike amylose, amylopectin,which is
cells (3), where they form a complex network practically insoluble, is branched.On average,
onein 20–25 glucoseresidues is linkedto
with other polysaccharides. The linking poly-
saccharides include hemicellulose,which is a another chain via an α1 6bond. This leads
to an extended tree-like structure, which—
mixture of predominantly neutral heterogly- like amylose—contains only one anomeric
cans (xylans, xyloglucans, arabinogalactans,
etc.). Hemicellulose associates with the cellu- OH group (a “reducing end”). Amylopectin
molecules can contain hundreds of thousands
lose fibrils via noncovalent interactions. These
complexes are connected by neutral and of glucose residues; their mass can be more
8
than 10 Da.
acidic pectins, which typically contain galac-
turonic acid. Finally, a collagen-related
protein, extensin, is also involved in the for-
mation of primary walls.
In the higher animals, including humans,
cellulose is indigestible,but importantas
roughage (see p. 273). Many herbivores (e. g.,
the ruminants) have symbiotic unicellular or-
ganisms in their digestive tracts that break
down cellulose and make it digestible by the
host.

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