Plant polysaccharides

Plant polysaccharides

Two glucose polymers of plant origin are of

special importance among the polysaccharides:

β1
4-linked polymer cellulose

and starch, which ismostly α1
4-linked.

A. Cellulose 

Cellulose, a linear homoglycan of β1
4-

linked glucose residues, is the most abundant

organic substance in nature. Almost half of the

total biomass consists of cellulose. Some

40–50% of plant cell walls are formed by cellulose.

The proportion of cellulose in cotton

fibers, an important raw material, is 98%. Cellulose

molecules can contain more than 104

glucose residues (mass 1–2 106 Da) and can

reach lengths of 6–8 μm.

Naturally occurring cellulose is extremely

mechanically stable and is highly resistant to

chemical and enzymatic hydrolysis. These

properties are due to the conformation of

the molecules and their supramolecular organization.

The unbranched β1
4 linkage results

in linear chains that are stabilized by

hydrogen bonds within the chain and between

neighboring chains (1). Already during

biosynthesis, 50–100 cellulose molecules associate

to form an elementary fibril with a

diameter of 4 nm. About 20 such elementary

fibrils then form a microfibril (2), which is

readily visible with the electron microscope.

Cellulose microfibrils make up the basic

framework of the primary wall of young plant

cells (3), where they form a complex network

with other polysaccharides. The linking polysaccharides

include hemicellulose, which is a

mixture of predominantly neutral heteroglycans

(xylans, xyloglucans, arabinogalactans,

etc.). Hemicellulose associates with the cellulose

fibrils via noncovalent interactions. These

complexes are connected by neutral and

acidic pectins, which typically contain galacturonic

acid. Finally, a collagen-related

protein, extensin, is also involved in the formation

of primary walls.

In the higher animals, including humans,

cellulose is indigestible, but important as

roughage . Many herbivores (e. g.,

the ruminants) have symbiotic unicellular organisms

in their digestive tracts that break

down cellulose and make it digestible by the

host.

B. Starch 

Starch, a reserve polysaccharide widely distributed

in plants, is the most important carbohydrate

in the human diet. In plants, starch

is present in the chloroplasts in leaves, as well

as in fruits, seeds, and tubers. The starch content

is especially high in cereal grains (up to

75% of the dry weight), potato tubers (approximately

65%), and in other plant storage

organs.

In these plant organs, starch is present in

the form of microscopically small granules in

special organelles known as amyloplasts.

Starch granules are virtually insoluble in cold

water, but swell dramatically when the water

is heated. Some 15–25% of the starch goes

into solution in colloidal form when the mixture

is subjected to prolonged boiling. This

proportion is called amylose (“soluble

starch”).

Amylose consists of unbranched α1
4-

linked chains of 200–300 glucose residues.

Due the α configuration at C-1, these chains

form a helix with 6–8 residues per turn (1).

The blue coloring that soluble starch takes on

when iodine is added (the “iodine–starch reaction”)

is caused by the presence of these

helices—the iodine atoms form chains inside

the amylose helix, and in this largely nonaqueous

environment take on a deep blue

color. Highly branched polysaccharides turn

brown or reddishbrown in the presence of

iodine.

Unlike amylose, amylopectin, which is

practically insoluble, is branched. On average,

one in 20–25 glucose residues is linked to

another chain via an α1
6 bond. This leads

to an extended tree-like structure, which—

like amylose—contains only one anomeric

OH group (a “reducing end”). Amylopectin

molecules can contain hundreds of thousands

of glucose residues; their mass can be more

than 108 Da.