Polysaccharides

PolysaccharidesJames N BeMiller, Purdue University, Indiana, USA

Polysaccharides are carbohydrate polymers containing

from approximately 35 (usually more than 100) to as

many as 60 000 monosaccharide units. Polysaccharides

have a range of general structures (from linear to various

branched structures) and shapes. They are structural

components of cell walls of bacteria, fungi, algae and

higher plants and of the exoskeletons of insects and

crustaceans, are energy- and carbon-storage substances,

and serve various other functions as extracellular materi-

als of plants, animals and microorganisms. They are the

most abundant (by mass) of all organic substances in

living organisms, comprising about two-thirds of the dry

weight of the total biomass. Some have commercial value

as isolated substances.

Nature, Occurrence and Classification

Polysaccharides are carbohydrate polymers whose mono-mer units are simple aldose and/or ketose sugars (mono-saccharides). The number of monosaccharide units inpolysaccharides varies fromabout 35 (although it is usuallyat least 100) to approximately 60 000. Themonosaccharide(glycosyl) units are in either five-membered (furanosyl) orsix-membered (pyranosyl) ring forms,most often the latter.These units are joined together in a head-to-tail fashion byglycosidic linkages, a glycosidic linkage being one-half ofan acetal structure, the other half being that which formsthe pyranosyl or furanosyl ring. An example of a glycosidiclinkage is given in Figure 1. See also: Monosaccharides

Polysaccharides may be linear or branched. There are avariety of branched structures, including structures withonly a few, very long branches, linear structures with shortbranches regularly spaced, irregularly spaced or in clusters,and branch-on-branch structures with branches clusteredor positioned to produce bush-like structures with orwithout decoration with short branches. Representativesof the general kinds of branching found in polysaccha-ride molecules are shown in Figure 2. Each polysaccharidehas one, and only one, reducing end, which is the endterminating in a hemiacetal (or carbonyl) group (desig-nated asf in Figure 2). Each branch generates an additionalnon-reducing end, which consists of a glycosyl unit that isattached to another through its anomeric hydroxyl groupbut has no glycosyl unit attached to it, making it a chainend with no carbonyl group. Therefore, a polysaccharidemolecule may have many non-reducing ends. In terms ofmass, linear polysaccharides are the most abundant, beingstructural components of higher plants and marine algae.However, there are many more branched polysaccharidesthan linear ones.In addition to monosaccharide units, polysaccharides

may contain ester, ether and/or cyclic acetalmoieties. Ester

OCH2OH

HOHO

O

OH

OHO

CH2OHO

Figure 1 Repeating unit structure of cellulose showing how the

b-D-glucopyranosyl units are joined by (1!4) glycosidic linkages. The carbon

and hydrogen atoms bonded to the carbon atoms are omitted for clarity. The

carbon atoms of each glycosyl unit are numbered, C1 being the anomeric

carbon atom (the one on the extreme right of each unit in this structure), C2

being the carbon atom attached to C1, C3 being the next and so on to C6 in

this case. The oxygen atoms attached to each carbon atom are numbered O1,

O2, O3, and so on. A cellulose molecule will contain approximately

150–5000 of these repeating units. The right-hand end (the reducing end)

will be terminated in an –OH group, making the structure a hemiacetal (a

potential aldehydo group) and that end the reducing end. The left-hand

end (the non-reducing end) will be terminated in a –H atom.

Introductory article

Article Contents

. Nature, Occurrence and Classification

. Determination of Structure

. Higher-Plant Cell-Wall Polysaccharides

. Energy Storage Polysaccharides

. Chitin and Other Fungal Cell-Wall Polysaccharides

. Bacterial Exopolysaccharides

. Glycosaminoglycans

. Polysaccharides as Industrial Gums

Online posting date: 15th September 2009

ELS subject area: Biochemistry

How to cite:BeMiller, James N (September 2009) Polysaccharides. In: Encyclopedia ofLife Sciences (ELS). John Wiley & Sons, Ltd: Chichester.

DOI: 10.1002/9780470015902.a0000693.pub2

ENCYCLOPEDIA OF LIFE SCIENCES & 2009, John Wiley & Sons, Ltd. www.els.net 1

http://dx.doi.org/10.1038/npg.els.0000691

groups include acetate, glycolate, succinate, sulphate andphosphate groups on the polysaccharide’s hydroxylgroups. Methyl and ethyl ether groups may be present.Pyruvic acid may be present as a cyclic acetal. In thosepolysaccharides containing amino sugars, the aminogroups are usually not free but are present as amidogroups, with the acid moiety being acetic, glycolic orsulphuric acid. In polysaccharides containing uronic acidunits (sugar units with a carboxylic acid group in place ofthe hydroxymethyl group), the carboxylic acid functionsmay be present as methyl esters.

Glycose is the generic term for amonosaccharide (simplesugar). A glycosyl (saccharide) unit is the group formed byremoving the OH group from the anomeric (originally thecarbonyl) carbon atomof a pyranose or furanose ring formof the sugar. A glycosamine is a monosaccharide thatcontains an amino group in place of a hydroxyl group. In

formal carbohydrate nomenclature, it is a deoxyaminosugar.Glycan is the generic term for a polysaccharide. For

example, a xylan is a polysaccharide made up primarily ofD-xylopyranosyl units (it may contain minor amounts ofother sugars); a b-glucan is constructed of b-D-glucopyr-anosyl units; an arabinoxylan has L-arabinose andD-xyloseas itsmonomeric units; a glucuronoxylomannan consists ofD-glucuronic acid, D-xylose and D-mannose; and so on. Apolysaccharide composed of D-galacturonic acid units maybe correctly called a galacturonan, a galacturonoglycanor poly(D-galacturonic acid). If the polysaccharide hasa backbone structure, it forms the latter part of thesystematic name; for example, galactomannans have amain mannan chain to which D-galactopyranosyl units areattached; the same is true of arabinoxylans. Polysacchar-ides that were named before the systematic names weredeveloped may have names that do not follow the system-atic nomenclature rules; examples are cellulose, amylose,pectin, glycogen and alginic acid.Polysaccharides may be attached to proteins, in which

case they are known as protein-polysaccharides whenthey originate from plants, or proteoglycans when theyoriginate from animals. The different proteoglycans havedifferent overall structures, but for the most part consist ofa large number of polysaccharide (glycan) chains on oneend of a polypeptide backbone to form a structure like abottle brush, with the carbohydrate portion accountingfor as much as 90% of the molecular weight of theproteoglycan. The polysaccharide chains contain aminosugars and are known as glycosaminoglycans. They arealso acidic polysaccharides, containing uronic acid unitsand/or sulphate half-ester groups. See also: ProteoglycanLipopolysaccharides are cell-wall components of Gram-

negative bacteria. Many bacteria also produce polysac-charides that are excreted outside the cell wall. They maybe components of a capsule (capsular polysaccharides) orsoluble in the extracellular medium. In either case, they arecalled exopolysaccharides (see section on Bacterial Exo-polysaccharides). See also: LipopolysaccharidesA polymer called peptidoglycan is present in most,

perhaps all, bacterial cell walls as the major structuralcomponent. Other carbohydrate polymers present inbacterial cell walls are teichoic acids (polymers of alditolphosphates) and teichuronic acids, which are linked topeptidoglycan. See also: PeptidoglycanFungal, including yeast, cell walls may contain one or

more of the following polysaccharides: chitin, cellulose,other b-glucans and mannans. See also: Polysaccharides:Bacterial and FungalBecause their synthesis does not involve a template

molecule, polysaccharides are polydisperse, that is mole-cules of a specific polysaccharide from a single source arepresent in a range of molecular weights. In addition, thedegree of polydispersity, the average molecular weightand the range of molecular weights in a polysaccharidepreparation vary from source to source. Most polysac-charides are also polymolecular, that is their fine structures

Figure 2 Portions of polysaccharide molecules showing the different kinds

of branching. F indicates the reducing end.

Polysaccharides

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vary from molecule to molecule. With the exception ofcellulose and a few other plant polysaccharides, onlybacterial polysaccharides have repeating-unit structuresand even those may vary, especially with the number andplacement of non-carbohydrate substituents. Structures ofother polysaccharides, and even of bacterial polysacchar-ides with regard to non-carbohydrate components, canvary between taxa, with growth conditions of the plant ormicroorganism, and evenbetween tissues of the sameplant.

Polysaccharides are present in most living organisms. Infact, polysaccharides comprise at least 75%of the dryweightof the total biomass. They serve a variety of functions, not allofwhich areknown.Theyaremost abundant inhigher plants(1) as structural components of primary and secondary cellwalls and in the middle lamella, (2) as reserve food materialsin leaves, seeds, stems, roots, tubers, rhizomes and othertissues, providing carbon sources for energy and biosynthesisof other substances, (3) as exudates of unknown function and(4) as extractable material of unknown function. They arecell-wall constituents, non-cell-wall constituents and storagematerials in algae. In microorganisms, they may be extra-cellular in addition to being cellular constituents. Thepolysaccharide chitin is a structural component in theexoskeletons of crustaceans and insects. See also: Plant CellWalls; Polysaccharides: Plant Noncellulosic

There is no ideal system of polysaccharide classification.Thebest system shouldbe that basedon chemical structure.However, because of their polymolecularity, which limitsdescriptions to statistical structures in many cases, andbecause of the great variety of structures, classifyingpolysaccharides in this way is problematic. Combinationsof the following categories are used:

1. Branching: (a) unbranched; (b) linear with shortbranches or side units (i) regularly spaced or (ii)irregularly spaced; (c) short branches or side units inclusters; (d) highly branched (branch-on-branch)structures

2. Different kinds of monomer units: (a) one (homogly-can); (b) two (diheteroglycan); (c) three (triheterogly-can); (d) four (tetraheteroglycan); (e) five(pentaheteroglycan)

3. Charge: (a) neutral; (b) anionic (acidic); (c) cationic

Specific sugars and linkage types may then be usedwithin each of these general groups. Non-carbohydrateconstituents may also be present in varying amounts.Polysaccharides used industrially are most often classifiedby source. Examples of a very limited number ofpolysaccharides classified in these two ways are given inTable 1 and Table 2.

Determination of Structure

Because polysaccharides other than bacterial polysacchar-ides and a very few plant polysaccharides are chemically

Table 1 Classification of unmodified polysaccharides by

structure, with examples

I. Linear molecules

(A) Unbranched

1. Neutral homoglycans(a) Cellulose(b) Laminaransa

(c) Yeast glucans(d) Cereal b-glucans(e) Amyloses(f) Inulins(g) Yeast mannans

2. Neutral diheteroglycans(a) Konjac glucomannan(b) Agarose component of agar

3. Anionic/acidic homoglycans(a) Lambda-carrageenans(b) Pectins, pectic acidsb

4. Anionic/acidic diheteroglycans(a) Algins/alginates(b) Kappa-carrageenans(c) Iota-carrageenans

5. Anionic/acidic triheteroglycan(a) Gellan gum

6. Cationic/basic homoglycan(a) Chitosan

(B) Linear with short branches/side units

1. Irregularly spaced branches(a) Neutral homoglycans

i. Fungal (mushroom) b-glucans(b) Neutral diheteroglycans

i. Galactomannans (guar gum, locust beangum, tara gum)

ii. Wheat flour arabinoxylansiii. Larch arabinogalactan

(c) Neutral tetraheteroglycans

i. Xyloglucans2. Regularly spaced branches

(a) Neutral homoglycan

i. Yeast mannan(b) Anionic/acidic triheteroglycan

i. Xanthan gumII. Nonlinear molecules

(A) Branches in clusters, homoglycan

1. Amylopectins

(B) Highly branched/branch-on-branch structures, anionic/

acidic

1. Tetraheteroglycans(a) Gum karayas(b) Okra gum

2. Pentaheteroglycans(a) B-type hemicelluloses of cereal brans (arabinoxylans)(b) Gum arabic(c) Psyllium seed gum

aSome laminarans contain a small percentage of D-mannitol units.bPrincipal structure in commercial high- and low-methoxyl pectins.Pectins are actually complex, heterogeneous polysaccharidescontaining some percentage of a-L-rhamnopyranosyl units in themain chain and perhaps arabinogalactan side chains.

Polysaccharides





heterogeneous (i.e. their structures vary in proportions ofmonosaccharide constituents and/or in proportions oflinkage types and in non-carbohydrate groups, if present,frommolecule to molecule, as well as in molecular weight),a variety of chemical and instrumental methods are usedtogether to determine structures, and a most probablestatistical structure is deduced.Polysaccharide preparation, whether in the laboratory

for characterization or in commercial production, beginswith extraction from the source in the case of a plantpolysaccharide, or with isolation from a fermentationculture medium in the case of an extracellular bacterialpolysaccharide. In laboratory preparations, extractionsfrom a plant tissue are usually preceded by removal oflipids and lignin. Extractionmaybedonewithwater in a fewcases, but most often involves an alkaline solution. Bothextraction and recovery from a fermentation medium arefollowed by purification and fractionation to separate thedesired polysaccharide from other polysaccharides andfrom water- or alkali-soluble non-carbohydrate materials,such as proteins. Purification most often involves precipita-tion, sometimes fractional precipitation. Precipitation isusually achieved by addition of awater-soluble alcohol suchas ethanol (in the laboratory) or 2-propanol (industrially).Structural analysis of a polysaccharide may be under-

taken once it is obtained in an acceptable degree of purity.Polysaccharides have a great variety of structures; theonly common feature being that each is composed,at least primarily, of monosaccharide units. Structural

Table 2 Classification of commercial polysaccharides by

source

I. Plant polysaccharidesa

(A) Higher plants

1. Cell wall constituents

(a) Cellulose

(b) Native and extracted/commercial pectins

2. Storage polysaccharides

(a) Starches (amyloses and amylopectins)

(b) Galactomannans (guar gum, locust bean/carob

gum, tara gum)

(c) Konjac glucomannan

(d) Inulin

3. Other non-cell-wall polysaccharides

(a) Exudate gums

i. Gum arabicsii. Gum tragacanthsiii. Gum ghattisiv. Gum karayas

(b) Extractives

i. Larch arabinogalactanii. Psyllium seed gum

(B) Algae

1. Cell-wall constituents

(a) Agars

(b) Alginates

(c) Carrageenans

(d) Furcellarans (Danish agars)

2. Non-cell-wall constituent

(a) Laminaran

(C) Microorganisms

1. Extracellular

(a) Curdlan

(b) Dextrans

(c) Diutan

(d) Gellan

(e) Pullulan

(f) Welan

(g) Xanthan

II. Animal

(A) Crustacean

1. Chitinb

III. Derived from native polysaccharides

(A) From cellulose

1. Carboxymethylcelluloses

2. Cellulose acetates

3. Cellulose acetate butyrates

4. Cellulose acetate propionates

5. Ethylcelluloses

6. Hydroxyethylcelluloses

7. Hydroxypropylcelluloses

8. Hydroxypropylmethylcelluloses

9. Methylcelluloses

10. Microcrystalline celluloses

Table 2 Continued

(B) From starch

1. Starch acetates

2. Starch 1-octenylsuccinates

3. Starch phosphates

4. Starch succinates

5. Starch adipates

6. Hydroxyethylstarches

7. Hydroxypropylstarches

8. Dextrins

(C) From guar gum

1. Carboxymethylguar gum

2. Carboxymethylhydroxypropylguar gum

3. Hydroxypropylguar gum

4. 2-Hydroxy-3-(trimethylammonium chloride)-

propylguar gum

(D) From native pectins

1. Pectic acids (low-methoxyl pectins)

2. Amidated pectins

3. Propylene glycol alginates

aPlant polysaccharides are generally restricted to those polysacchar-ides that are isolated in at least a semipurified form. They do notinclude those that are present as constituents of other materials;for example, they do not include the arabinoxylans of flours or theb-glucans of brans.bChitin is not produced in the native form, but rather in thedeacetylated form (chitosan).

(Continued )

Polysaccharides


characterization involves determination of (1) monosac-charide composition; (2) linkage types; (3) ring size, that ispyranose or furanose; (4) anomeric configurations, that isconfiguration at carbon atom 1 (C1) of an aldose unit or atcarbon atom 2 (C2) of a ketose unit; (5) presence andlocation of non-carbohydrate substituent groups; and (6)degree of polymerization/molecular weight. Because thereis such great variability in structures, there is somevariability in methods used; however, some generalitiescan be described.

Determination of the monosaccharide compositionbegins with acid-catalysed hydrolysis. The monosacchar-ides released are then determined both qualitatively andquantitatively by high-performance liquid chromatogra-phy (HPLC) or by gas–liquid chromatography (GLC)after conversion to volatile, thermostable derivatives (oftenalditol acetates). See also: Liquid Chromatography

Linkages are determined by methylation analysis, whichcan reveal the linkageposition, the ring size and thenatureofthe monosaccharide. In methylation analysis, all hydro-xyl groups of the polysaccharide are converted into methylethers. Acid-catalysed hydrolysis of the completely methy-lated polysaccharide and reduction of the carbonyl group ofeach released methylated monosaccharide to producepartially methylated alditols then exposes the hydroxylgroups involved in glycosidic linkages as free (unmethy-lated) hydroxyl groups. The anomeric hydroxyl group ofeach unit will always be involved in a glycosidic bond, soonly the other hydroxyl groups are significant. Each of theother hydroxyl groups involved in a glycosidic linkagebefore hydrolysis will be unmethylated, a characteristic thatmarks the location of a linkage. Units that are non-reducingend units are completely methylated, that is methyl ethersare formed at all hydroxyl groups of the pyranose orfuranose ring formof the sugar unit, that is hydroxyl groupsother than the one at C1 and the one involved in ringformation (e.g. 0–4 for a furanose ring of an aldose and 0–5for a pyranose ring of an aldose). One other free hydroxylgroup indicates a single linkage to that unit. Two other freehydroxyl groups indicate a single branch point at that chainunit. Thus, methylation analysis indicates the position oflinkages to each monosaccharide unit, but not the sequenceor any other structural information except for units that areat non-reducing termini.

To obtain information about sequences, the polysac-charide is partially depolymerized using specific enzyme-and/or acid-catalysed hydrolysis to yield oligosaccharides,the structures of which are then determined.

All known polysaccharides have at least chain segmentswithin their structures that have some kind of helixconformation.

Higher-Plant Cell-WallPolysaccharidesDifferences in cell-wall compositions occur within phyla,classes, families and genera of plants,with locationwithin a

given plant (because different cells have different functionsand exist in different environments), and with stage ofdevelopment of the tissue or organism. Nevertheless, somegeneral features can be described.Polysaccharides are the primary constituents of plant cell

walls. Because cellulose is the principal component of the cellwalls of higher plants, comprising about one-third of all plantmaterial, it is the most abundant organic compound onEarth. Cellulose is also present in brown, red and green algaeand in oomycetes and is excreted extracellularly by certainbacteria. Even certain animals (tunicates and protozoans)synthesize cellulose. Thus, cellulose is synthesized by bothplants and animals, though almost exclusively by plants, andby both prokaryotes and eukaryotes, but primarily byeukaryotes. The amount of cellulose in a plant varies greatlyfrom species to species. Wood, which is about one-halfcelluloseonadryweightbasis (db), has thehighest percentageof cellulose, except for the fibrous seed hairs of cotton, whichareapproximately90%cellulose. Inwood, themajorityof thecellulose is found in thickened, secondary cell walls. See also:Cellulose: Structure and Distribution; Plant Cell WallsCellulose differs frommostother polysaccharides inbeing

a homoglycan with a single type of linkage and in occurringnaturally as a paracrystalline, but largely crystalline,polymer. It also has a higher average molecular weight thanmost other polysaccharides. Most cellulose exists in naturein the form of strong fibres. Chemically, all celluloses are thesame; they all are polymers of b-D-glucopyranosyl unitslinked (1,4), but these (1,4)-b-glucans differ in physicalorganization into fibrillar structures from source to source.See also: Cellulose: Biogenesis and Biodegradation; PlantCell: Overview; Plant Cell Wall BiosynthesisInbothprimaryandsecondarycellwalls, cellulose ismixed

with hemicellulose(s) (Table 3) and lignin. The term hemi-cellulose indicates a polysaccharide closely associated withcellulose in plant cell walls, not structurally, but physically.Hemicelluloses may be composed primarily of a single typeof monosaccharide unit (e.g. a D-xylan), but are usuallyheteroglycans. Their structures vary from linear to highlybranched, bush-like polymers. The hemicellulose content ofwoods varies in the approximate range 17–23% (db).Pectic substances, another family of polysaccharides, are

located in middle lamellae and primary cell walls. Pecticsubstances comprise 1–4% of woody tissue. Xyloglucansare also cell-wall components of many higher plant cells.See also: Pectic Substances; Polysaccharides: PlantNoncellulosicThe polysaccharides found in cell walls are specific to

specific cell types. Most flowering plants have type I cellwalls. These walls have a cellulose–xyloglucan interloc-king framework (approximately 50% of the wall mass)embedded in a matrix of pectic polysaccharides (approxi-mately 30%) composed of rhamnogalacturonan I pluslesser amounts of galacturonans (partiallymethyl esterifiedpoly-D-galacturonic acids). Grasses have type II cell walls,the characteristic components of which are cellulose,glucuronoarabinoxylans, galacturonans and mixed-link-age b-glucans (1,3:1,4-b-glucans).

Polysaccharides












Structural polysaccharides of algae vary greatly betweenphyla. Included as cell-wall components of algae arecellulose, alginates, sulphated galactans (carrageenans,agars, and so on), lichenan (a1,3:1,4-b-glucan), xylans,mannans and pectic substances. See also: Algal Cell Walls

Energy Storage Polysaccharides

Serving as a reserve energy supply is one function ofpolysaccharides in living systems (Table 4). Glycogen, apolysaccharide that serves in this role, is ubiquitous inmammals, fish, molluscs, insects, other animals, bacteria,fungi and some plants, and exists in different forms andamounts in these organisms. Glycogen is present in mostmammalian tissues; the liver contains the highest

concentration; skeletal muscle contains the greatestamount. Glycogen is well constructed to be an energystorage molecule: it stores a large number of units ofD-glucose while producing a negligible increase in osmoticpressure because of its high molecular weight and/or inviscosity because of its compactness; and its highlybranched, bush-like structure makes a large number ofglucosyl end units simultaneously available to enzymemolecules that release them. Liver glycogen is used tomaintain blood glucose levels.Muscle glycogen is used as asupply of energy, even under less than fully aerobicconditions. Glycogen pools are generally in dynamicstates. See also: Glycogen Storage Diseases; Polysacchar-ides: Energy StorageStarch is the principal carbohydrate energy storage

substance of higher plants and, after cellulose, is the secondmost abundant carbohydrate end product of photosynth-esis. In addition to being a reserve food substance for mosthigher plants, starch is an energy source for animals thatfeed on those plants. Starch is found in leaves, where itserves as a transient D-glucose storage material, and mayoccur in seeds (especially those of cereal grains), fruits,roots, rhizomes, stems, tubers and trunks for long-termstorage. Starches provide at least 70% of human caloricintake on a worldwide basis.Starch is unique among carbohydrates in that it occurs

in discrete particles called granules. Starch granules arerelatively dense and insoluble. Most starch granules con-tain two polymers: an essentially linear polysaccharidecalled amylose and a highly branched polysaccharidecalled amylopectin. Amylopectin is perhaps the largestnatural polymer with a distinct structure, having averagemolecular weight values of at least 10 000 kDa. Each starchfrom each plant source and tissue is unique in terms of

Table 3 Cell wall polysaccharidesa

Higher land plants

Cellulose

Hemicelluloses

Arabinoxylans

Galactoglucomannans

b-Glucans

Glucomannans

Mannans

Xylans

Xyloglucans

Pectic polysaccharides

Arabinans

Arabinogalactans

Galactans

Galacturonans

Rhamnogalacturonans

Marine algae

Algins

Cellulose

L-Fucans

Galactans

Agars

Carrageenans

Furcellarans

b-Glucans

Mannans

Xylans

Fungi and yeasts

Cellulose

Chitin

b-Glucans

a-Glucans

Mannans

aMany of these polysaccharides may contain monosaccharide unitsother than those indicated in the name. For example, xylans oftencontain uronic acid units, and polysaccharides named L-fucans maycontain, in addition to the principal sugar (L-fucose), D-galactose,D-glucuronic acid, D-mannose and D-xylose.

Table 4 Energy storage polysaccharides

Higher land plants

Fructans

Galactans

Galactomannans

Glucomannans

Mannans

Starches

Xyloglucans

Marine algae

Fructans

a-Glucansa

b-Glucans

Xylans

Freshwater algae

a-Glucansa

b-Glucans

Fungi and yeasts

a-Glucansa

b-Glucans

aStarch-like and glycogen-like polymers.

Polysaccharides






granule morphology, composition, polysaccharide struc-tures and characteristic properties. See also: Starch andStarch Grains

Other energy storage polysaccharides include inulin andother fructans in roots, tubers and stems; galactomannansin legume seeds; mannans in certain palm seeds; galactansin lupine; and starch-type and laminaran-type polysac-charides of green and brown algae, respectively. (Laminar-an is a (1,3)-b-glucan.) See also: Polysaccharides: EnergyStorage

Chitin and Other Fungal Cell-WallPolysaccharides

Fungal cell walls often contain 80–90% (db) polysacchar-ide. Fungal cell walls have been categorized into eighttypes by polysaccharide composition: cellulose–glycogen,cellulose–b-glucan, cellulose–chitin, chitosan–chitin, chitin–b-glucan (the most common), mannose-containing poly-saccharide–b-glucan, mannan–chitin and galactosamino-glycan–chitin. Chitin is also a primary matrix constituentof the exoskeletons of insects and crustaceans. Its structureis identical to that of cellulose (Figure 1), with the exceptionof having an acetamido (–NH–CO–CH3) group in place ofthe hydroxyl group at C2 of each monomer unit. Thus,chitin is (1,4)-linked poly(N-acetyl-b-D-glucosamine) ormore properly poly(2-acetamido-2-deoxy-b-D-glucopyra-nose). Like cellulose, chitin is highly crystalline and in-soluble. See also: Chitin; Fungal Cell Walls

Yeast cell walls may contain a (1,3)-b-glucan, a (1,6)-b-glucan, various mannans or a galactan. Other cell-wallcomponents of certain fungi include rhamnomannans,glucomannans, galactomannans, xylomannans, glucuro-noxylomannans and other polysaccharides.

Bacterial Exopolysaccharides

Carbohydrate-containing polymers are components ofbacterial cell walls and capsules and may also be secretedinto the culture medium. Of the cell-wall polysaccharides,the lipopolysaccharides of Gram-negative bacteria shouldbe mentioned. These materials consist of three parts: acompound called lipidA, a core polysaccharide section andside chains known as O-specific side chains, which areattached to the core polysaccharide. The O-specific chainsare also polysaccharides and are immunogenic. Lipopoly-saccharide structures are quite diverse and often containsugar units that are rarely or never found in otherpolysaccharides. Gram-positive bacteria do not synthesizelipopolysaccharides of the type present in Gram-negativebacteria, but do make other types of cell-wall polysac-charides. Some also produce capsules of polysaccharide.Intracellular polysaccharides are present in some bacteria.See also: Bacterial Cells; Bacterial Cell Wall; Lipopoly-saccharides

Aswith eukaryotes, the great diversity of types of bacterialeads to a great diversity in polysaccharide structures, butthe overwhelming majority of bacterial polysaccharides areheteroglycans that are polymers of oligosaccharide repeat-ing units. These repeating units generally contain three toeight glycosyl units. The polymers often also contain non-carbohydrate components.Bacterial exopolysaccharides (extracellular polysacchar-

ides) include those that are made from sucrose, namelydextrans and fructans. Dextrans are branched a-glucanscontaining (1,3:1,6) and occasionally (1,2) linkages.Fructans contain b-D-fructofuranosyl units linked (2,6)or (2,1).Several extracellular microbial polysaccharides are

commercial products (see section on Polysaccharides asIndustrial Gums).

Glycosaminoglycans

Glycosaminoglycans are found primarily in animal tissues,mostly in connective tissue, and often as part of a largersupermolecular structure known as a proteoglycan. As thename implies, they contain amino sugars. These polysac-charides usually contain alternatingunits of an amino sugar(2-amino-2-deoxy-D-glucose (D-glucosamine) or 2-amino-2-deoxy-D-galactose (D-galactosamine)) and a uronic acid(D-glucuronic acid or L-iduronic acid); they usually containN-acetyl or N- or O-sulphate groups. An exception to thisoverall general structure is keratan sulphate, which has aD-galactopyranosyl unit in place of a uronic acid unitalternating with an amino sugar. The glycosaminoglycanheparin, unlike the others, originates in mast cells in manynonconnective tissues. Glycosaminoglycans that do havethe usual structure and are present in connective tissue,either free or as part of a proteoglycan, are hyaluronic acid,chondroitin 4- and 6-sulphates, dermatan sulphate andheparan sulphate. See also: Glycosaminoglycans: Structureand Biological Functions; Proteoglycan

Polysaccharides as Industrial Gums

Many polysaccharides obtained from plants or micro-organisms are used in their native state in various industrialapplications, including in food products, but theymay alsobe modified before use. Their common characteristic istheir hydrophilic nature. Many are water-soluble; thosethat are not, like cellulose, absorb water (hydrate). Thewater-soluble polysaccharides and modified polysacchar-ides used in industrial applications are known as gums.When used in food products, they (and also the proteingelatin) may be referred to as hydrocolloids. Gums/hydrocolloids are used primarily to thicken and/or gelaqueous systems and otherwise to modify and/or controlthe rheological properties of aqueous systems, but theymay also be used for a wide range of other attributes,including, but not limited to, their ability to function as

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adhesives, crystallization inhibitors, emulsion and suspen-sion stabilizers, film formers and texturizing agents. Table 2

lists commercial, water-soluble polysaccharides and poly-saccharide derivatives by source.See also: PlantCellWalls:Economic Significance; Plant Gums

Further Reading

Aspinall GO (ed.) (1982) The Polysaccharides, vol. 1. New York,

NY: Academic Press.


NY: Academic Press.


NY: Academic Press.

BeMiller JN (1999) Structure–property correlations of non-starch

food polysaccharides. Macromolecular Symposia 140: 1–15.

Klemm D, Philipp B, Heinze T, Heinze U and Wagenknecht W

(1998) Comprehensive Cellulose Chemistry, vols 1 and 2.

Weinheim, Germany: Wiley VCH.

ShimizuK (1991)Chemistry of hemicelluloses. In:HonDN-S and

Shiraishi N (eds) Wood and Cellulosic Chemistry. New York,

NY: Marcel Dekker.

SuzukiM and ChattertonNJ (eds) (1993) Science and Technology

of Fructans. Boca Raton, FL: CRC Press.

Varki A, Cummings R, Esko J et al. (1999) Essentials of

Glycobiology. Cold Spring Harbor, NY: Cold Spring Harbor

Laboratory Press.

Walter RH (ed.) (1991) The Chemistry and Technology of Pectin.

San Diego, CA: Academic Press.

Whistler RL and BeMiller JN (2009) Starch: Chemistry and

Technology. Orlando, FL: Academic Press.

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Polysaccharides