Carbohydrates: Classification and Importance

Carbohydrates

• Aldehyde or ketone derivatives of polyhydric alcohols containing carbon, hydrogen, and oxygen
• Contains equal amounts of carbon and water molecules

Importance

• Plant metabolites contain sugar as essential features
• Plays an ecological role in plant-animal interaction
• Protects against wound and infection and detoxifies foreign substances
• Sugars and starches are used as food and in pharmaceuticals

Classification of Carbohydrates

• Monosaccharides cannot be hydrolyzed into simple sugars
• Disaccharides are hydrolyzed into two monosaccharides
• Trisaccharides yield three monosaccharides
• Tetrasaccharides yield four monosaccharides
• Polysaccharides contain more than ten monosaccharide units

Monosaccharides

• Classified by the number of carbon atoms they contain

Monosaccharides: Carbon Atom Count

• 3C: Triose
• 4C: Tetrose
• 5C: Pentose
• 6C: Hexose is the most important monosaccharide found in plants
• Acts as a building block for many polysaccharides such as starches and glycogen

Disaccharides

• Consist of "two sugars"
• Commonly found in nature as sucrose, lactose, and maltose
• Formed by a condensation reaction where water is released during the joining of two monosaccharides
• The bond formed between the two sugars is a glycosidic bond (alpha & beta)

Disaccharides: Maltose (Malt Sugar)

• Composed of two glucose molecules
• Joined by an a-glycosidic bond
• Its use is in alcohol production
• Formed through the breakdown of starch by enzymes in the mouth, though uncommon in nature
• Found in sweet potatoes (Ipomea batatas)

Disaccharides: Sucrose (Table Sugar)

• Functions as an excellent preservative
• Lacks a "reducing end" or reactive group unlike other sugars
• Hydrolysis yields glucose and fructose, known as "invert sugars" which are sweeter than sucrose alone
• Functions as a sweetening agent

Disaccharides: Lactose (Milk Sugar)

• Joined by beta glycosidic bond
• Many people are lactose intolerant, which causes intestinal cramping and bloating due to incomplete digestion

Oligosaccharides

• Important ones are raffinose and stachyose, found in beans and legumes
• Their glycosidic bonds prevent breakdown into simple sugars
• Cannot be absorbed by the small intestine
• Metabolized by bacteria in the large intestine, producing unwanted gaseous byproducts

Trisaccharides: Raffinose

• With acid yields sucrose + galactose
• With yeast yields melibiose + fructose

Trisaccharides: Gentianose

• With acid produces 2 glucose + fructose
• With emulsin produces sucrose + glucose
• With invertase produces fructose + gentiabiose

Tetrasaccharides: Stachyose

• With acid it yields 2 galactose + glucose + fructose

Polysaccharides: Glycans

• Monosaccharide polymers
• Homoglycans consist of one type of monosaccharide unit
• Heteroglycans consist of more than one type of monosaccharide unit

Homoglycans: Starch

• Two forms: amylose and amylopectin
• Amylose is linear, non-branching, less common, and more soluble in water
• Amylopectin is branching, abundant, soluble in water, and has branched a-1,4 glycosidic bonds

Homoglycans: Hetastarch

• Semisynthetic that contains 90% amylopectin
• Used as a plasma expander in the treatment of shock caused by hemorrhage, burns, surgery, and sepsis

Homoglycans: Inulin

• Consists only of fructosan
• The D-fructofuranose is abundant in Asteraceae
• Used in culture media to identify certain bacteria and to evaluate renal function

Homoglycans: Dextrans

• From Leuconostoc mesenteroides
• Used as a 6% solution as a plasma expander in cases of trauma and burns to reduce blood viscosity

Homoglycans: Cellulose

• The most abundant organic compound
• The primary structural element of higher plant cell walls
• Linear with β-1,4, glycosidic bonds

Homoglycans: Purified Cotton/Absorbent Cotton

• From Gossypium hirsutum or G. herbaceum
• Used as a surgical dressing to absorb blood, pus, mucus, and keep bacteria from infecting wounds
• Raw cotton is for textile production
• Powdered Cellulose's purity is defined by its solubility in NaOH solution
• Cellulose that remains insoluble is called α-cellulose
• Used as a pharmaceutical excipient

Celluloses Derivative

• Methylcellulose is used as artificial tears or contact lens solution
• Ethylcellulose is used as a tablet binder and film coating
• Hydroxyethylcellulose is a thickening agent and ingredient in artificial tear formulations
• Hydroxypropylcellulose is used as a stabilizer, thickener, and binder
• Pyroxylin or soluble guncotton, is obtained by mixing nitric and sulfuric acid in cotton
• Purified Rayon is fibrous, bleached, regenerated cellulose used as a surgical aid

Other Homoglycans

• Glycogen is animal starch and reserved carbohydrates of animal tissue
• Xylans, mannans, and galactans are difficult to isolate in pure form
• Hemicellulose occurs in the cell wall with cellulose and pectic substances
• Lichenin or lichen starch resembles cellulose but contains 25% β-1,3 glycosidic bonds

Heteroglycans

• Readily dissolve in water
• Plant hydrocolloids classified as anionic or non-ionic polysaccharides Consist of linear polymer or branched polymer
• Linear polymers are less soluble, yield more viscous solutions, and are less stable (tend to precipitate)
• Branched hydrocolloids form gels instead of viscous solutions and tend to be tacky

Heteroglycans: Sources

• Exudates from trees and shrubs seal bark damage and preserve from dehydration
• Seed gums serve as polysaccharide food reservoirs
• Microbial gums result from fermentation
• Marine gums serve as reserve food
• Plant extracts: pectins

Examples of Gum: Tragacanth (Persian Gum)

• From Astragalus gummifer, Fabaceae
• Contains 60-70% bassorin that swell in water but do not dissolve
• Contains 30% tragacanthin, which is more water soluble
• Used as emulsifying, suspending agent demulcent, and emollient

Examples of Gum: Acacia (Gum Arabic)

• From Acacia senegal, Fabaceae.
• Contains arabin (Ca, Mg, and K salts of Arabic acid).
• In use as suspending agent, demulcent, emollient, adhesive & binder
• Functions as substitute for tragacanth

Examples of Gum: Ghatti Gum (Indian Gum)

• From Anogeissus latifolia, Combretaceae
• Functions as a substitute for acacia
• Forms a more viscous dispersion with cold water
• Contains gallotanins

Examples of Gum: Karaya Gum

• From Sterculia urens, Sterculiaceae
• Least soluble among exudates
• Used as a bulk laxative and suspending agent
• Contains galactose and rhamnose etc

Examples of Gum: Sodium Alginate

• Algin (alginic acid) principal constituent of brown algae)
• From Macrocystis pyrifera, Lessoniaceae
• Use as a suspending agent, tablet binder, thickening agent formation of a firm gel

Examples of Gum: Agar (Japanese Isinglas)

• From Gelidium cartilagineum, Gelidiaceae and Gracilaria confervoides, Gelidiaceae
• Use as a laxative, suspending and emulsifying agent
• Functions as a tablet excipient & disintegrant

Examples of Gum: Carrageenan (Irish Moss)

• From Chondrus crispus / Gigartina mamillosa, Gigartinaceae
• Consists of galactans and carrageenans
• Forms a gel and gives stability to emulsions and suspensions, demulcent bulk laxative

Examples of Gum: Plantago seed

• From Plant Plantago psyllium, Plantaginaceae
• Consists of xylose and arabinose
• The husk is cathartic due to swelling, gives bulk & lubrication when taken with water

Examples of Gum: Xanthan Gum

• From Xanthomonas campestris, Xanthomonadaceae
• Exhibits pseudoplastic properties (baking)

Examples of Gum: Guar Gum

• From Cyamopsis tetragonolobus, Fabaceae
• Consists of Galactomannan from powdered seed
• Functions as as a bulk laxative, tablet and disintegrator

Marshmallow

• Part used is Leaf/roots
• From Althea officinalis, Malvaceae
• Its constituent is mucilage
• Functions as an emollient and laxative

Mullein

• Part used is Flower
• From Verbascum thapsus, V. deusiflorum, Srophulariaceae
• Consists of Flavonoids and apigenin
• Aids in the management of lung conditions and is an expectorant

Couch grass

• Part used is Rhizome
• From Agropyron repens, Poaceae
• Consists of Mucilage and Sugar alcohols
• Functions as a demulcent, diuretic, anti-gout

Aloe vera

• Part used is Latex
• From Aloe vera, A. barbadensis, Liliaceae
• Consists of Glucomannan, Galactorans, and Organic acids
• Functions as a demulcent, diuretic, and anti-gout

Quince

• Part used is Seeds
• From Cydonra oblouga, Rosaceae
• Contains Mulicaginous components and other carbohydrates
• Functions as a demulcent and emulsifying agent

Slippery Elm

• Part used is Bark
• From Ulmus rubra, Ulmaceae
• Contains Mulicaginous components
• Functions as a demulcent and emulsifying agent

Honey

• From Apis meliffera, Apidae
• Contains Invert sugars
• Functions as a sweetening agent

Figs

• From Ficus carica, Moraceae
• Consists of Glucose and is a source of vitamin A,B,C and D
• Functions as a Laxative

Fucus

• From Fucus resicolosus, Fucaceae
• Contains Mucilaginous substances
• Functions as a Bulk laxative

Cetraria

• Aka Iceland moss
• From Cetraria islandica, Parmeliaceae
• Contains Carbohydrates like lichenin
• Functions as a Bitter tonic

Pectin

• Found in the middle lamella of seed bearing plants
• Insoluble form is called protopectin
• Obtained by dilute acid extraction of the inner portion of citrus fruit.

Molisch's Test

• A general tests for carbohydrates
• Reagent used is a-napthol a dehydrating reagent
• A positive result will return a purple color at the junction of two layers.