In this article, we are discussing Oligosaccharides and Polysaccharides another classification of Biomolecules Carbohydrates. In the previous article, we discussed Monosaccharides. This is another important section to pay attention from the Chapter Biomolecules from which questions are asked every year. Let's begin with a brief intro of Oligosaccharides.
Biomolecules Carbohydrates Notes on Oligosaccharides and Polysaccharides
Carbohydrates consisting of (2 - 9) of monosaccharides are called as oligosaccharides. They can be trisaccharides, tetrasaccharides, hexasaccharide etc.
Two monosaccharides (usually hexose) joined by a glycosidic bond (condensation reaction). It normally forms between C-atoms 1 and 4 of neighboring units (1,4 bond). When Disaccharides are hydrolyzed, they release their respective constituent monosaccharides (e.g., hydrolysis sucrose yields one molecule each of glucose and fructose).
|Disaccharides||Individual Residue||Physiological Role|
|Sucrose (Non- reducing disaccharide)||Glucose α (1→2) β Fructose||End product of Photosynthesis|
|Maltose (reducing disaccharide)||Glucose α (1→4) Glucose||Dimer derived from starch & Glycogen|
|Lactose (reducing disaccharide)||Galactose β (1→4) Glucose||Energy Source in Animals|
|Cellobiose||Glucose β (1→2) Glucose||Dimer of cellulose polymer|
|Gentiobiose||Glucose β (1→6) Glucose||Constituent of plant Glycosides and some Polysaccharides|
|Trehalose||Glucose α (1→1) α Glucose||Major Circulatory sugar in insects|
Three monosaccharide units joined together are called as trisaccharides (E.g. Raffinose: found in plants. Upon hydrolysis, it yields one molecule each of glucose, fructose, and galactose.)
Note: Larger oligosaccharides are present in the cell membrane and play an important role in cell recognition & in antigen specificity.
- Polymers of monosaccharides and are branched or unbranched linear molecular chains.
- Insoluble carbohydrates and are commonly called non—sugars.
Classification of Polysaccharides: Based on the type of Monosaccharide present
- Homopolysaccharides — only one type of monosaccharide present e.g. starch, glycogen and cellulose, fructan, xylan, arabinan.
- Heteropolysaccharides — more than one type of monosaccharide present e.g. chitin, agar, arabinogalactans, arabinoxylan etc.
Classification of Polysaccharides: Based on their Function
Polysaccharides can also be categorized as — storage (e.g. starch and glycogen), structural (e.g. chitin, cellulose) and mucopolysaccharides (e.g. keratin sulfate, chondroitin sulfate, hyaluronic acid, agar, alginic acid, carrageen, and heparin). Their formation requires the removal of many molecules of water from monosaccharides. This helps in condensing the bulk to be stored. Unlike small carbohydrates, polysaccharides are relatively easy to store. When necessary polysaccharides are broken down by enzymes for the release of energy.
1. Storage Polysaccharides - act as food storage polysaccharides
- The polymer of glucose.
- Major reserve food in plants.
- Starch has two components — amylose (an unbranched polymer- straight chain of 200 -1000 glucose units; 1-4 glycosidic linkage between a - D glucose molecules.) and amylopectin (a branched polymer -2000 – 200,000 glucose molecules branched after 25 glucose units; branching point has 1-6 glycosidic linkage.)
- found abundantly in rice, wheat and other cereal grains legumes, potato, tapioca and bananas
- Animal equivalent of starch; fungi also store it. Glycogen turns red-violet with iodine. It consists of 30,000 glucose units joined by a, 1 - 4 bonds, much more branched than starch. Branching point has a, 1 - 6 linkage, branching occurs after 10 - 14 glucose units.
- found in liver and muscles stores energy in mammals.
- Unusual polysaccharide (polymer of fructose). It is used as a fructose, particularly in roots and tubers (e.g. Dahlia tubers).
- dietary fiber; used as a prebiotic agent to stimulate the growth of good intestinal bacteria.
2. Structural Polysaccharides - important components of cell wall & cell membrane
- Main structural unbranched homopolysaccharide of plants.
- Cellulose is the most abundant organic compound in the biosphere
- One molecule of cellulose has about 6000 glucose residues.
- Used for building the cell wall. Wood and cotton contain large quantities of cellulose.
- similar to cellulose but its basic unit is not glucose, but a nitrogen-containing similar molecule (N-acetylglucosamine)
- A polysaccharide found in the exoskeletons of fungi, insects & crustaceans.
- Soft and leathery, it becomes hard when impregnated with calcium carbonate or certain proteins.
- Insoluble- helps to retain the form and to strengthen the structure of organisms.
c) Pectin and hemicellulose
- Made up of arabinose, galactose, and galacturonic acid. Pectic acid is an acidic polysaccharide Of the methyl ester of D—galacturonic acid.
Some Important Polysaccharides
|Murein||N-acetyl glucosamine + N -acetyl muramic acid (β1,4 Linkages)|
|Dextran||D- Glucose (β1,4 Linkages); Branch Linkages - β1,3 Linkages|
|Inulin||D-fructose (β2,6 Linkages)|
|Chitin||N- acetyl-glucosamine (β1,4 Linkages)|
|Glycogen||D- Glucose α1,4 Linkages; Branch Linkages - α1,6 Linkages|
|Callose||D- Glucose (β1,3 Linkages)|
|Keratin sulfate||D- Galactose, N- acetylglucosamine - provides strength and flexibility to skin and cornea.|
D-glucuronic acid & N- acetyl glucosamine. It is anticoagulant present in human blood. The husk of Plantago ovata and mucilage of Aloe barbedense are medicinally used.
D-Glucuronic acid and N- acetyl galactosamine- present in the vitreous humor of the eye, synovial fluid, and cerebrospinal fluid etc.
All the best!