a) Storage polysaccharides
- Starch, galactomanans and inulin are important storage polysaccharides in plants.
Starch
- The principal food-reserve polysaccharide in the plant kingdom is starch. It forms the major source of carbohydrate in the human diet. Starch has been found in some protozoa, bacteria and algae. But the major source is plants where it occurs in the seeds, fruits, leaves, tubers and bulbs in varying amount from a few percent to over 74%. Starch is an alpha-glucan that has structurally distinct components called amylose and amylopectin. A third component referred as the intermediate fraction has also been identified in some starches.
- Starch molecules are organized into quasicrystalline macromolecular aggregates called granules. The shape of the granules are characteristics of the source of the starch. The two components, amylose and amylopectin, vary in amount among the different sources from less than 2% of amylose in waxy rice or waxy maize to about 80% amylose in amylomaize. The majority of starches contain 15 to 35% of amylose. The ratio of amylose and amylopectin is a function of the enzymes, granulosis bound starch synthase (GBSS) and soluble starch synthase (SSS). GBSS is able to synthesise amylose in a form that is not a substrate for branching enzyme to form amylopectin. Waxy mutants containing only amylopectin lack the GBSS but still contain soluble starch synthase.
- Amylose is made up of α - D-glucose units linked mostly in a linear way by 1–4 linkages. It has a molecular weight of 150,000 to 1,000,000 depending on its biological origin. It has been shown that amylose has some elements of nonlinearity. It consists of a mixture of linear molecules with limited, long-chain branching involving œ1 -6 linkages. The branches contain several hundred glucose residues. Amylose gives a characteristic blue color with iodine due to the ability of the iodine to occupy a position in the interior of a helical coil of glucose units. Pure amylose binds 20% of iodine at 20ºC
Amylopectin
- Amylopectin is a branched, water-insoluble polymer comprised of thousands of D-glucose residues. The main chain of amylopectin consists of D-glucose residues joined by α (1 →4) glycosidic bonds. Side chains of glucose residues are attached to the main chain by α (1→6) glycosidic bond. Each chain contains 15-25 glucose residues joined by α (1→4) bonds. It contains 94-96% α 1→4 and 4-6% α - 1→6 linkages. The molecular weight of amylopectin is in the order of 107 - 108. Robin and co-workers have proposed a model for amylopectin.In this model, A and B chains are linear and have degree of polymerization as 15 and 45 respectively.
- The B chain form the backbone of the amylopectin molecule and extend over two or more clusters. Each cluster of A chain are primarily responsible for the crystalline regions within the granule. The intercrystalline regions occur at regular intervals (60 - 70 ºA) containing the majority of α - 1 - 6 linkages. The amylopectin molecule is 100 - 150 A in diameter and 1200-4000 A long. Within the granule, amylose may be located between amylopectin molecules and associated with the linear regions of the amylopectin molecule. Amylopectin produces a purple to red color with iodine
Inulin
- Inulin is a non-digestible fructosyl oligosaccharide found naturally in more than 36000 types of plants. It is a storage polysaccharide found in onion, garlic, chicory, artichoke, asparagus, banana, wheat and rye. It consists of mainly,if not exclusively, of ß- 2→1 fructosyl-fructose links. A starting glucose moiety can be present,but is not necessary. Inulin is a soluble fibre that helps maintain normal bowel . Function,decreases constipation,lowers cholesterol and triglycerides. It is used for fat replacement and fibre enrichment in processed foods
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Last modified: Tuesday, 27 March 2012, 6:53 PM
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