What is Polysaccharide? — Biology

Definition 10.7 University Biology — Year 1 · Chapter 10 — Carbohydrates

A polysaccharide is a chain of hundreds to tens of thousands of monosaccharides joined by glycosidic bonds, linear or branched. Storage polysaccharides: starch in plants — amylose, unbranched $\alpha(1{\to}4)$ glucose, $20\,\%$ , and amylopectin, $\alpha(1{\to}4)$ with $\alpha(1{\to}6)$ branches every $24\text{ to }30\,$ residues, $80\,\%$ — packed in grains; glycogen in animals and fungi, like amylopectin but branched every $8\text{ to }12\,$ residues, in cytosolic granules of the liver and muscles. Structural polysaccharides: cellulose, unbranched $\beta(1{\to}4)$ glucose, $2000\text{ to }15\,000\,$ residues, in plant walls; chitin, $\beta(1{\to}4)$ N-acetylglucosamine, in the walls of fungi and the cuticles of arthropods; peptidoglycan, alternating N-acetylglucosamine and N-acetylmuramic acid cross-linked by short peptides, the one-molecule mesh that is the bacterial wall.

Left: potato starch grains under the microscope, stained by iodine; the concentric rings are layers of amylopectin laid down day by day. Right: a stag beetle’s cuticle — chitin fibres in a protein matrix, hardened and dark: the same (1 4) design as cellulose, on a different sugar. — Left: potato starch grains under the microscope, stained by iodine; the concentric rings are layers of amylopectin laid down day by day. Right: a stag beetle’s cuticle — chitin fibres in a protein matrix, hardened and dark: the same $\beta(1{\to}4)$ design as cellulose, on a different sugar.

Examples

Example 10.9 (Why glycogen is branched)

Glycogen is degraded from its non-reducing ends, one glucose at a time, by glycogen phosphorylase. An unbranched chain of $50\,000$ glucoses would have one such end and release one glucose per enzyme turnover; a glycogen particle of the same size, branched every twelve residues, has some $2000$ ends and releases two thousand at once. Branching also keeps the particle compact and soluble. The liver’s hundred grams of glycogen can be mobilised at ten grams an hour; the same glucose stored as one long chain would take years to unwind.

Example 10.10 (Why not store glucose itself)

A hundred grams of glucose ( $0.55\,\mathrm{mol}$ ) dissolved in the $1\,\mathrm{L}$ of water of a liver’s cells would add $0.55\,\mathrm{osmol}/\mathrm{L}$ to a cytosol at $0.3\,\mathrm{osmol}/\mathrm{L}$ : the cells would swell to three times their volume and burst. As glycogen the same glucose is $7 \times 10^{18}$ particles, $1 \times 10^{-5}\,\mathrm{mol}/\mathrm{L}$ , osmotically invisible. A polymer is a way of storing a great many molecules as one.

Example 10.4 (Sugars that are not $(\mathrm{CH_2O})_n$ )

Deoxyribose lacks the hydroxyl at C2 (Chapter 11); glucosamine and N-acetylglucosamine carry an amine instead of the C2 hydroxyl (chitin, peptidoglycan); glucuronic acid has a carboxyl at C6 (the matrix polysaccharides); sugar phosphates (glucose-6-phosphate, ribose-5-phosphate) are the forms in which sugars enter metabolism; sugar alcohols (glycerol, sorbitol) have no carbonyl. The core is the same: a small polyhydroxylated carbon chain that water loves.

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Examples

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