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Describe polysaccharides and identify two types of polysaccharides:
- Polysaccharides are also called Glycans. They differ in degrees of chain length, branching, and types of bonds.
- Homopolysaccharides: structural elements and energy storage.
- Heteropolysaccharides: Extracellular support in all kingdoms of life.
Name some homopolysaccharides that are used for energy storage:
Starch and Glycogen
Name some homopolysaccharides that are used for structure:
cellulose and chitin.
Name a structural heteropolysaccharide:
Peptidoglycan of the bacteria cell wall.
Overall review of Starch and Glycogen:
Starch and Glycogen: stored fuel; occurs in nature as large cellular aggregates (gains, granules), and energy storage in plants (starch) and animals (glycogen). Starch and glycogen molecules are heavily hydrated, because they have many exposed hydroxyl groups available to hydrogen-bond with water.
What does starch consist of?
Two glucose polymers: amylose and amylopectin.
Amylose: long,unbranched glucose chain with (a1à4) linkages. (a1à4)linkages form a coil.
Amylopectin: highly branched (branch points = (a1-->6) linkages).
What is the most stable 3D strucuture for starch?
The most stable three-dimensional structure for the (α1→ 4)-linked chains of starch and glycogen is a tightly coiled helix (Fig. 7-19), stabilized by interchain hydrogen bonds. For amylose, the core of the helix is of precisely the right dimensions to accommodate iodine as complex ions ( and ), giving an intensely blue complex. This interaction is a common qualitative test for amylose.
Describe glycogen: what kind of polymer does it have, what is similar to its structure?
- Glycogen: Glucose polymer with (a1à4) bonds and (a1à6) branches similar to amylopectin, but more branches: one per 8-12 residues. Each branch ends with a non-reducing glucose à no free anomeric carbon. Because each branch in glycogen ends with a nonreducing sugar unit, a glycogen molecule with n branches has n + 1 nonreducing ends, but only one reducing end.
- Note: metabolism- sugars are removed or added as needed from the non-reducing ends.
Describe cellulose: what polymer is it like? how are they different?
- Cellulose: Fibrousand water insoluble. It is a major component of plant cell walls. It is linear,unbranched homopolysaccharide with 10,00-15,000 monomers per molecule. Eachmonomer is turned 180 degrees around the glycosidic bond. (B1à4)linkage, not a coil. Many interchain and interchain hydrogen bonds, but nointerchain covalent bonds. Straight, stable supermolecular structure: hightensile strength and low water content (no place for water hydrogen bonds).
- Like amylose, the cellulose molecule is a linear, unbranched homopolysaccharide, consisting of 10,000 to 15,000 D-glucose units. But there is a very important difference: in cellulose the glucose residues have the β configuration (Fig. 7-14), whereas in amylose the glucose is in the α configuration. The glucose residues in cellulose are linked by (β1→4) glycosidic bonds, in contrast to the (α1→4) bonds of amylose. This difference causes individual molecules of cellulose and amylose to fold differently in space, giving them very different macroscopic structures and physical properties.
Define chitin: how is it different from cellulose?
Chitin: linearhomopolysaccharide of N-acetylglucosamine with beta-linkgages. Only differenceto cellulose is that there is an acetylated amino group instead of hydroxylgroup at C-2. It forms long, fibrous chains like cellulose and forms the hard,water-tight exoskeleton of arthropods.
StrucuralHeteropolysaccharides: Peptidoglycans of the bacterial cell wall definition.
Alternating N-acetylglucosamine and N-acetylmuraminc acid in (B1->4) linkage. Linear polymers arranged side-by-side. Cross linked by short peptides (species specific). Antibiotic penicillin prevents synthesis of peptide cross-links => osmotic lysis of bacteria.