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How does the structure of polysaccharides help them to carry out their function?
- Chains may be folded, making them compact and ideal for storage.
- The size of the molecules make them insoluble, making them ideal for storage as they are osmotically inactive/do not affect water potential of the cell and do not easily diffuse out of the cell.
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Starch has monomers of ___________, and its function is to ___________ formed from any excess __________.
alpha-glucose, reserve food, glucose
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Starch comprises of 20% amylose, 79% amylopectin & 1% phosphates and fatty acids.
- Compact shape means many glucose residues can be stored in a small volume.
- By converting small flucose molecules into a polysaccharide, small molecules are stored in large macromolecules.
- Less water will enter by osmosis as the cell is less concentrated.
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Structure of Amylose
- 300 alpha-glucose units, bonded by alpha-1,4 glycosidic linkages, making a 109 degree angle between 2 C-O bonds
- causing the polymer to twist into a helix.
- OH group of C2 projects out into the middle and form H bonds with each other, stabilising the shape.
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Structure of Amylopectin
- Highly branched chains of 1300-1500 alpha-glucose subunits
- alpha-1,4 glycosidic linkages give helical chains
- alpha-1,6 glycosidic linkages give branch points
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In starch, amylopectin and amylose fit together to form a complex 3D structure in which amylose helices entangle in the branches of amylopectin molecules.
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Glycogen
- comprises of alpha-glucose units with alpha-1,4 glycosidic linkages in helical chains and alpha-1,6 glycosidic linkages where branching occurs.
- similar to amylopectin but has shorter chains and is more highly branched,
- heavier, more compact and insoluble
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Cellulose
- high Mr - 10000 beta-glucose monomers
- glucose molecule rotated 180 degrees, upside down relative to adj molecule, giving a beta-1,4 glycosidic bond
- successive molecules linked 180 degrees to each other by beta-1,4 glycosidic bonds
- which are unreactive and can only by hydrolysed by cellulase.
- (-OH) groups project from both sides of chain form H bonds with each other, forming cross-links
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- In a cell wall, 60-70 molecules become tightly cross-linked forming bundles of microfibrils which run roughly parallel to each other
- Microfibrils are held together by H bonds to form fibres
- Fibres embedded in gel-like organic matrix containing hemicelluloses and pectins, which help transfer the stress to fibres, increasing resistance to compression
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Where are starch, glycogen and cellulose found?
- Starch granules in chlorophyll
- Glycogen granules in cytoplasm of skeletal muscles and liver
- Cell wall of plants
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