polysaccharides

• a polysaccharide is formed when more than 2 monosaccharides are joined together by condensation reactions
• polysaccharides can be broken don into their constituent monosaccharides by hydrolysis reactions
• starch - cells get energy from glucose - plants store excess glucose as starch (when a plant needs more glucose for energy, it breaks down starch to release the glucose)
• starch is a mixture of alpha glucose - amylose and amylopectin
• amylose is a long, unbranched chain of alpha glucose. the angles of the glycosidic bonds give it a coiled structure, almost like a cylinder - makes it compact - good for storage - can fit more in a small space
• amylopectin is a long, branched chain of alpha glucose - side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily - this means that glucose can be released easily
• starch is insoluble in water - doesn't affect water potential so it doesn't cause water to ender cells by osmosis which would make them swell - makes it good for storage
• glycogen - animals get energy from glucose too but store excess glucose as glycogen - another polysaccharide of alpha glucose. structure very similar to amylopectin but has more side branches coming off it. more branches means that stored glucose can be released quickly which is important for energy release in animals - very compact molecule - good for storage
• cellulose - made of long unbranched chains of beta-glucose. when beta-glucose molecules bond they form straight cellulose chains. the cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils - strong fibres mean cellulose provides structural support for cells (e.g. plant cell walls)
• starch is a large molecule - can't leave the cell - good storage molecule
• glycogen acts as an energy store/ reserve
• hydrogen bonds between alpha glucose molecules help to hold amylose in its helical structure
• if you want to test for the presence of starch - iodine test
• add iodine dissolved in potassium iodide solution to the test sample - if starch is present sample changes from browny-orange to dark blue-black colour