polysaccharides

polysaccharide formation and break down

- a polysaccharide is formed when two or more monosaccharides are joined together by condensation reactions. e.g. lots of a-glucose molecules are joined together by glycosidic bonds to form amylose. - polysaccharides can be broken down into their constituent monosaccharides by condensation reactions. e.g. amylose is hydrolysed into a-glucose molecules.

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 two polysaccharides of a-glucose, amylose and amylopectin :  amylose is a long, unbranched chain of a-glucose. the angles of the glycosidic bonds give it a coiled structure, almost like a cylinder. this makes it compact, so it's really good for storage because you can fir more into a small space.  amylopectin is a long, branched chain of a-glucose. its side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily. this means glucose can be released quickly.  - starch is insoluble in water and doesn't affect water potential so it doesn't cause water to enter the cells by osmosis, which would make them swell. this makes it good for storage. 

glycogen

- animal cells get energy from glucose too. but animals store excess glucose as glycogen - another polysaccharide of alpha-glucose. its structure is very similar to amylopectin, except that it has loads more side branches coming off it. loads of branches means that stored glucose can be released quickly, which is important for energy release in animals. it's also a very compact molecule, so it's good for storage. 

cellulose

- cellulose is 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 fibers called microfibrils. the strong fibers mean cellulose provides structural support for cells (e.g. in plant cell walls). 

the iodine test for starch

- if you want to test for the presence of starch in a sample, you'll need to do the iodine test. - add iodine to dissolved in potassium iodide solution to the test sample. if there is starch present, the sample changes from browny-orange to a dark, blue-black colour.