A polysaccharide is a polymer made of many monosaccharide units, often thousands, joined by glycosidic bonds formed in condensation reactions. Each new bond releases a molecule of water.Polysaccharides can be unbranched chains or branched, and the type of monomer and the type of glycosidic bond decide the shape and properties:• starch: made of α-glucose; the energy store of plants
• glycogen: made of α-glucose; the energy store of animals and fungi
• cellulose: made of β-glucose; a structural material in plant cell wallsWhy polysaccharides make good stores. Glucose itself is soluble. Storing lots of it in a cell would lower the water potential, so water would enter by osmosis and the cell could swell or burst. Polysaccharides are very large molecules, so they are insoluble (or only slightly soluble) and osmotically inactive. They are also too big to diffuse out of the cell, and can be packed into a small space.When energy is needed, the glycosidic bonds are broken by hydrolysis, catalysed by enzymes such as amylase, releasing glucose for respiration.Polysaccharides are not sugars: they are not sweet and they do not give a positive Benedict's test.Worked example: a polysaccharide chain contains 3000 glucose units in a single unbranched chain. The number of glycosidic bonds is 3000 − 1 = 2999, and complete hydrolysis would use 2999 molecules of water.
• glycogen: made of α-glucose; the energy store of animals and fungi
• cellulose: made of β-glucose; a structural material in plant cell wallsWhy polysaccharides make good stores. Glucose itself is soluble. Storing lots of it in a cell would lower the water potential, so water would enter by osmosis and the cell could swell or burst. Polysaccharides are very large molecules, so they are insoluble (or only slightly soluble) and osmotically inactive. They are also too big to diffuse out of the cell, and can be packed into a small space.When energy is needed, the glycosidic bonds are broken by hydrolysis, catalysed by enzymes such as amylase, releasing glucose for respiration.Polysaccharides are not sugars: they are not sweet and they do not give a positive Benedict's test.Worked example: a polysaccharide chain contains 3000 glucose units in a single unbranched chain. The number of glycosidic bonds is 3000 − 1 = 2999, and complete hydrolysis would use 2999 molecules of water.