Disaccharides (double sugars)
Dissacharides are formed from the condensation of two monosaccharides.
e.g. glucose + glucose = maltose (malt sugar)
glucose + fructose = sucrose (cane sugar)
glucose + galactose =lactose (milk sugars)
These three are disaccharides and have the basic formula C12H22O11.
Some disaccharides, e.g. maltose will reduce Benedicts reagent and some e.g. sucrose are non reducing sugars. All disaccharides are sweet, solublein water and are readily converted into monosaccharides when water molecules are added by a process called hydrolysis.
fig 2.1 formation of disaccharides - click on picture to enlarge
Lactose
Lactose is a disaccharide sugar which consists of a glucose residue linked to galactose (another hexose sugar [isomer of glucose]) residue by a glycosidic bond. It is the sugar found in milk. When milk is allowed to go sour, the lactobacillus bacteria uses the lactose in their metabolism and produce LACTIC ACID. The more acidic conditionstake the milk protien out of their normal pH range and they denature and precipitate out as curds - when the liqour is strained off, the curds can then be futher 'matured' by bacterial activity to make cheese.
Polysaccharides (long chains)
Polysaccharides form when many monosaccharides join together through condensation. Polysaccharides are non reducing, not sweet and insoluble. In the same way that two monosaccharides may combine to pair to give disaccharides, many monosaccharides may combine by condensation reactions to give a polysaccharides. The number of monosaccharides which combine is variable and the chain produced can be branched and unbranched. The chain may be folded, thus, making them compact and more easier for storage. The size of the molecules make them insoluble - another feature which suites them for storage as they exert no osmotic influence and do not diffuse out of th cell. Upon hydrolysism, polysaccharides can be converted to their constituent substrates. Starch and glycogen are an example of storage polysaccharides. Not all polysaccharides are used for storage; cellulose, for example, is a structural polysaccharide giving support and strength to cell walls.
Starch
Starch is a polysaccharide which is found inmost parts of the plant in the form of small granules. It is reserved food formed from any excess glucose produced through photosynthesis. It is common in the seeds of some plants e.g. maize, where it forms the food supply for germination. Indirectly these starch stores form an important food supply for animals.
Starch is a mixture of two substances; amylos and amylopectin. Starch differs from one plant species to the next, but on the hole comprise 20% amylose and 79%amylopectin and 1% of other substances such as phosphates and fatty acids. A comparison of amylose and amylopectin is shown here:
Glycogen
Glycogen is a major polysaccharide storage in animals and fungi and is often called 'animal starch'. It is stored mainly in the liver and muscles. Like starch it is made up of a glucose moleculesand exist as granules. It is similar to amylopectin in structure but it has shorter chains and is highly branched.
Cellulose
Cellulose typacilly comprises upto 50% of a plant cell wall and in cotton it makes up 90%. It is a polymeme of around 10,000 b glucose molecular forming a long unbranched chain. Many chains run parrallel to each other and have cross linkage between them. These help to give celluloseits considerable stability which makes it a valuble structural material. The stability of cellulose makes it difficult to digest and therefore not such a valuble food sourceto animals, which only rarely produce cellulose digesting enzymes. Some however, have formed symbolic relationships with organism which major componentsof their diet. Celluloses structural strenght has long been recognised by man. Cottonis used in the manufacture of fabrics. Deyon is produced from cellulose extracted from wood and its remarkable tensile strength makes it especially useful in the manufacture of tyre cords. Also cellophane, used in packaging.