Examples of some common disaccharides and how they are formed are shown below:

Maltose is a disaccharide formed by condensation of two glucose molecules.

Sucrose is a disaccharide formed by condensation of glucose & fructose.

• Lactose is a disaccharide formed by condensation of glucose & galactose.

Polysaccharides

Polysaccharides are formed from many glucose units joined together and include:

• Glycogen and starch which are both formed by the condensation of alpha glucose.

• Cellulose formed by the condensation of beta glucose.

Glycogen is the main energy storage molecule in animals and is formed from many molecules of alpha glucose joined together by 1, 4 and 1, 6 glycosidic bonds. It has a large number of side branches meaning that energy can be released quickly as enzymes can act simultaneously on these branches. Moreover, it is a relatively large but compact molecule thus maximising the amount of energy it can store. Finally being insoluble means it will not affect the water potential of cells and cannot diffuse out of cells.

Starch stores energy in plants and is a mixture of two polysaccharides called amylose and amylopectin:

• Amylose - amylose is an unbranched chain of glucose molecules joined by 1, 4 glycosidic bonds, and as a result amylose is coiled and thus a very compact molecule storing a lot of energy.

• Amylopectin is branched and is made up of glucose molecules joined by 1, 4 and 1, 6 glycosidic bonds. Due to the presence of many side branches these can be acted upon simultaneously by many enzymes and thus broken down to release its energy.

• Some key properties of starch that make it suitable are that; its insoluble so will not affect cell water potential, it is compact so a lot of energy can be stored in a small space and when it is hydrolysed the released alpha glucose can be transported easily.

Cellulose is a component of cells walls in plants and is composed of long, unbranched chains of beta glucose which are joined by glycosidic bonds. Microfibrils are strong threads which are made of long cellulose chains running parallel to one another that are joined together by hydrogen bonds forming strong cross linkages. Cellulose is important in stopping the cell wall from bursting under osmotic pressure. This is because it exerts inward pressure that stops the influx of water. This means that cells stay turgid and rigid, helping to maximise the surface area of plants for photosynthesis.