1.1.8 Starch & Glycogen

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.

Comparison

Feature	Glycogen	Starch
Found in	Animals	Plants
Function	Main energy storage molecule in animals	Main energy storage molecule in plants
Structure	Highly branched with 1,4 and 1,6 glycosidic bonds	Made of two components: amylose (unbranched) and amylopectin (branched)
Branching	More extensively branched than starch	Amylopectin has some branching; amylose is unbranched
Storage Capacity	Compact, stores energy efficiently	Compact, but stores less energy compared to glycogen
Solubility	Insoluble in water (does not affect water potential)	Insoluble in water (does not affect water potential)
Breakdown Speed	Quick release of glucose due to many branch points	Slower breakdown; amylopectin is broken down faster than amylose
Bond Types	1,4 and 1,6 glycosidic bonds in amylose; 1,4 and 1,6 glycosidic bonds in amylopectin