Saccharide is a very important organic compound in cells. It contains C, H and O, and their ratio is generally 1:2:1. Its chemical formula is written as Cn(H₂O)n. sometimes, so it is also called carbohydrate. Several monosaccharide molecules are linked into a polysaccharide chain by glycosidic bonds(dehydration reaction), such as starch, glycogen cellulose and chitin.
|Starch||Branched aligned (amylopectin) or linearly aligned (amylose) glucose||It can store energy in the plant.|
|Glycogen||Glycogen has more branches than starch.||It can store energy in the animal, especially in liver and muscle.|
|Cellulose||Linearly, parallelly arranged glucose||Cell wall, protection and support|
|Chitin||nitrogen containing glucose||Exoskeleton of insects, crabs and shrimps|
Glucose produced by photosynthesis is converted to sucrose or starch for storage in plant tissues. Starch is divided into amylose and amylopectin. Amylose is a long unbranched spiral chain that contains a few hundreds or a few thousands of glucose. Amylose is not easily soluble in water. It can form a hydrocolloid when the heated aqueous solution is cooled.
Amylopectin consists of a few thousands or duzens thousands of glucose molecules and is a polysaccharide with branchings; amylopectin is more soluble in water than amylose. Its heated aqueous solution forms a sticky paste when cooled. Natural starch contains these two kinds of starch together.
Amylopectin accounts for about 80% and the remaining is amylose. In the confectionery industry, natural or modified starch is often used as a gelling agent for candies.
Excess glucose in body is converted to glycogen in animals, which is also known as animal starch. Glycogen is similar to amylopectin, but it has more branchings than amylopectin. It is very widely distributed in animals and is most abundant in liver and muscle. Liver glycogen can be interconverted with glucose in the blood, muscle glycogen only provide energy to muscle.
Cellulose is the most abundant organic compound on earth and is the main component of plant cell walls. It is an unbranched chain-like macromolecule composed of glucose. Parallel arranged cellulose molecules are linked by hydrogen bonds to form a strong microfibril. These microfibrils then form strong fibers that provide support and protection to the plant. Cellulose is not digestible by animals directly, but some probiotics in animal (cattle and sheep) bodies can break down it into glucose.
Chitin is a polymer of nitrogenous glucose (one of its hydroxyl groups has been replaced with an acetyl amino group). It is the second most abundant biopolymer on earth after cellulose. It forms the exoskeleton of insects, shrimps and crabs. In the shells of shrimps and crabs calcium ions combine with chitin to form a more rigid exoskeleton. It is also a component of the fungus cell wall.