Difference Between Amylose and Amylopectin

Amylose vs Amylopectin
 

Starch is a carbohydrate which is categorized as a polysaccharide. When ten or higher numbers of monosaccharides are joined by glycosidic bonds, they are known as polysaccharides. Polysaccharides are polymers and, therefore, have a larger molecular weight, typically more than 10000. Monosaccharide is the monomer of this polymer. There can be polysaccharides made out of a single monosaccharide and these are known as homopolysaccharides. These can be also classified based on the type of monosaccharide. For example, if the monosaccharide is glucose, then the monomeric unit is called a glucan. Starch is a glucan like that. Depending on the way the glucose molecules attach to each other, there are branched and unbranched parts in starch. Broadly starch is said to be made of amylose and amylopectin which are larger chains of glucose.

Amylose

This is a part of starch, and it is a polysaccharide. D-glucose molecules are linked to each other in order to form a linear structure called amylose. Large amounts of glucose molecules can participate in forming an amylose molecule. This number can be ranging from 300 to several thousand.  When the D-glucose molecules are in cyclic form, number 1 carbon atom can form a glycosidic bond with the 4^th carbon atom of another glucose molecule. This is called a α-1,4-glycosidic bond. Because of this linkage amylose has obtained a linear structure. There can be three forms of amylose. One is a disordered amorphous form, and there are two other helical forms. One amylose chain can bind with another amylose chain or with another hydrophobic molecule like amylopectin, fatty acid, aromatic compound, etc. When only amylose is in a structure, it is tightly packed because they don’t have branches. So the rigidity of the structure is high.

Amylose makes 20-30% of the structure of starch. Amylose is insoluble in water. Amylose is also the reason for the insolubility of starch. It also reduces the crystallinity of amylopectin.  In plants, amylose is functioning as an energy storage. When amylose is degraded into smaller carbohydrate forms as maltose, they can be used as a source of energy. When performing the iodine test for starch, the iodine molecules are fit into the helical structure of amylose, hence give the dark purple/blue color.

Amylopectin

Amylopectin is a highly branched polysaccharide which is also a part of starch. 70-80% of the starch consists of amylopectin. As in amylose, there are some glucose molecules linked with α-1,4-glycosidic bonds forming a linear structure of amylopectin. However, at some points α-1,6-glycosidic bonds are also formed. These points are known as branching points. Branching is taking place every 24 to 30 glucose units. 2,000 to 200,000 glucose units are participating in the formation of a single amylopectin molecule. Because of this, branching rigidity of amylopectin is lower, and it is soluble in water. Amylopectin can be easily degraded using enzymes. This is a plant energy storage molecule and also a source of energy.