A starch is a complex polysaccharide made up of a large number of glucose as monomeric units joined together by glycosidic bonds.
Starch is long chains of sugar molecules linked together like a chain. A single sugar molecule is a monosaccharide.
Many sugar molecules linked together is a polysaccharide.
Starch, therefore, is a polysaccharide.
Chemical Formula of Starch：
Starch is a chain of glucose molecules, but the chain isn't always straight. Sometimes, the sugar molecules branch off from the main chain and form their own, just like a tree has a main trunk and then branches. As such, starch actually has two forms: one form has no branches while the other form does.
The branchless form is amylose.（直链淀粉）
Amylose is an unbranched chain of glucose molecules joined by 1, 4 glycosidic bonds, as a result of that amylose is coiled and thus it is very compact molecule meaning it can store a lot of energy.
The branched form is Amylopectin.(支链淀粉)
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 it is rapidly digested by enzymes therefore energy is released quickly. It is also a compact molecule.