In chemical terms starch is a polysaccharide, i.e. it has large molecules composed of long chains of smaller sugar molecules joined end to end. About three-quarters of the total, depending on the source of the starch, is in the form of amylopectin; this consists of short, interconnected chains making up a many-branched mass comprising thousands of sugar units. The rest is amylose, consisting of single chains from 70 to 350 units long. The length of the chains, and the proportion of amylose present, both affect the performance of the starch.