What makes up the largest portion of any cereal grain?

The largest portion of any cereal grain is typically the endosperm, which constitutes about 70-80% of the grain's weight. The endosperm is primarily composed of starch and serves as the main energy source for the germinating seed. In addition to starch, it contains proteins and small amounts of vitamins and minerals. The outer bran layer and the inner germ or embryo make up the remaining portions of the grain.