It makes up about 3 percent of a wheat kernel by weight. The germ will grow into a new wheat plant if the kernel is planted. It is made up of tissues high in fiber that protect the kernel, and tissues high in protein that digest starch. It is comprised of starch and protein. The endosperm is the food source for the embryonic plant and becomes the material we commonly call flour.
Millers talk about extraction rates.
An extraction rate is the amount of flour derived from 100 pounds of grain. It can be thought of as a percent. The theoretical maximum pure flour yield is roughly 82 percent - the amount of endosperm in a kernel. The red or white coloration that a kernel has comes from a cell layer that is found in the bran. Red wheat bran flecks are much more obvious in a white flour than those from white wheat bran.