As related to HIV: HIV is spherical in shape with a diameter of 1/10,000 of a millimeter. The outer coat, or
envelope, is composed of two layers of fat-like molecules called lipids, taken from the membranes of human cells. Embedded in the
envelope are numerous cellular
proteins, as well as mushroom-shaped HIV
proteins that protrude from the surface. Each mushroom is thought toconsist of a cap made of four
glycoprotein molecules called
gp120, and a stem consisting of four
gp41 molecules embedded in the
envelope. The
virus uses these
proteins to attach to and infect cells.