3.3 Eukaryotic Cells - Concepts of Biology - 1st Canadian Edition Notice that the nuclear envelope consists of two phospholipid bilayers (membranes)—an outer membrane and an inner membrane—in contrast to the plasma membrane, which consists of only one phospholipid bilayer. The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus (Figure 3.11). Both the ...