Nuclear Membrane The nuclear membrane, also called the nuclear envelope, is actually two separate bilayer
membranes, one inside the other. The outer membrane is continuous with the endoplasmic reticulum of
the cell cytoplasm, and the space between the two nuclear membranes is also continuous with the
space inside the endoplasmic reticulum, as shown in Figure 2-9. The nuclear membrane is penetrated
by several thousand nuclear pores. Large complexes of protein molecules are attached at the edges of
the pores so that the central area of each pore is only about 9 nanometers in diameter. Even this size is
large enough to allow molecules up to 44,000 molecular weight to pass through with reasonable ease.
