Mammalian cells are eukaryotic this means that all of their cells have common components, membrane bound organelles which prokaryotic cells do not this is shown in figure 1. Organelles are purpose made to carry out a specific function that is necessary within a cell. Due to the specificity of both conditions and concentrations with in these organelles these are compartmentalized. Encasing the organelle in a membrane allows is to do this. In most cases a single phospholipid bilayer is used however in certain organelles such as the mitochondria and the nucleus both an inner and outer membrane are used for greater control and regulation of movement in and out of the organelle.
Figure 1: A diagram of both a prokaryotic and a eukaryotic cell http://www.bacterialphylogeny.com/eukaryotes.html In general the largest organelle within a cell is the nucleus, which is an enclosed by a nuclear envelope which is comprised of two membrane separated by 10-50 nanometers (nm), the inner most of which gives the nucleus its distinctive round shape. This membrane therefore prevents the mixing of genetic material with the rest of the cell and allows it to stay separate and prevent easy diffusion of materials from nucleoplasm to cytoplasm. The outer of these two membranes is uninterrupted with membrane of the rough endoplasmic reticulum and thusly has ribosomes scattered along it. The gap between the inner and outer nuclear membrane is called the perinuclear space and also conjoins with the lumen of the rough Endoplasmic reticulum.
This double membrane layer as well as containing many channel proteins also has many perforations where the two membranes join together to form holes called nuclear pores. These holes are important as they allow the continuous movement of mRNA, rRNA and tRNA out of and into the nucleus in order to translate proteins from an original