Editing Endomembrane system (section)

=== Nuclear envelope ===
[[Image:Diagram human cell nucleus.svg|thumb|350px|Diagram of the nucleus with the nuclear envelope shown as the orange portion]]
{{main|Nuclear envelope}}

The [[nuclear envelope]] surrounds the [[cell nucleus|nucleus]], separating its contents from the cytoplasm. It has two membranes, each a [[lipid bilayer]] with associated proteins.<ref>{{cite web| last = Childs| first = Gwen V. | name-list-style = vanc | title = Nuclear Envelope| publisher = UTMB| year = 2003| url = http://cellbio.utmb.edu/CELLBIO/nuclear_envelope.htm| access-date = 2008-09-28|archive-url = https://web.archive.org/web/20060620093536/http://cellbio.utmb.edu/CELLBIO/nuclear_envelope.htm |archive-date = June 20, 2006|url-status=dead}}</ref> The outer nuclear membrane is continuous with the rough endoplasmic reticulum membrane, and like that structure, features [[ribosome]]s attached to the surface. The outer membrane is also continuous with the inner nuclear membrane since the two layers are fused together at numerous tiny holes called [[nuclear pore]]s that perforate the nuclear envelope. These pores are about 120 [[Nanometre|nm]] in diameter and regulate the passage of molecules between the nucleus and cytoplasm, permitting some to pass through the membrane, but not others.<ref name=Cooper>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Nuclear%20Envelope&rid=cooper.section.1323#1324|title=The Nuclear Envelope and Traffic between the Nucleus and Cytoplasm|access-date=2008-12-09|last=Cooper|first=Geoffrey| name-list-style = vanc |year=2000|work= The Cell: A Molecular Approach|publisher=Sinauer Associates, Inc}}</ref> Since the nuclear pores are located in an area of high traffic, they play an important role in [[cell physiology]]. The space between the outer and inner membranes is called the [[perinuclear space]] and is joined with the lumen of the rough ER.

The nuclear envelope's structure is determined by a network of intermediate filaments (protein filaments). This network is organized into a mesh-like lining called the [[nuclear lamina]], which binds to [[chromatin]], integral membrane proteins, and other nuclear components along the inner surface of the nucleus. The nuclear lamina is thought to help materials inside the nucleus reach the nuclear pores and in the disintegration of the nuclear envelope during [[mitosis]] and its reassembly at the end of the process.<ref name=Davidson />

The nuclear pores are highly efficient at selectively allowing the passage of materials to and from the nucleus, because the nuclear envelope has a considerable amount of traffic. [[RNA]] and ribosomal subunits must be continually transferred from the nucleus to the cytoplasm. [[Histone]]s, gene regulatory proteins, [[DNA polymerase|DNA]] and [[RNA polymerase]]s, and other substances essential for nuclear activities must be imported from the cytoplasm. The nuclear envelope of a typical mammalian cell contains 3000–4000 pore complexes. If the cell is synthesizing DNA each pore complex needs to transport about 100 histone molecules per minute. If the cell is growing rapidly, each complex also needs to transport about 6 newly assembled large and small ribosomal subunits per minute from the nucleus to the cytosol, where they are used to synthesize proteins.<ref name=Alberts2>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Nuclear%20Envelope&rid=mboc4.section.2154#2156|title=Nuclear Pore Complexes Perforate the Nuclear Envelope|access-date=2008-12-09|last=Alberts|first=Walter | name-list-style = vanc |year=2002|work= Molecular Biology of the Cell 4th edition|publisher=Garland Science|display-authors=etal}}</ref>