Editing Chloroplast (section)

=== Outer chloroplast membrane ===

{{main|Chloroplast membrane}}

The outer chloroplast membrane is a semi-porous membrane that small molecules and [[ions]] can easily diffuse across.<ref>{{cite journal | vauthors=Koike H, Yoshio M, Kashino Y, Satoh K | title=Polypeptide composition of envelopes of spinach chloroplasts: two major proteins occupy 90% of outer envelope membranes | journal=Plant & Cell Physiology | volume=39 | issue=5 | pages=526–32 | date=May 1998 | pmid=9664716 | doi=10.1093/oxfordjournals.pcp.a029400 | doi-access=free }}</ref> However, it is not permeable to larger [[protein]]s, so chloroplast [[polypeptides]] being synthesized in the cell [[cytoplasm]] must be transported across the outer chloroplast membrane by the [[TOC complex]], or ''[[translocon|'''t'''ranslocon]] on the '''o'''uter '''c'''hloroplast'' membrane.<ref name="Soll-2004" />

The chloroplast membranes sometimes protrude out into the cytoplasm, forming a [[stromule]], or [[stroma (fluid)|'''strom'''a]]-containing tub'''ule'''. Stromules are very rare in chloroplasts, and are much more common in other [[plastids]] like [[chromoplasts]] and [[amyloplasts]] in petals and roots, respectively.<ref name="Köhler-2000">{{cite journal | vauthors=Köhler RH, Hanson MR | title=Plastid tubules of higher plants are tissue-specific and developmentally regulated | journal=Journal of Cell Science | volume=113 | issue=Pt 1 | pages=81–9 | date=January 2000 | doi=10.1242/jcs.113.1.81 | pmid=10591627 | url=http://jcs.biologists.org/cgi/pmidlookup?view=long&pmid=10591627 | url-status=live| archive-url=https://web.archive.org/web/20160920012721/http://jcs.biologists.org/cgi/pmidlookup?view=long&pmid=10591627 | archive-date=20 September 2016 }}</ref><ref name="Gray-2001">{{cite journal |vauthors=Gray JC, Sullivan JA, Hibberd JM, Hansen MR |title=Stromules: mobile protrusions and interconnections between plastids |journal=Plant Biology |volume=3 |issue=3|pages=223–33 |year=2001 |doi=10.1055/s-2001-15204|bibcode=2001PlBio...3..223G |s2cid=84474739 }}</ref> They may exist to increase the chloroplast's [[surface area to volume ratio|surface area]] for cross-membrane transport, because they are often branched and tangled with the [[endoplasmic reticulum]].<ref name="Schattat-2011">{{cite journal | vauthors=Schattat M, Barton K, Baudisch B, Klösgen RB, Mathur J | title=Plastid stromule branching coincides with contiguous endoplasmic reticulum dynamics | journal=Plant Physiology | volume=155 | issue=4 | pages=1667–77 | date=April 2011 | pmid=21273446 | pmc=3091094 | doi=10.1104/pp.110.170480 }}</ref> When they were first observed in 1962, some plant biologists dismissed the structures as artifactual, claiming that stromules were just oddly shaped chloroplasts with constricted regions or [[dividing chloroplasts]].<ref name="Schattat-2012">{{cite journal | vauthors=Schattat MH, Griffiths S, Mathur N, Barton K, Wozny MR, Dunn N, Greenwood JS, Mathur J | display-authors=6 | title=Differential coloring reveals that plastids do not form networks for exchanging macromolecules | journal=The Plant Cell | volume=24 | issue=4 | pages=1465–77 | date=April 2012 | pmid=22474180 | pmc=3398557 | doi=10.1105/tpc.111.095398 | bibcode=2012PlanC..24.1465S }}</ref> However, there is a growing body of evidence that stromules are functional, integral features of plant cell plastids, not merely artifacts.<ref>{{cite journal | vauthors=Brunkard JO, Runkel AM, Zambryski PC | title=Chloroplasts extend stromules independently and in response to internal redox signals | journal=Proceedings of the National Academy of Sciences of the United States of America | volume=112 | issue=32 | pages=10044–9 | date=August 2015 | pmid=26150490 | pmc=4538653 | doi=10.1073/pnas.1511570112 | bibcode=2015PNAS..11210044B | doi-access=free }}</ref>