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{{main|Protocell}}
[[Tiedosto:Phospholipids aqueous solution structures.svg|thumb|upright|The three main structures [[:en:phospholipid]]s form spontaneously in solution: the [[:en:liposome]] (a closed bilayer), the [[:en:micelle]] and the bilayer.]]
A protocell is a self-organized, endogenously ordered, spherical collection of [[:en:lipid]]s proposed as a stepping-stone to the origin of life.<ref name="Chen 2010">{{Lehtiviite,Irene A. |Tekijä=Chen,Peter Irene A.; Walde, Peter |Otsikko=From Self-Assembled Vesicles to Protocells |www=http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2890201/pdf/cshperspect-ORI-a002170.pdf |Tiedostomuoto=PDF |Julkaisu=Cold Spring Harbor Perspectives in Biology |Julkaisupaikka=Cold Spring Harbor, NY |Julkaisija=Cold Spring Harbor Laboratory Press |AjankohtaVuosi=July 2010 |Vuosikerta=2 |Numero=7 |Sivu=a002170 |Doi=10.1101/cshperspect.a002170 |Issn=1943-0264 |Pmc=2890201 |Pmid=20519344 |Viitattu=2015-06-1519.9.2016 |Kieli={{en}} }}</ref> A central question in evolution is how simple protocells first arose and differed in reproductive contribution to the following generation driving the evolution of life. Although a functional protocell has not yet been achieved in a laboratory setting, the goal appears well within reach.<ref name="Exploring">{{Verkkoviite |www=http://exploringorigins.org/protocells.html |Otsikko=Exploring Life's Origins: Protocells |Sivusto=Exploring Life's Origins: A Virtual Exhibit |Julkaisija=National Science Foundation |Julkaisupaikka=Arlington County, VA |Viitattu=2014-03-18}}</ref><ref name="Chen 2006">{{Lehtiviite |Tekijä=Chen,Irene A. |Ajankohta=8 December 2006 |Otsikko=The Emergence of Cells During the Origin of Life |www=http://www.sciencemag.org/content/314/5805/1558.full |Julkaisu=Science |Julkaisupaikka=Washington, D.C. |Julkaisija=American Association for the Advancement of Science |Vuosikerta=314 |Numero=5805 |Sivut=1558–1559 |Doi=10.1126/science.1137541 |Issn=0036-8075 |Pmid=17158315 |Viitattu=2015-06-15}}</ref><ref name="Discover 2004">{{Lehtiviite |Tekijä=Zimmer,Carl |authorlink=Carl Zimmer |Ajankohta=26 June 2004 |Otsikko=What Came Before DNA? |www=http://discovermagazine.com/2004/jun/cover |Julkaisu=Discover |Julkaisupaikka=Waukesha, WI |Julkaisija=Kalmbach Publishing |Issn=0274-7529}}</ref>
 
Self-assembled vesicles are essential components of primitive cells.<ref name="Chen 2010" /> The [[:en:second law of thermodynamics]] requires that the Universe move in a direction in which disorder (or [[:en:entropy]]) increases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate [[:en:Metabolism|life processes]] from non-living matter.<ref name="SciAm 2007">{{Lehtiviite |Tekijä=Shapiro,Robert |authorlink=Robert Shapiro (chemist) |Ajankohta=June 2007 |Otsikko=A Simpler Origin for Life |www=http://www.scientificamerican.com/article/a-simpler-origin-for-life/ |Julkaisu=Scientific American |Julkaisupaikka=Stuttgart |Julkaisija=Georg von Holtzbrinck Publishing Group |Vuosikerta=296 |Numero=6 |Sivut=46–53 |Doi=10.1038/scientificamerican0607-46 |Issn=0036-8733 |Pmid=17663224 |Viitattu=2015-06-15}}</ref> Researchers Irene A. Chen and Jack W. Szostak amongst others, suggest that simple physicochemical properties of elementary protocells can give rise to essential cellular behaviors, including primitive forms of differential reproduction competition and energy storage. Such cooperative interactions between the membrane and encapsulated contents could greatly simplify the transition from replicating molecules to true cells.<ref name="Chen 2006" /> Furthermore, competition for membrane molecules would favor stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids and even the [[:en:phospholipid]]s of today.<ref name="Chen 2006" /> This [[:en:micro-encapsulation]] allowed for metabolism within the membrane, exchange of small molecules and prevention of passage of large substances across it.<ref>{{harvnb|Chang|2007}}</ref> The main advantages of encapsulation include increased [[:en:solubility]] of the cargo and storing energy in the form of a [[:en:Electrochemical gradient|chemical gradient]].
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