Editing Chloroplast (section)

==== Thylakoid structure ====

[[File:Thylakoid Structure.jpg|thumb|660px|'''Granum-stroma assembly structure''' The prevailing model of the granum-stroma assembly is stacks of granal thylakoids wrapped by right-handed helical stromal thylakoids which are connected to large parallel sheets of stromal thylakoids and adjacent right-handed helices by left-handed helical structures. (Based on<ref name="Bussi-2019" />).]]

Using a [[light microscope]], it is just barely possible to see tiny green granules—which were named [[Thylakoid|grana]].<ref name="Burgess-1989a" /> With [[electron microscopy]], it became possible to see the thylakoid system in more detail, revealing it to consist of stacks of flat [[thylakoid]]s which made up the grana, and long interconnecting stromal thylakoids which linked different grana.<ref name="Burgess-1989a" />
In the [[transmission electron microscope]], thylakoid membranes appear as alternating light-and-dark bands, 8.5 nanometers thick.<ref name="Burgess-1989a" />

The three-dimensional structure of the thylakoid membrane system has been disputed. Many models have been proposed, the most prevalent being the [[Helix|helical]] model, in which granum stacks of thylakoids are wrapped by helical stromal thylakoids.<ref name="Paolillo-1970">{{cite journal | author1=Paolillo Jr, DJ | title=The three-dimensional arrangement of intergranal lamellae in chloroplasts | journal= J Cell Sci | year=1970 | pmid=5417695 | volume=6 | issue=1| pages=243–55| doi=10.1242/jcs.6.1.243 }}</ref> Another model known as the 'bifurcation model', which was based on the first electron tomography study of plant thylakoid membranes, depicts the stromal membranes as wide lamellar sheets perpendicular to the grana columns which bifurcates into multiple parallel discs forming the granum-stroma assembly.<ref name="Reich Z-2005">{{cite journal | title=Three-dimensional organization of higher-plant chloroplast thylakoid membranes revealed by electron tomography | journal=Plant Cell | volume=17 | issue=9 | pages=2580–6 | year=2005 | pmid=16055630 | doi=10.1105/tpc.105.035030 | author1=Shimoni E | author2=Rav-Hon O | author3=Ohad I | author4=Brumfeld V | author5=Reich Z | pmc=1197436| bibcode=2005PlanC..17.2580S }}</ref> The helical model was supported by several additional works,<ref name="Mustárdy-2008">{{cite journal | vauthors=Mustárdy L, Buttle K, Steinbach G, Garab G | title=The three-dimensional network of the thylakoid membranes in plants: quasihelical model of the granum-stroma assembly | journal=The Plant Cell | volume=20 | issue=10 | pages=2552–7 | date=October 2008 | pmid=18952780 | pmc=2590735 | doi=10.1105/tpc.108.059147 | bibcode=2008PlanC..20.2552M }}</ref><ref name="Austin-2011">{{cite journal | vauthors=Austin JR, Staehelin LA | title=Three-dimensional architecture of grana and stroma thylakoids of higher plants as determined by electron tomography | journal=Plant Physiology | volume=155 | issue=4 | pages=1601–11 | date=April 2011 | pmid=21224341 | pmc=3091084 | doi=10.1104/pp.110.170647 }}</ref> but ultimately it was determined in 2019 that features from both the helical and bifurcation models are consolidated by newly discovered left-handed helical membrane junctions.<ref name="Bussi-2019">{{cite journal | title=Fundamental helical geometry consolidates the plant photosynthetic membrane | journal=Proc Natl Acad Sci USA | volume=116 | issue=44 | pages=22366–22375 | year=2019 | pmid=31611387 | doi=10.1073/pnas.1905994116 | author1=Bussi Y | author2=Shimoni E | author3=Weiner A | author4=Kapon R | author5=Charuvi D | author6=Nevo R | author7=Efrati E | author8=Reich Z | pmc=6825288| bibcode=2019PNAS..11622366B | doi-access=free }}</ref> Likely for ease, the thylakoid system is still commonly depicted by older "hub and spoke" models where the grana are connected to each other by tubes of stromal thylakoids.<ref>{{cite web | url=https://www.sciencephoto.com/media/911533/view/chloroplast-in-a-plant-cell | title=Chloroplast in a plant cell | publisher=TUMEGGY / SCIENCE PHOTO LIBRARY | access-date=19 August 2020}}</ref>

Grana consist of a stacks of flattened circular granal thylakoids that resemble pancakes. Each granum can contain anywhere from two to a hundred thylakoids,<ref name="Burgess-1989a" /> though grana with 10–20 thylakoids are most common.<ref name="Mustárdy-2008" /> Wrapped around the grana are multiple parallel right-handed helical stromal thylakoids, also known as frets or lamellar thylakoids. The helices ascend at an angle of ~20°, connecting to each granal thylakoid at a bridge-like slit junction.<ref name="Mustárdy-2008" /><ref name="Austin-2011" /><ref name="Bussi-2019" />

The stroma lamellae extend as large sheets perpendicular to the grana columns. These sheets are connected to the right-handed helices either directly or through bifurcations that form left-handed helical membrane surfaces.<ref name="Bussi-2019" /> The left-handed helical surfaces have a similar tilt angle to the right-handed helices (~20°), but ¼ the pitch. Approximately 4 left-handed helical junctions are present per granum, resulting in a pitch-balanced array of right- and left-handed helical membrane surfaces of different radii and pitch that consolidate the network with minimal surface and bending energies.<ref name="Bussi-2019" /> While different parts of the thylakoid system contain different membrane proteins, the thylakoid membranes are continuous and the thylakoid space they enclose form a single continuous labyrinth.<ref name="Mustárdy-2008" />