Editing Chloroplast (section)

=== Specialized chloroplasts in {{C4}} plants ===
{{See also|Photosynthesis|C4 carbon fixation|label 2={{C4}} photosynthesis}}
{{Plain image with caption|File:C4 photosynthesis is less complicated.svg|Many [[C4 plant|{{C4}} plants]] have their [[Mesophyll tissue|mesophyll cells]] and [[bundle sheath cells]] arranged radially around their [[leaf veins]]. The two types of cells contain different types of chloroplasts specialized for a particular part of [[photosynthesis]].|500px|right|bottom|triangle|#00cd4c}}

To fix [[carbon dioxide]] into sugar molecules in the process of [[photosynthesis]], chloroplasts use an enzyme called [[RuBisCO]]. RuBisCO has trouble distinguishing between [[carbon dioxide]] and [[oxygen]], so at high oxygen concentrations, RuBisCO starts accidentally adding oxygen to sugar precursors. This has the result of [[Adenosine triphosphate|ATP]] energy being wasted and {{CO2}} being released, all with no sugar being produced. This is a big problem, since O{{sub|2}} is produced by the initial [[light reactions]] of photosynthesis, causing issues down the line in the [[Calvin cycle]] which uses RuBisCO.<ref name="Campbell-2009d">{{cite book | vauthors=Campbell NA, Reece JB, Urry LA, Cain ML, Wasserman, Minorsky PV, Jackson RB |title=Biology | edition=8th | year=2009 |publisher=Benjamin Cummings (Pearson) | pages=200–201 | isbn=978-0-8053-6844-4 }}</ref>

[[C4 plants|{{C4}} plants]] evolved a way to solve this—by spatially separating the light reactions and the Calvin cycle. The light reactions, which store light energy in [[Adenosine triphosphate|ATP]] and [[NADPH]], are done in the [[mesophyll tissue|mesophyll]] cells of a {{C4}} leaf. The Calvin cycle, which uses the stored energy to make sugar using RuBisCO, is done in the [[bundle sheath cells]], a layer of cells surrounding a [[vein (botany)|vein]] in a [[leaf]].<ref name="Campbell-2009d" />

As a result, chloroplasts in {{C4}} mesophyll cells and bundle sheath cells are specialized for each stage of photosynthesis. In mesophyll cells, chloroplasts are specialized for the light reactions, so they lack [[RuBisCO]], and have normal [[Thylakoid|grana]] and [[thylakoid]]s,<ref name="Gunning-1996a">{{cite book| first1=Brian E S | last1=Gunning | first2=Martin W | last2=Steer | name-list-style=vanc |title=Plant cell biology: structure and function|year=1996|publisher=Jones and Bartlett Publishers|location=Boston, Mass.|isbn=0-86720-504-0|page=[https://archive.org/details/plantcellbiology00gunn_0/page/n137 24]|url=https://archive.org/details/plantcellbiology00gunn_0| url-access=registration }}</ref> which they use to make ATP and NADPH, as well as oxygen. They store {{CO2}} in a four-carbon compound, which is why the process is called [[C4 carbon fixation|''{{C4}} photosynthesis'']]. The four-carbon compound is then transported to the bundle sheath chloroplasts, where it drops off {{CO2}} and returns to the mesophyll. Bundle sheath chloroplasts do not carry out the light reactions, preventing oxygen from building up in them and disrupting RuBisCO activity.<ref name="Campbell-2009d" /> Because of this, they lack thylakoids organized into [[Thylakoid|grana]] stacks—though bundle sheath chloroplasts still have free-floating thylakoids in the stroma where they still carry out [[cyclic electron flow]], a light-driven method of synthesizing [[Adenosine triphosphate|ATP]] to power the Calvin cycle without generating oxygen. They lack [[photosystem II]], and only have [[photosystem I]]—the only protein complex needed for cyclic electron flow.<ref name="Gunning-1996a" /><ref name="Campbell-2009d" /> Because the job of bundle sheath chloroplasts is to carry out the Calvin cycle and make sugar, they often contain large [[starch]] grains.<ref name="Gunning-1996a" />

Both types of chloroplast contain large amounts of [[chloroplast peripheral reticulum]],<ref name="Gunning-1996a" /> which they use to get more [[surface area to volume ratio|surface area]] to transport stuff in and out of them.<ref name="Whatley-1994" /><ref name="Wise-2007a">{{cite book|last=Wise|first=Robert R | name-list-style=vanc |title=The Structure and Function of Plastids|year=2007|publisher=Springer|isbn=978-1-4020-6570-5|pages=17–18|url=https://books.google.com/books?id=FKeCVPbJ3asC&pg=PA17}}</ref> Mesophyll chloroplasts have a little more peripheral reticulum than bundle sheath chloroplasts.<ref name="Lawton-1988">{{cite journal|last=Lawton|first=June R | name-list-style=vanc |title=Ultrastructure of Chloroplast Membranes in Leaves of Maize and Ryegrass as Revealed by Selective Staining Methods|journal=New Phytologist|date=March 1988|volume=108|issue=3|pages=277–283|jstor=2433294|doi=10.1111/j.1469-8137.1988.tb04163.x|pmid=33873933 |doi-access=free|bibcode=1988NewPh.108..277L }}</ref>