View source for Crista

{{Short description|Fold in the inner membrane of a mitochondrion}}
{{Other uses}}
{{Cell biology|mitochondrion=yes|background color=#bd0926}}
A '''crista''' ({{IPAc-en|ˈ|k|r|ɪ|s|t|ə}}; {{plural form}}: '''cristae''') is a fold in the [[inner mitochondrial membrane|inner membrane]] of a [[mitochondrion]]. The name is from the Latin for ''crest'' or ''plume'', and it gives the inner membrane its characteristic wrinkled shape, providing a large amount of [[surface area]] for [[chemical reaction]]s to occur on. This aids [[cellular respiration#Aerobic respiration|aerobic cellular respiration]], because the mitochondrion requires [[oxygen]]. Cristae are studded with [[protein]]s, including [[ATP synthase]] and a variety of [[cytochrome]]s.

==Background==

With the discovery of the dual-membrane nature of mitochondria, the pioneers of mitochondrial [[ultrastructure|ultrastructural]] research proposed different models for the organization of the mitochondrial inner membrane.<ref name=Griparic2001>{{cite journal|last1=Griparic|first1=L|last2=van der Bliek|first2=AM|s2cid=9500863|title=The many shapes of mitochondrial membranes.|journal=Traffic|date=August 2003|volume=2|issue=4|pages=235–44|pmid=11285133|doi=10.1034/j.1600-0854.2001.1r008.x| doi-access = free }}</ref> Three models proposed were:

*'''Baffle model''' – According to [[George Emil Palade|Palade]] (1953), the mitochondrial inner membrane is convoluted in a baffle-like manner with broad openings towards the intra-cristal space. This model entered most textbooks and was widely believed for a long time.
*'''Septa model''' – [[Fritiof S. Sjöstrand|Sjöstrand]] (1953) suggested that sheets of inner membrane are spanned like septa (plural of [[septum]]) through the matrix, separating it into several distinct compartments.<ref>{{cite journal|last1=Sjostrand|first1=F|title=Systems of double membranes in the cytoplasm of certain tissue cells|journal=Nature|date=Jan 3, 1953|volume=171|issue=4340|pages=31–32|doi=10.1038/171031a0|s2cid=6765607}}</ref>
*'''Crista junction model''' – Daems and Wisse (1966) proposed that cristae are connected to the inner boundary membrane via tubular structures characterized by rather small diameters, termed crista junctions (CJs). In the middle of 1990s these structures were rediscovered by EM tomography, leading to the establishment of this currently widely accepted model.<ref>{{cite journal|last1=Zick|first1=M|last2=Rabl|first2=R|last3=Reichert|first3=AS|title=Cristae formation-linking ultrastructure and function of mitochondria.|journal=Biochimica et Biophysica Acta (BBA) - Molecular Cell Research|date=January 2009|volume=1793|issue=1|pages=5–19|pmid=18620004|doi=10.1016/j.bbamcr.2008.06.013|doi-access=free}}</ref>

More recent research (2019) finds rows of [[ATP synthase]] dimers (formerly known as "elementary particles" or "oxysomes") forming at the cristae. These membrane-curving dimers have a bent shape, and may be the first step to cristae formation.<ref>{{cite journal | vauthors = Blum TB, Hahn A, Meier T, Davies KM, Kühlbrandt W | title = Dimers of mitochondrial ATP synthase induce membrane curvature and self-assemble into rows | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 116 | issue = 10 | pages = 4250–4255 | date = March 2019 | pmid = 30760595 | doi = 10.1073/pnas.1816556116 | pmc = 6410833| bibcode = 2019PNAS..116.4250B | doi-access = free }}</ref> They are situated at the base of the crista. A mitochondrial contact site cristae organizing system (MICOS) protein complex occupies the crista junction. Proteins like [[Dynamin-like 120 kDa protein|OPA1]] are involved in cristae remodeling.<ref name=baker>{{cite journal |last1=Baker |first1=Nicole |last2=Patel |first2=Jeel |last3=Khacho |first3=Mireille |title=Linking mitochondrial dynamics, cristae remodeling and supercomplex formation: How mitochondrial structure can regulate bioenergetics |journal=Mitochondrion |date=November 2019 |volume=49 |pages=259–268 |doi=10.1016/j.mito.2019.06.003|pmid=31207408 |doi-access=free }}</ref>

Crista are traditionally sorted by shapes into lamellar, tubular, and vesicular cristae.<ref>{{cite journal | vauthors = Hanaki M, Tanaka K, Kashima Y | year = 1985 | title = Scanning electron icroscopic study on mitochondrial cristae in the rat adrenal cortex | journal = Journal of Electron Microscopy | volume = 34 | issue = 4 | pages = 373–380 | pmid = 3837809 | language = en }}</ref> They appear in different cell types. It is debated whether these shapes arise by different pathways.<ref>{{cite journal |last1=Stephan |first1=Till |last2=Roesch |first2=Axel |last3=Riedel |first3=Dietmar |last4=Jakobs |first4=Stefan |title=Live-cell STED nanoscopy of mitochondrial cristae |journal=Scientific Reports |date=27 August 2019 |volume=9 |issue=1 |page=12419 |doi=10.1038/s41598-019-48838-2|pmid=31455826 |pmc=6712041 |bibcode=2019NatSR...912419S }}</ref>

==Electron transport chain of the cristae==
[[File:Blausen_0644_Mitochondria.png|thumb|A [[mitochondrion]], with labeled cristae.]]
{{Main|Electron transport chain}}

[[NADH]] is oxidized into [[NAD+|NAD<sup>+</sup>]], H<sup>+</sup> [[ions]], and [[electrons]] by an [[enzyme]]. [[Flavin adenine dinucleotide|FADH<sub>2</sub>]] is also oxidized into H<sup>+</sup> ions, electrons, and [[Flavin adenine dinucleotide|FAD]]. As those [[electron]]s travel farther through the [[electron transport chain]] in the inner membrane, energy is gradually released and used to pump the hydrogen ions from the splitting of NADH and FADH<sub>2</sub> into the space between the inner membrane and the outer membrane (called the [[intermembrane space]]), creating an [[electrochemical gradient]].

This [[electrochemical gradient]] creates potential energy (see ''{{section link|potential energy|chemical potential energy}}'') across the inner mitochondrial membrane known as the [[proton-motive force]]. As a result, [[chemiosmosis]] occurs, and the enzyme [[ATP synthase]] produces [[Adenosine triphosphate|ATP]] from [[Adenosine diphosphate|ADP]] and a [[phosphate group]]. This harnesses the [[potential energy]] from the concentration gradient formed by the amount of H<sup>+</sup> ions. H<sup>+</sup> ions passively pass into the mitochondrial [[matrix (biology)|matrix]] by the ATP synthase, and later help to re-form H<sub>2</sub>O (water).

The [[electron transport chain]] requires a varying supply of electrons in order to properly function and generate ATP. However, the electrons that have entered the electron transport chain would eventually pile up like cars traveling down a blocked one-way street. Those electrons are finally accepted by [[oxygen]] (O<sub>2</sub>). As a result, they form two molecules of [[water]] (H<sub>2</sub>O). By accepting the electrons, oxygen allows the electron transport chain to continue functioning. The chain is organized in the cristae lumen membrane, i.e. the membrane inside the junction.<ref name=baker/>

The electrons from each NADH molecule can form a total of 3 ATP's from ADPs and phosphate groups through the electron transport chain, while each FADH<sub>2</sub> molecule can produce a total of 2 ATPs.

As a result, 10 NADH molecules (from [[glycolysis]] and the [[Krebs cycle]]), along with 2 FADH<sub>2</sub> molecules, can form a total of 34 ATPs during [[aerobic respiration]] (from a single electron transport chain). This means that combined with the Krebs Cycle and [[glycolysis]], the efficiency for the electron transport chain is about 65%, as compared to only 3.5% efficiency for glycolysis alone.

==Function==
The cristae greatly increase the surface area of the [[inner mitochondrial membrane|inner membrane]] on which the above-mentioned reactions may take place. A widely accepted hypothesis for the function of the cristae is that the high surface area allows an increased capacity for ATP generation. However, the current model is that active [[ATP synthase]] complexes localize preferentially in dimers to the narrow edges of the cristae. Thus, the surface area of mitochondrial membranes allocated to ATP syntheses is actually quite modest.

Mathematical modelling suggested that the optical properties of the cristae in filamentous mitochondria may affect the generation and propagation of light within the tissue.<ref>Thar, R.and M. Kühl (2004). "Propagation of electromagnetic radiation in mitochondria?". ''J.Theoretical Biology'', '''230'''(2), 261-270. [http://www.mbl.ku.dk/MKuhl/pages/PDF/Thar&Kuhl2004.pdf] {{Webarchive|url=https://web.archive.org/web/20130718201720/http://www.mbl.ku.dk/MKuhl/pages/PDF/Thar%26Kuhl2004.pdf |date=2013-07-18 }}</ref>

==References==
<references/>

[[Category:Cellular respiration]]
[[Category:Membrane biology]]