Editing Starch (section)

==Energy store of plants==
[[File:Starch granules of potato02.jpg|thumb|Potato starch granules in [[Cell (biology)|cells]] of the potato]]
[[File:Starch Granules in Endosperm of Zea Mays Embryo by Phase Contrast (40725881153).jpg|thumb|Starch in endosperm in embryonic phase of maize seed]]
Plants produce [[glucose]] from [[carbon dioxide]] and water by [[photosynthesis]]. The glucose is used to generate the chemical energy required for general [[metabolism]] as well as a precursor to myriad organic building blocks such as [[nucleic acids]], [[lipids]], [[proteins]], and structural polysaccharides such as [[cellulose]]. Most green plants store any extra glucose in the form of starch, which is packed into semicrystalline granules called starch granules or [[amyloplast]]s.<ref name="Zobel starch review">{{cite journal |last1=Zobel |first1=H.F. |title=Molecules to granules: a comprehensive starch review |journal=Starch - Stärke |date=1988 |volume=40 |issue=2 |pages=44–50 |doi=10.1002/star.19880400203 }}</ref>  Toward the end of the growing season, starch accumulates in twigs of trees near the buds. [[Fruit]], [[seed]]s, [[rhizome]]s, and [[tuber]]s store starch to prepare for the next growing season. Young plants live on this stored energy in their roots, seeds, and fruits until they can find suitable soil in which to grow.<ref>{{cite journal |last1=Bailey |first1=E.H.S. |last2=Long |first2=W.S. |title=On the occurrence of starch in green fruits |journal=Transactions of the Kansas Academy of Science |date=Jan 14, 1916 – Jan 13, 1917 |volume=28 |pages=153–155 |doi=10.2307/3624346 |jstor=3624346 }}</ref>  The starch is also consumed at night when photosynthesis is not occurring.<!-- An exception is the family [[Asteraceae]] (asters, daisies and sunflowers), where starch is replaced by the [[fructan]] [[inulin]]. Inulin-like fructans are also present in grasses such as [[wheat]], in [[onion]]s and [[garlic]], [[bananas]], and [[asparagus]].<ref name="Vijn">{{cite journal |title=Fructan: more than a reserve carbohydrate? |first1=Irma |last1=Vijn | first2=Sjef |last2=Smeekens |journal=Plant Physiology |date=1999 |volume=120 |issue=2 |pages=351–360 |doi=10.1104/pp.120.2.351 |pmid=10364386 |doi-access=free|pmc=1539216 }}</ref>-->

Green algae and land-plants store their starch in the [[plastid]]s, whereas [[red algae]], [[glaucophyte]]s, [[cryptomonad]]s, [[dinoflagellate]]s and the parasitic [[apicomplexa]] store a similar type of polysaccharide called [[floridean starch]] in their [[cytosol]] or [[periplast]].<ref>{{cite journal | pmid=19940244 | year=2009 | last1=Dauvillée | first1=D. | last2=Deschamps | first2=P. | last3=Ral | first3=J. P. | last4=Plancke | first4=C. | last5=Putaux | first5=J. L. | last6=Devassine | first6=J. | last7=Durand-Terrasson | first7=A. | last8=Devin | first8=A. | last9=Ball | first9=S. G. | title=Genetic dissection of floridean starch synthesis in the cytosol of the model dinoflagellate Crypthecodinium cohnii | journal=Proceedings of the National Academy of Sciences of the United States of America | volume=106 | issue=50 | pages=21126–21130 | doi=10.1073/pnas.0907424106 | pmc=2795531 | bibcode=2009PNAS..10621126D | doi-access=free }}</ref>

Especially when hydrated, glucose takes up much space and is [[osmosis|osmotically]] active. Starch, on the other hand, being insoluble and therefore osmotically inactive, can be stored much more compactly. The semicrystalline granules generally consist of concentric layers of amylose and amylopectin which can be made bioavailable upon cellular demand in the plant.<ref name="Blennow review">{{cite journal |last1=Blennow |first1=Andreas |last2=Engelsen |first2=Soren B |title=Helix-breaking news: fighting crystalline starch energy deposits in the cell |journal=Trends in Plant Science |date=10 Feb 2010 |volume=15 |issue=4 |pages=236–40 |doi=10.1016/j.tplants.2010.01.009 |pmid=20149714 |bibcode=2010TPS....15..236B }}</ref>

Amylose consists of long chains derived from glucose molecules connected by α-1,4-[[glycosidic bond|glycosidic linkage]]. Amylopectin is highly branched but also derived from glucose interconnected by α-1,6-[[glycosidic bond|glycosidic linkages.]] The same type of linkage is found in the animal reserve polysaccharide [[glycogen]]. By contrast, many structural polysaccharides such as [[chitin]], cellulose, and [[peptidoglycan]] are linked by [[glycosidic bond|β-glycosidic bonds]], which are more resistant to hydrolysis.<ref>{{cite journal|last1=Zeeman|first1=Samuel C.|last2=Kossmann|first2=Jens|last3=Smith|first3=Alison M.|title=Starch: Its Metabolism, Evolution, and Biotechnological Modification in Plants|journal=Annual Review of Plant Biology|date=June 2, 2010|volume=61|issue=1|pages=209–234|doi=10.1146/annurev-arplant-042809-112301|pmid=20192737}}</ref>

===Structure of starch particles===
Within plants, starch is stored in semi-crystalline granules. Each plant species has a distinctive starch granular size: rice starch is relatively small (about 2&nbsp;μm), [[potato starch]]es have larger granules (up to 100&nbsp;μm) while wheat and tapioca fall in-between.<ref name="Rosicka-Kaczmarek wheat chapter">{{cite book |last1=Rosicka-Kaczmarek |first1=Justyna |last2=Kwasniewska-Karolak |first2=Izabella |last3=Nebesny |first3=Ewa |last4=Komisarczyk |first4=Aleksandra |chapter=The Functionality of Wheat Starch |title=Starch in Food |date=2018 |publisher=Woodhead Publishing |location=Duxford, United Kingdom |isbn=978-0-08-100868-3 |page=331 |url=https://www.elsevier.com/books/starch-in-food/sjoo/978-0-08-100868-3 |access-date=2022-02-27 |archive-date=2022-02-27 |archive-url=https://web.archive.org/web/20220227211012/https://www.elsevier.com/books/starch-in-food/sjoo/978-0-08-100868-3 |url-status=live }}</ref>  Unlike other botanical sources of starch, wheat starch has a bimodal size distribution, with both smaller and larger granules ranging from 2 to 55&nbsp;μm.<ref name="Rosicka-Kaczmarek wheat chapter" />

Some cultivated plant varieties have pure amylopectin starch without amylose, known as ''waxy starches''. The most used is [[waxy corn|waxy maize]], others are [[glutinous rice]] and [[waxy potato starch]]. Waxy starches undergo less [[Retrogradation (starch)|retrogradation]], resulting in a more stable paste. A maize cultivar with a relatively high proportion of amylose starch, [[amylomaize]], is cultivated for the use of its gel strength and for use as a [[resistant starch]] (a starch that resists digestion) in food products.

===Biosynthesis===
Plants synthesize starch in two types of tissues. The first type is storage tissues, for example, cereal endosperm, and storage roots and stems such as cassava and potato. The second type is green tissue, for example, leaves, where many plant species synthesize transitory starch on a daily basis. In both tissue types, starch is synthesized in plastids (amyloplasts and chloroplasts).

The biochemical pathway involves conversion of [[glucose 1-phosphate]] to [[Adenosine diphosphate|ADP]]-glucose using the enzyme [[glucose-1-phosphate adenylyltransferase]]. This step requires energy in the form of [[Adenosine triphosphate|ATP]]. A number of [[starch synthase|starch synthases]] available in plastids then adds the ADP-glucose via α-1,4-[[glycosidic bond]] to a growing chain of glucose residues, liberating [[Adenosine diphosphate|ADP]]. The ADP-glucose is almost certainly added to the non-reducing end of the amylose polymer, as the UDP-glucose is added to the non-reducing end of glycogen during [[Glycogenesis|glycogen synthesis]].<ref>Nelson, D. (2013) Lehninger Principles of Biochemistry, 6th ed., W.H. Freeman and Company (p. 819)</ref> The small glucan chain, further agglomerate to form initials of starch granules. 

The biosynthesis and expansion of granules represent a complex molecular event that can be subdivided into four major steps, namely, granule initiation, coalescence of small granules,<ref>{{Cite journal |last1=Bürgy |first1=Léo |last2=Eicke |first2=Simona |last3=Kopp |first3=Christophe |last4=Jenny |first4=Camilla |last5=Lu |first5=Kuan Jen |last6=Escrig |first6=Stephane |last7=Meibom |first7=Anders |last8=Zeeman |first8=Samuel C. |date=2021-11-26 |title=Coalescence and directed anisotropic growth of starch granule initials in subdomains of Arabidopsis thaliana chloroplasts |journal=Nature Communications |language=en |volume=12 |issue=1 |page=6944 |doi=10.1038/s41467-021-27151-5 |issn=2041-1723 |pmc=8626487 |pmid=34836943|bibcode=2021NatCo..12.6944B }}</ref> phase transition, and expansion. Several proteins have been characterized for their involvement in each of these processes. For instance, a chloroplast membrane-associated protein, MFP1, determines the sites of granule initiation.<ref>{{Cite journal |last1=Sharma |first1=Mayank |last2=Abt |first2=Melanie R. |last3=Eicke |first3=Simona |last4=Ilse |first4=Theresa E. |last5=Liu |first5=Chun |last6=Lucas |first6=Miriam S. |last7=Pfister |first7=Barbara |last8=Zeeman |first8=Samuel C. |date=2024-01-16 |title=MFP1 defines the subchloroplast location of starch granule initiation |journal=Proceedings of the National Academy of Sciences |language=en |volume=121 |issue=3 |pages=e2309666121 |doi=10.1073/pnas.2309666121 |issn=0027-8424|doi-access=free |pmid=38190535 |pmc=10801857 |bibcode=2024PNAS..12109666S }}</ref> Another protein named PTST2 binds to small glucan chains and agglomerates to recruit starch synthase 4 (SS4).<ref>{{Cite journal |last1=Seung |first1=David |last2=Boudet |first2=Julien |last3=Monroe |first3=Jonathan |last4=Schreier |first4=Tina B. |last5=David |first5=Laure C. |last6=Abt |first6=Melanie |last7=Lu |first7=Kuan-Jen |last8=Zanella |first8=Martina |last9=Zeeman |first9=Samuel C. |date=July 2017 |title=Homologs of PROTEIN TARGETING TO STARCH Control Starch Granule Initiation in Arabidopsis Leaves |journal=The Plant Cell |language=en |volume=29 |issue=7 |pages=1657–1677 |doi=10.1105/tpc.17.00222 |issn=1040-4651 |pmc=5559754 |pmid=28684429}}</ref> Three other proteins, namely, PTST3, SS5, and MRC, are also known to be involved in the process of starch granule initiation.<ref>{{Cite journal |last1=Seung |first1=David |last2=Schreier |first2=Tina B. |last3=Bürgy |first3=Léo |last4=Eicke |first4=Simona |last5=Zeeman |first5=Samuel C. |date=July 2018 |title=Two Plastidial Coiled-Coil Proteins Are Essential for Normal Starch Granule Initiation in Arabidopsis |journal=The Plant Cell |language=en |volume=30 |issue=7 |pages=1523–1542 |doi=10.1105/tpc.18.00219 |issn=1040-4651 |pmc=6096604 |pmid=29866647}}</ref><ref>{{Cite journal |last1=Vandromme |first1=Camille |last2=Spriet |first2=Corentin |last3=Dauvillée |first3=David |last4=Courseaux |first4=Adeline |last5=Putaux |first5=Jean-Luc |last6=Wychowski |first6=Adeline |last7=Krzewinski |first7=Frédéric |last8=Facon |first8=Maud |last9=D'Hulst |first9=Christophe |last10=Wattebled |first10=Fabrice |date=January 2019 |title=PII1: a protein involved in starch initiation that determines granule number and size in Arabidopsis chloroplast |url=https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.15356 |journal=New Phytologist |language=en |volume=221 |issue=1 |pages=356–370 |doi=10.1111/nph.15356 |pmid=30055112 |bibcode=2019NewPh.221..356V |issn=0028-646X}}</ref><ref>{{Cite journal |last1=Abt |first1=Melanie R. |last2=Pfister |first2=Barbara |last3=Sharma |first3=Mayank |last4=Eicke |first4=Simona |last5=Bürgy |first5=Léo |last6=Neale |first6=Isabel |last7=Seung |first7=David |last8=Zeeman |first8=Samuel C. |date=August 2020 |title=STARCH SYNTHASE5, a Noncanonical Starch Synthase-Like Protein, Promotes Starch Granule Initiation in Arabidopsis |journal=The Plant Cell |language=en |volume=32 |issue=8 |pages=2543–2565 |doi=10.1105/tpc.19.00946 |issn=1040-4651 |pmc=7401018 |pmid=32471861}}</ref> Furthermore, two proteins named ESV and LESV play a role in the aqueous-to-crystalline phase transition of glucan chains.<ref>{{Cite journal |last1=Liu |first1=Chun |last2=Pfister |first2=Barbara |last3=Osman |first3=Rayan |last4=Ritter |first4=Maximilian |last5=Heutinck |first5=Arvid |last6=Sharma |first6=Mayank |last7=Eicke |first7=Simona |last8=Fischer-Stettler |first8=Michaela |last9=Seung |first9=David |last10=Bompard |first10=Coralie |last11=Abt |first11=Melanie R. |last12=Zeeman |first12=Samuel C. |date=2023-05-26 |title=LIKE EARLY STARVATION 1 and EARLY STARVATION 1 promote and stabilize amylopectin phase transition in starch biosynthesis |journal=Science Advances |language=en |volume=9 |issue=21 |pages=eadg7448 |doi=10.1126/sciadv.adg7448 |issn=2375-2548 |pmc=10219597 |pmid=37235646|bibcode=2023SciA....9G7448L }}</ref> Several catalytically active starch synthases, such as SS1, SS2, SS3, and GBSS, are critical for starch granule biosynthesis and play a catalytic role at each step of granule biogenesis and expansion.<ref>{{Cite journal |last1=Pfister |first1=Barbara |last2=Zeeman |first2=Samuel C. |date=July 2016 |title=Formation of starch in plant cells |journal=Cellular and Molecular Life Sciences |language=en |volume=73 |issue=14 |pages=2781–2807 |doi=10.1007/s00018-016-2250-x |issn=1420-682X |pmc=4919380 |pmid=27166931}}</ref>

In addition to above proteins, [[Starch branching enzyme|starch branching enzymes (BEs)]] introduces α-1,6-glycosidic bonds between the glucose chains, creating the branched amylopectin. The starch debranching enzyme (DBE) [[isoamylase]] removes some of these branches. Several [[isoform]]s of these enzymes exist, leading to a highly complex synthesis process.<ref>{{cite journal |doi=10.1021/bm000133c |title=The Biosynthesis of Starch Granules |date=2001 |last1=Smith |first1=Alison M. |journal=Biomacromolecules |volume=2 |issue=2 |pages=335–41 |pmid=11749190}}</ref>

===Degradation===
The starch that is synthesized in plant leaves during the day is transitory: it serves as an energy source at night. Enzymes catalyze release of glucose from the granules. The insoluble, highly branched starch chains require [[phosphorylation]] in order to be accessible for degrading enzymes. The enzyme [[glucan, water dikinase]] (GWD) installs a phosphate at the C-6 position of glucose, close to the chain's 1,6-alpha branching bonds. A second enzyme, [[phosphoglucan, water dikinase]] (PWD) phosphorylates the glucose molecule at the C-3 position. After the second phosphorylation, the first degrading enzyme, [[beta-amylase]] (BAM) attacks the glucose chain at its non-reducing end. [[Maltose]] is the main product released. If the glucose chain consists of three or fewer molecules, BAM cannot release maltose. A second enzyme, [[disproportionating enzyme-1]] (DPE1), combines two maltotriose molecules. From this chain, a glucose molecule is released. Now, BAM can release another maltose molecule from the remaining chain. This cycle repeats until starch is fully degraded.  If BAM comes close to the phosphorylated branching point of the glucose chain, it can no longer release maltose. In order for the phosphorylated chain to be degraded, the enzyme isoamylase (ISA) is required.<ref name=Smith>{{cite journal| doi = 10.1146/annurev.arplant.56.032604.144257| pmid = 15862090| url = http://www.ccrc.uga.edu/~dmohnen/bcmb8020/Smith2005.pdf| journal = Annual Review of Plant Biology| volume = 56| pages = 73–98|date= 2005| last1 = Smith| first1 = Alison M.| title = Starch Degradation| last2 = Zeeman| first2 = Samuel C.| last3 = Smith| first3 = Steven M.| access-date = 2014-02-13| archive-url = https://web.archive.org/web/20150412040521/http://www.ccrc.uga.edu/~dmohnen/bcmb8020/Smith2005.pdf| archive-date = 2015-04-12| url-status = dead}}</ref>

The products of starch degradation are predominantly maltose<ref>{{cite journal |pmid=14566561 | doi=10.1007/s00425-003-1128-y | volume=218 | issue=3 | title=Maltose is the major form of carbon exported from the chloroplast at night |date=2004 | journal=Planta | pages=474–82 | last1 = Weise | first1 = SE | last2 = Weber | first2 = AP | last3 = Sharkey | first3 = TD| bibcode=2004Plant.218..474W | s2cid=21921851 }}</ref> and smaller amounts of glucose. These molecules are exported from the plastid to the cytosol, maltose via the maltose transporter and glucose by the [[plastidic glucose translocator]] (pGlcT).<ref>{{cite journal | pmc = 139927 | pmid=10810150 | volume=12 | issue=5 | title=Identification, purification, and molecular cloning of a putative plastidic glucose translocator | date=May 2000 | journal=Plant Cell | pages=787–802 | doi=10.1105/tpc.12.5.787 | last1 = Weber | first1 = A | last2 = Servaites | first2 = JC | last3 = Geiger | first3 = DR | display-authors = etal  }}</ref>  These two sugars are used for sucrose synthesis. [[Sucrose]] can then be used in the oxidative pentose phosphate pathway in the mitochondria, to generate ATP at night.<ref name=Smith/>