Editing Wood (section)

==Chemistry==
[[File:Lignin.png|thumb|right|upright=1.35|Chemical structure of [[lignin]], which makes up about 25% of wood dry matter and is responsible for many of its properties.]]
The chemical composition of wood varies from species to species, but is approximately 50% carbon, 42% oxygen, 6% hydrogen, 1% nitrogen, and 1% other elements (mainly [[calcium]], [[potassium]], [[sodium]], [[magnesium]], [[iron]], and [[manganese]]) by weight.<ref>{{cite book|title=Mémotech Bois et Matériaux Associés|date=1996|publisher=Éditions Casteilla|location=Paris|isbn=978-2-7135-1645-0|page=22|author= Jean-Pierre Barette|author2= Claude Hazard et Jérôme Mayer}}</ref> Wood also contains [[sulfur]], [[chlorine]], [[silicon]], [[phosphorus]], and other elements in small quantity.

Aside from water, wood has three main components. [[Cellulose]], a crystalline polymer derived from glucose, constitutes about 41–43%. Next in abundance is [[hemicellulose]], which is around 20% in deciduous trees but near 30% in conifers. It is mainly [[pentose|five-carbon sugars]] that are linked in an irregular manner, in contrast to the cellulose. [[Lignin]] is the third component at around 27% in coniferous wood vs. 23% in deciduous trees. Lignin confers the hydrophobic properties reflecting the fact that it is based on [[aromatic ring]]s. These three components are interwoven, and direct covalent linkages exist between the lignin and the hemicellulose. A major focus of the paper industry is the separation of the lignin from the cellulose, from which paper is made.

In chemical terms, the difference between hardwood and softwood is reflected in the composition of the constituent [[lignin]]. Hardwood lignin is primarily derived from [[sinapyl alcohol]] and [[coniferyl alcohol]]. Softwood lignin is mainly derived from coniferyl alcohol.<ref name="Boerjan-2003">{{cite journal| author = W. Boerjan| author2 =  J. Ralph| author3 = M. Baucher|date=June 2003| title = Lignin biosynthesis| journal = Annu. Rev. Plant Biol.| volume = 54| issue = 1| pages = 519–549| doi = 10.1146/annurev.arplant.54.031902.134938| pmid = 14503002}}</ref>

===Extractives===
Aside from the structural [[polymer]]s, i.e. [[cellulose]], [[hemicellulose]] and [[lignin]] ([[Lignocellulosic biomass|lignocellulose]]), wood contains a large variety of non-structural constituents, composed of low [[molecular weight]] [[organic compound]]s, called ''extractives''. These compounds are present in the [[extracellular space]] and can be extracted from the wood using different neutral [[solvent]]s, such as [[acetone]].<ref name="Ek-2009">{{cite book |last1=Ek |first1=Monica |last2=Gellerstedt |first2=Göran |last3=Henriksson |first3=Gunnar |title=Pulp and Paper Chemistry and Technology. Volume 1, Wood Chemistry and Wood Biotechnology |date=2009 |publisher=Walter de Gruyter |location=Berlin |isbn=978-3-11-021339-3 |chapter=Chapter 7: Wood extractives}}</ref> Analogous content is present in the so-called ''exudate'' produced by trees in response to mechanical damage or after being attacked by [[insect]]s or [[fungi]].<ref name="Sjöström-2013">{{cite book |last1=Sjöström |first1=Eero |title=Wood Chemistry: Fundamentals and Applications |publisher=Elsevier Science |location=San Diego |isbn=978-0-08-092589-9 |edition=Second |chapter=Chapter 5: Extractives|date=October 22, 2013 }}</ref> Unlike the structural constituents, the composition of extractives varies over wide ranges and depends on many factors.<ref>{{cite book |last1=Ansell |first1=Martin P. |title=Woodhead Publishing Series in Composites Science and Engineering: Number 54. Wood Composites |date=2015 |publisher=Woodhead Publishing |location=Cambridge, UK |isbn=978-1-78242-454-3 |chapter=Chapter 11: Preservation, Protection and Modification of Wood Composites}}</ref> The amount and composition of extractives differs between tree species, various parts of the same tree, and depends on genetic factors and growth conditions, such as climate and geography.<ref name="Ek-2009" /> For example, slower growing trees and higher parts of trees have higher content of extractives. Generally, the [[softwood]] is richer in extractives than the [[hardwood]]. Their concentration increases from the [[cambium]] to the [[pith]]. [[Bark (botany)|Bark]]s and [[branch]]es also contain extractives. Although extractives represent a small fraction of the wood content, usually less than 10%, they are extraordinarily diverse and thus characterize the chemistry of the wood species.<ref name="Hon-2001">{{cite book |last1=Hon |first1=David N.-S. |last2=Shiraishi |first2=Nubuo |title=Wood and Cellulosic Chemistry |date=2001 |publisher=Marcel Dekker |location=New York |isbn=0-8247-0024-4 |edition=2nd, rev. and expanded |chapter=Chapter 6: Chemistry of Extractives}}</ref> Most extractives are secondary metabolites and some of them serve as precursors to other chemicals. Wood extractives display different activities, some of them are produced in response to wounds, and some of them participate in natural defense against insects and fungi.<ref>{{cite book |last1=Rowell |first1=Roger M. |title=Handbook of Wood Chemistry and Wood Composites |date=2013 |publisher=Taylor & Francis |location=Boca Raton |isbn=9781439853801 |edition=2nd |chapter=Chater 3: Cell Wall Chemistry}}</ref>

[[File:Forchem Rauma 2.jpg|thumb|Forchem [[tall oil]] refinery in [[Rauma, Finland|Rauma]], Finland]]
These compounds contribute to various physical and chemical properties of the wood, such as wood color, fragnance, durability, acoustic properties, [[hygroscopicity]], adhesion, and drying.<ref name="Hon-2001" /> Considering these impacts, wood extractives also affect the properties of [[Pulp (paper)|pulp]] and paper, and importantly cause many problems in [[Papermaking|paper industry]]. Some extractives are surface-active substances and unavoidably affect the surface properties of paper, such as water adsorption, friction and strength.<ref name="Ek-2009" /> [[Lipophilicity|Lipophilic]] extractives often give rise to sticky deposits during [[kraft pulping]] and may leave spots on paper. Extractives also account for paper smell, which is important when making [[food contact materials]].

Most wood extractives are [[Lipophilicity|lipophilic]] and only a little part is water-soluble.<ref name="Sjöström-2013" /> The lipophilic portion of extractives, which is collectively referred as wood [[resin]], contains [[fat]]s and [[fatty acid]]s, [[sterol]]s and steryl esters, [[terpene]]s, [[terpenoid]]s, [[resin acid]]s, and [[wax]]es.<ref>{{cite book |last = Mimms |first = Agneta |author2=Michael J. Kuckurek |author3=Jef A. Pyiatte |author4=Elizabeth E. Wright |title = Kraft Pulping. A Compilation of Notes |publisher = TAPPI Press |date = 1993 |pages = 6–7 |isbn = 978-0-89852-322-5 }}</ref> The heating of resin, i.e. [[distillation]], vaporizes the [[Volatility (chemistry)|volatile]] terpenes and leaves the solid component – [[rosin]]. The concentrated liquid of volatile compounds extracted during [[steam distillation]] is called [[essential oil]]. Distillation of [[oleoresin]] obtained from many [[pines]] provides [[rosin]] and [[turpentine]].<ref>{{cite book |doi=10.1002/14356007.a23_073 |chapter=Resins, Natural |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2000 |last1=Fiebach |first1=Klemens |last2=Grimm |first2=Dieter |isbn=978-3-527-30673-2}}</ref>

Most extractives can be categorized into three groups: [[aliphatic compound]]s, [[terpene]]s and [[Phenols|phenolic compounds]].<ref name="Ek-2009" /> The latter are more water-soluble and usually are absent in the resin.
*[[Aliphatic compound]]s include fatty acids, [[fatty alcohol]]s and their esters with [[glycerol]], fatty alcohols (waxes) and sterols (steryl esters). [[Hydrocarbon]]s, such as [[alkane]]s, are also present in the wood. [[Suberin]] is a polyester, made of suberin acids and glycerol, mainly found in [[Bark (botany)|bark]]s. Fats serve as a source of energy for the wood cells.<ref name="Sjöström-2013" /> The most common wood sterol is [[sitosterol]], and less commonly [[sitostanol]], citrostadienol, [[campesterol]] or [[cholesterol]].<ref name="Ek-2009" />
*The main [[terpene]]s occurring in the softwood include [[Monoterpene|mono-]], [[Sesquiterpene|sesqui-]] and [[diterpene]]s.<ref name="Sjöström-2013" /> Meanwhile, the terpene composition of the hardwood is considerably different, consisting of [[Triterpene#Triterpenoids|triterpenoid]]s, [[polyprenol]]s and other higher terpenes. Examples of mono-, di- and sesquiterpenes are [[alpha-Pinene|α-]] and [[beta-Pinene|β-pinene]]s, [[3-carene]], [[β-myrcene]], [[limonene]], [[thujaplicin]]s, α- and β-[[phellandrene]]s, α-muurolene, [[δ-cadinene]], [[alpha-cadinol|α-]] and [[delta-cadinol|δ-cadinol]]s, α- and β-[[cedrene]]s, juniperol, [[longifolene]], ''cis''-abienol, [[borneol]], pinifolic acid, nootkatin, chanootin, [[phytol]], geranyl-linalool, β-epimanool, manoyloxide, pimaral and pimarol. Resin acids are usually [[tricyclic]] [[terpenoid]]s, examples of which are [[pimaric acid]], sandaracopimaric acid, [[isopimaric acid]], [[abietic acid]], [[levopimaric acid]], palustric acid, neoabietic acid and dehydroabietic acid. [[Bicyclic molecule|Bicyclic]] resin acids are also found, such as lambertianic acid, communic acid, mercusic acid and secodehydroabietic acid. [[Cycloartenol]], [[betulin]] and [[squalene]] are [[Triterpene#Triterpenoids|triterpenoid]]s purified from hardwood. Examples of wood polyterpenes are [[rubber]] (''cis''-polypren), [[gutta percha]] (''trans''-polypren), gutta-balatá (''trans''-polypren) and betulaprenols ([[Open-chain compound|acyclic]] polyterpenoids).<ref name="Ek-2009" /><ref name="Sjöström-2013" /> The mono- and sesquiterpenes of the softwood are responsible for the typical smell of [[pine]] forest.<ref name="Ek-2009" /> Many monoterpenoids, such as [[β-myrcene]], are used in the preparation of [[Flavoring|flavor]]s and [[Perfume|fragrances]].<ref name="Sjöström-2013" /> [[Tropolone]]s, such as [[hinokitiol]] and other [[thujaplicin]]s, are present in [[Wood-decay fungus|decay]]-resistant trees and display [[fungicidal]] and [[Insecticide|insecticidal]] properties. Tropolones strongly bind metal ions and can cause digester [[corrosion]] in the process [[kraft pulping]]. Owing to their [[Chelation|metal-binding]] and [[Ionophore|ionophoric]] properties, especially thujaplicins are used in physiology experiments.<ref>{{cite book |title=Cell physiology sourcebook: essentials of membrane biophysics |location=London, UK |isbn=978-0-12-387738-3 |edition=Fourth |chapter=Chapter 4: Ionophores in Planar Lipid Bilayers |chapter-url=https://www.sciencedirect.com/science/article/pii/B9780123877383000044 |last1=Sperelakis |first1=Nicholas |last2=Sperelakis |first2=Nick |date=January 11, 2012 |access-date=September 27, 2020 |archive-date=June 28, 2020 |archive-url=https://web.archive.org/web/20200628201941/https://www.sciencedirect.com/science/article/pii/B9780123877383000044 |url-status=live }}</ref> Different other ''in-vitro'' biological activities of thujaplicins have been studied, such as insecticidal, anti-browning, anti-viral, anti-bacterial, anti-fungal, anti-proliferative and anti-oxidant.<ref>{{cite journal |last1=Saniewski |first1=Marian |last2=Horbowicz |first2=Marcin |last3=Kanlayanarat |first3=Sirichai |title=The Biological Activities of Troponoids and Their Use in Agriculture A Review |journal=Journal of Horticultural Research |date=10 September 2014 |volume=22 |issue=1 |pages=5–19 |doi=10.2478/johr-2014-0001|s2cid=33834249 |doi-access=free }}</ref><ref>{{cite journal |last1=Bentley |first1=Ronald |title=A fresh look at natural tropolonoids |journal=Nat. Prod. Rep. |date=2008 |volume=25 |issue=1 |pages=118–138 |doi=10.1039/b711474e|pmid=18250899 }}</ref>
*[[Phenols|Phenolic compounds]] are especially found in the hardwood and the bark.<ref name="Sjöström-2013" /> The most well-known wood phenolic constituents are [[stilbenes]] (e.g. [[pinosylvin]]), [[lignans]] (e.g. [[pinoresinol]], conidendrin, [[plicatic acid]], [[hydroxymatairesinol]]), norlignans (e.g. [[nyasol]], puerosides A and B, hydroxysugiresinol, sequirin-C), [[tannin]]s (e.g. [[gallic acid]], [[ellagic acid]]), [[flavonoid]]s (e.g. [[chrysin]], [[taxifolin]], [[catechin]], [[genistein]]). Most of the phenolic compounds have fungicidal properties and protect the wood from [[Wood-decay fungus|fungal decay]].<ref name="Sjöström-2013" /> Together with the neolignans the phenolic compounds influence on the color of the wood. Resin acids and phenolic compounds are the main toxic contaminants present in the untreated [[effluent]]s from [[Pulp (paper)|pulping]].<ref name="Ek-2009" /> [[Polyphenolic]] compounds are one of the most abundant biomolecules produced by plants, such as [[flavonoid]]s and [[tannin]]s. Tannins are used in [[leather]] industry and have shown to exhibit different biological activities.<ref name="Hon-2001" /> [[Flavonoid]]s are very diverse, widely distributed in the [[plant]] kingdom and have numerous biological activities and roles.<ref name="Sjöström-2013" />