View source for Cellulose

'''Cellulose''' is the most abundant biopolymer on earth. It occurs in plant cell walls and in bacteria. Common materials containing high amounts of cellulose are wood, paper, and cotton. Cellulose is a water-insoluble [[Polysaccharides|polysaccharide]] that humans can not digest. It is a linear polymer of beta-1,4 linked [[hexoses|glucose]] building blocks, with chains arranged in microfibrils held together by [[hydrogen bonds]] and hydrophobic interactions. Cellulose is related to but distinct from starch, a water-soluble carbohydrate containing alpha-1,4 linked glucose building blocks that is digestible by humans.

==Structure==
<StructureSection load='' size='489' side='right' scene='82/824000/Cellobiose/4'>
Glucose, the building block of cellulose and starch, can form six-membered rings with two distinct stereoisomers called the alpha and beta anomer. The only difference between alpha and beta glucose is at carbon C1. The [[disaccharides|disaccharide]] cellobiose  (reload <scene name='82/824000/Cellobiose/4'>initial scene</scene>) is a breakdown product of cellulose which shows the beta 1,4 linkage between two glucose molecules also present in cellulose. "beta 1,4" refers to a glycosidic link between the anomeric carbon (<jmol><jmolLink><script> spin off; select 823.C1; selectionHalos ON; delay 0.5;selectionHalos OFF;</script><text>☼</text></jmolLink> </jmol>) in beta configuration of one glucose molecule with carbon 4 (<jmol><jmolLink><script> select 823.C4'; selectionHalos ON; delay 0.5;selectionHalos OFF;</script><text>☼</text></jmolLink> </jmol>) of the other glucose molecule. In contrast, starches (specifically the linear form [[amylose]]) can be broken down to [[disaccharides|maltose]], a stereoisomer of cellobiose showing an alpha 1,4 linkage. Thus, it is the type of glycosidic linkage that distinguishes cellulose from starches at the molecular level.

Longer chains of beta 1,4 linked glucoses are found in cellulose. When cellulose is synthesized, these chains are made individually (cellulose chain during <scene name='82/824000/Cellulose/2'>biosynthesis</scene>)<ref>DOI:10.1098/rsta.2017.0048</ref>. Again, the linkages are all of the beta 1,4 type (<jmol><jmolLink><script> select BGC and *.C1; selectionHalos ON; delay 0.5;selectionHalos OFF;</script><text>☼</text></jmolLink> </jmol>). In this structure, monomers are added to polymer chain inside the cell and secreted through the membrane, surrounded by the <scene name='82/824000/Cellulose/4'>enzyme</scene> throughout.

Once secreted, individual cellulose chains self-assemble to from semi-crystalline cellulose microfibrils. There are multiple forms of cellulose (I alpha and beta, II, III) which differ in the orientation and the detailed interactions between linear polymers. A model of a <scene name='82/824000/Microfibril/4'>cellulose type I beta micro-fibril</scene> shows a tightly packed structure. The model was made using cellulose builder (http://cces-sw.iqm.unicamp.br/cces/admin/cellulose, <ref>DOI:10.1002/jcc.22959</ref>) and is based on a fiber-diffraction study by Nishiyama et al <ref>DOI:10.1021/ja0257319</ref>. The <scene name='82/824000/Microfibril1d/1'>individual chains</scene> of cellulose form <scene name='82/824000/Microfibril2d/1'>layers</scene>, and multiple layers stack to form a <scene name='82/824000/Microfibril3d/1'>3D structure</scene> without any gaps. While interactions within layers are dominated by hydrogen bonding, <scene name='82/824000/Hydrophobic/1'>interactions between layers</scene> are hydrophobic<ref>DOI:10.1007/s10570-021-04325-4</ref>. You can use the buttons below to explore the 1D, 2D and 3D assembly of the microfibril model.

Repeating unit
Style <jmol>
  <jmolRadioGroup>
    <item>
      <script>select (4-5 and chain=M);spacefill only</script>
      <text>spacefill</text>
    </item>
    <item>
      <script>select (4-5 and chain=M);set bondmode OR; wireframe only</script>
      <text>bonds</text>
    </item>
     <item>
      <script>select 4-5 and chain=M;set bondmode OR; wireframe only; wireframe 0.2; spacefill 0.5</script>
      <text>ball-and-stick</text>
      <checked>true</checked>
    </item>
   </jmolRadioGroup>
</jmol>

Color <jmol>
  <jmolRadioGroup>
    <item>
      <script>select (4-5 and chain=M); color chain</script>
      <text>by chain</text>
    </item>
    <item>
      <script>select (4-5 and chain=M); color cpk</script>
      <text>by atomtype</text>
    </item>
     <item>
      <script>select (4-5 and chain=M); color black</script>
      <text>black</text>
      <checked>true</checked>
    </item>
   </jmolRadioGroup>
</jmol>


Rest of microfibril

Show <jmol>
  <jmolRadioGroup>
    <item>
      <script>display chain=M and 4-5</script>
      <text>none</text>
    </item>
    <item>
      <script>display chain=M</script>
      <text>1D</text>
    </item>
    <item>
      <script>display chain=B or chain=G or chain=M or chain=R</script>
      <text>2D</text>
    </item>
     <item>
      <script>display all</script>
      <text>3D</text>
      <checked>true</checked>
    </item>
   </jmolRadioGroup>
</jmol>

Style <jmol>
  <jmolRadioGroup>
    <item>
      <script>select not (4-5 and chain=M);spacefill only</script>
      <text>spacefill</text>
    </item>
    <item>
      <script>select not (4-5 and chain=M);wireframe only</script>
      <text>bonds</text>
    </item>
     <item>
      <script>select not (4-5 and chain=M);wireframe only; wireframe 0.2; spacefill 20%</script>
      <text>ball-and-stick</text>
      <checked>true</checked>
    </item>
   </jmolRadioGroup>
</jmol>

Color <jmol>
  <jmolRadioGroup>
    <item>
      <script>select not (4-5 and chain=M); color chain</script>
      <text>by chain</text>
    </item>
    <item>
      <script>select not (4-5 and chain=M); color cpk</script>
      <text>by atomtype</text>
    </item>
     <item>
      <script>select not (4-5 and chain=M); color black</script>
      <text>black</text>
      <checked>true</checked>
    </item>
   </jmolRadioGroup>
</jmol>

<jmol>
  <jmolCheckbox>
    <scriptWhenUnChecked>hbond off
       </scriptWhenUnChecked>
    <scriptWhenchecked>select all; hbond on
       </scriptWhenchecked>
    <checked>false</checked> 
    <text>hydrogen bonds</text>
  </jmolCheckbox>
</jmol>


</StructureSection>

==See also==
Lecture slides by Eero Kontturi, Aalto University, Espoo, Finland: https://mycourses.aalto.fi/pluginfile.php/148341/mod_folder/content/0/Lecture%202%20-%20Cellulose%20structure.pdf?forcedownload=1

== References ==
<references/>