Cellulose and Lignocellulose


Cell Walls

Cells walls are a tough support that is outside the cell membrane. They are somewhat flexible but prevent the cell from bursting due to pressure from water on the inside of the cell. Higher plants, bacteria, fungi, and algae have cell walls but animals do not.

The cell wall is composed of the glucose polysaccharide cellulose. The woody parts of trees and certain other plants have a secondary cell wall that contains another polymeric material called lignin.


Cellulose

Cellulose is the major part of plant cell walls. This polysaccharide is much like linear (amylose) starch but it can contain up to 10,000 glucose units The big difference between amylose starch and cellulose is orientation of the bond connecting the glucose units. In both structures, there is an oxygen bridging the anomeric carbon (C1) and C4.

When the hydroxyl groups on the anomeric carbon and the highest numbered carbon with a hydroxy group (C1 and C4 in glucose) are on the same side of the ring (alpha configuration), the formation of of an oxygen bridge between the two carbons results in a structure that curves in on itself. You can see this with only 4 glucose units in a chain below. With many glucose units the structure forms helices. This is amylose starch.



When the oxygen groups on the anomeric carbon and the highest numbered carbon with a hydroxy group (C1 and C4 in glucose) are on the opposite sides of the ring (beta configuration), the polysaccharide that forms has a linear structure. This is the structure in cellulose.



Although they have the same empirical formula and are made up of the same monomer units, the differences in shape and size make the properties of starch and cellulose very different.

Starch is food but neither humans nor any other mammal are able to digest cellulose.


Lignin

Lignin is a highly non-regular polymer of phenol sub-units. Phenol is a derivative of the very stable organic molecule benzene with an alcohol functional group. When phenol loses a proton, the benzene unit stabilizes the negative charge on the oxygen atom. This makes phenol a stronger acid than other alcohols.



A small piece of the lignin polymer is below. Can you locate the phenol units?




Humic Materials, Lignite, and Coal

When woody plants decompose, bacteria are able to break down the cellulose in the cell walls and use that for respiration. The lignin is much more difficult to decompose.

Pieces of lignin become humic material in soil. The humic material has basic groups and acidic groups. It serves to balance the acidity of soil and also to bind minerals that can be used for new plant growth. The humic materials are slowly oxidized to CO2 and water by oxygen.

When lignin is buried so that it can't react with oxygen, heat and pressure from within the Earth can condense the structure and form coal.

Lignite is a type of coal that closely resembles lignin. Further condensation leads to bituminous, then anthracite coal.

The coal that underlies most of the state of Illinois was derived from tree-ferns and other plants that date from 300 million years ago. At that time, the forest was abruptly covered by the sea (yes, sea-front property in Illinois) and buried in sediment.



Professor Patricia Shapley, University of Illinois, 2012