Cellulose is the most abundant macromolecule
. In plants it forms forms 20-30% of primary cell walls
and 40-90% of secondary cell walls
. It is a significant factor in human health
where it is often referred to as fibre
. It is also a key componant of wood
, used to manufacture plastics
and an abundant natural reservoir of organic carbon
and so has a profound environmental role.
Cellulose is a polymer of beta 1,4-linked glucose units, shown below:
__ || __
| H || H |
| | || | |
| H-C-OH || H-C-OH |
| | \||/ | |
| C -- O \/ C -- O |
| H /| \ |\ H /| \ |\ |
| |/ H \| \ |/ H \| \ |
| C C O C C O |
|\ |\ OH H /| \ |\ OH H /| \|
| \| \| |/ H \| \| |/ H |
| C -- C C -- C |
| | | | | |
| H OH H OH |
|__ Glucose Glucose __|n
Each molecule can be as short as 500 units long or as long as 15,000 units. The beta 1,4 glycosidic bonds between monomers cannot be hydrolysed by vertabrates making cellulose indigestible. However, some microorganisms contain the enzyme cellulase which can digest this linkage (albeit it slowly due to the dense packing of cellulose chains into microfibrils - see below) and have a symbiotic relationship with certain herbivores, living in the digestive tracks of their hosts. Interestingly, cellulose is an isomer of starch and glycogen but these two polymers differ from cellulose in that starch and glycogen have alpha 1,4 glycosidic bonds between glucose units. The result of the alpha 1,4 linkage is that in starch and glycogen chains each glucose unit is of the same orientation as its neighbours whereas in cellulose glucose units flip 180 degrees with respect to adjacent residues (in addition glycogen and starch contain branches from their chains formed by alpha 1,6 glycosidic bonds). In contrast starch and glycogen have entirely different physical properties, play no structural role in tissue and are easily degradable, constituting the principal food stores of plants and animals respectively.
Molecules of cellulose are stabilised by hydrogen bonds between glucose units to form a ribbon like structure which in turn can hydrogen bond to other cellulose molecules to form parallel bundles of 40-70 chains. These bundles are called microfibrils and have a tensile strength comparable to steel. Within the cell wall these microfibrils are assembled in layers or lamellae with each microfibril about 20-40nm apart, linked by cross-linking glycans (also called hemicelluloses).
Cellulose is synthesised at the extracellular surface of the plasma membrane by a membrane-bound enzyme complex called cellulose synthase which uses cytoplasmic UDP-glucose as its substrate. These complexes are arranged in rosettes such that many cellulose molecules are spun out from the cell surface together, presumably aiding microfibril formation. The orientation of newly synthesised cellulose molecules is determined by a cortical-array of microtubules which in turn determines the direction of cell-expansion and thus the shape of the cell and, ultimately, the shape of the plant as a whole.
As such cellulose, a relatively simple polymer, is also perhaps one of the most overlooked molecules in nature.
Sources: Lectures provided by Dr Paul knox at the University of Leeds supplemented by Molecular Biology of the Cell (4th edition), by Alberts et al., Garland Sciences, USA, (2002) and Biochemistry (2nd edition), by Voet D and Voet J G, John Wiley & Sons, Inc., 1995.
Thanks to Ladysun and wertperch for constructive criticism and advice.