Secondary cell wall
The secondary cell wall is a structure found in many plant cells, located between the primary cell wall and the plasma membrane. The cell starts producing the secondary cell wall after the primary cell wall is complete and the cell has stopped expanding.[1]
The secondary cell wall consists mainly of cellulose, but also other polysaccharides, lignin, and glycoproteins. It sometimes consists of three distinct layers - S1, S2 and S3 - where the direction of the Cellulose microfibrils differs between the layers.[1] Apparently there are no Structural proteins or enzymes in the secondary wall.[2]
The secondary cell wall has different ratios of wall constituents compared to the primary wall. An example of this is that wood secondary walls contain xylans, whereas the primary wall contains xyloglucans and the cellulose fraction is higher in the secondary wall.[3] Pectins may also be absent from the secondary wall and apparently it contain no Structural proteins or enzymes.[2]
The Cellulose microfibrils give tensile strength, whereas lignification in addition to making the secondary wall impermeable to water also give a "brittle" texture.[2] Conceptually this give lignified secondary wall properties resembling armored concrete, where the cellulose microfibrils act as the armoring and the lignin as concrete.
Lignification of the secondary wall confer resistance to pathogens by two mechanisms. As lignin repel water, hydrolytic enzymes are less likely to attack and successfully penetrate the wall and it lowers the nutritional value of the wall, providing less energy to pathogens.[3]
The secondary wall usually is absent under the regions of the primary wall, which contain pit pairs, giving rise to a pit cavity. (this is somewhat simplified and someone who know more about it than the author of this sentence should expand the section, e.g. describing simple and bordered pit cavities).
Wood consists mostly of secondary cell wall, and holds the plant up against gravity.[4]
Some secondary cell walls store nutrients, such as those in the cotyledons and the endosperm. These contain little cellulose, and mostly other polysaccharides.[1]
[edit] References
- ^ a b c Buchanan, Gruissem, Jones, Biochemistry & molecular biology of plants, 1st edition, American Society of Plant Physiology, 2000
- ^ a b c Raven, P. H., R. F. Evert, et al. (1999). Biology of plants. New York, W.H. Freeman : Worth Publishers.
- ^ a b Taiz, L. and E. Zeiger (2006). Plant physiology. Sunderland, Mass., Sinauer Associates.
- ^ Campbell, Reece, Biology, 7th edition, Pearson/Benjamin Cummings, 2005
