BIOL 2242 Lecture Notes - Lecture 11: The Plant Cell, Cell Potency, Middle Lamella
5 Feb 2018
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Secondary wall: after cell expansion complete, some cell (types with support function) produce secondary wall to the inside of primary wall, composition varies (but simpler) Matrix all hemicellulose (much lower water content: no pectin, enzymes, etc, constructed in similar way as primary but more organized. In 3 layers (s1, s2, s3) with s3 (newest) closest to plasma membrane. Layers deposited at angle to each other, like plywood: strengthens and makes more rigid but little plasticity, many become lignified (first in middle lamella > primary > secondary) Even more rigid and deters microbial breakdown. Cell then often dies, loses cytoplasmic contents. 23. 15, p. 547 a. shows both annular an helical: annular (separated) Common in vessel elements in young xylem tissue: helical (continuous) Also in vessels in young xylem tissue: double-helical. 23. 13b: pitted: continuous secondary wall except in pits. In tracheids, sclerenchyma fibres & some vessels in mature xylem tissue. Part b: cell and tissue types (chapter 23)
