Cell wall structure
The cell wall of growing eukaryotic plant cells is made up of two layers: the middle lamina and the primary wall. In many cells, when they mature and grow, a third layer, the secondary wall, appears between the plasma membrane and the primary wall.
- Middle foil. It is the outermost layer and the first to form, from vesicles of the Golgi apparatus when the cell is dividing, in telophase. It appears between the primary laminae of neighboring cells, except where there are plasmodesmata, which are channels that cross the cell wall and membrane. It is mainly composed of pectin (heteropolysaccharide) and proteins.
- The primary wall. Allows cell growth. It is a thin, flexible layer, allowing the cell to expand and grow. It is close to the plasma membrane. It is made up of cellulose fibers in a matrix of polysaccharides (hemicellulose and pectin), monosaccharides and some proteins.
- Secondary wall. It develops when cell growth has stopped. Only some types of cells have it, and it is the innermost layer, attached to the membrane. This third layer is thicker and stiffer than the primary layer. Chemically it is similar to the primary wall, but it has more cellulose and less pectin. It has a laminar structure, with the cellulose fibers very organized, forming a structure that gives it a lot of resistance. It can be composed, in addition to cellulose and hemicellulose, of suberin, cutin or lignin.
In many cells of the tissues specialized in the mechanical support of the plant (sclerenchyma) and in the conductors (such as xylem) when they die, lignin also appears, which provides the hardness to the wood.
Caroline Dahl [CC BY-SA 3.0], via Wikimedia Commons