Episode 15 - The Cell Wall
The cell wall is a rigid structure made of hard polymers that wraps around the cell membrane of plant cells, fungi cells, bacteria, archaea, and some algae (NOT animal cells and most protist cells).
(Wikimedia Commons)
Cell walls support and shape the cell, and can protect the cell from predators. The cell wall also maintains the pressure potential inside the plant cell (as cell contents squish against it) to equalize the solute potential and thus stop more water entering the cell. In addition, the strong, thick cell walls of collenchyma and sclerenchyma cells of a plant help to hold the plant up against gravity.
Plant cells exhibit the capacity for two kinds of cell walls: primary walls and secondary walls.
Primary cell walls are thin, flexible, and sticky, which helps to join other cells together and allows for easy cell division. Between primary cell walls of adjacent cells is the middle lamella, a thin layer of polysaccharides, especially pectin, which are very sticky and help glue cells together.
When a plant cell matures and stops growing, it strengthens its cell wall, either by secreting hardening substances into the primary wall, or adding a secondary wall between the cell membrane and the primary wall. Secondary cell walls are made up of several laminated layers thatare secreted from the inside of mature plant cells. They help to maintain the size of the cell and to reinforce the cell shape, protecting and supporting the cell (ex. Wood). Lignin is a complex, highly cross-linked aromatic polymer that acts as a hardening agent for secondary cell walls (ONLY secondary cell walls have lignin).
The cell wall constantly interacts with other cell walls, because they are cemented together by the plant’s secretions of pectin and other sticky polysaccharides. In addition, tiny channels, called plasmodesmata, run through the cell walls and connect the cytoplasm of neighboring plant cells.
(SEM of Diatoms with Cell walls visible - Wikimedia Commons)