A cell wall is a characteristic feature of plant cells, bacteria and fungal cell. The composition varies in all these organisms.
In plants cell wall surrounds the plasma membrane in all plant cells. It is a porous covering of about 0.1 milli microns to 10 milli microns in size.
The cell wall is mainly composed of insoluble polysaccharides. The most common constituent is cellulose. Certain other polysaccharides like pectin, hemicellulose lignin and xylan may also be present.
The cell wall has two layers- Primary wall and secondary wall
Primary wall: It is represented by the wall material deposited when the cell is young. It is thin and elastic and is composed of only cellulose. Meristematic cells and parenchyma cells in plants show only primary wall.
Secondary wall: It is represented by wall material deposited in mature cells. Secondary wall is always internal to the primary walls. It is thick, rigid and non elastic. It is usually composed of additional cellulose or hemicellulose or pectin or lignin.
The deposition of secondary wall material is not uniform. At places it is discontinuous, to enclose areas called pits. The pits may be simple or bordered with overhanging wall material.
Secondar wall are seen in specialized cells such as sclerenchyma, collenchyma, xylem vessels and tracheid's.
The term middle lamella is used for describing a thin sticky, amorphous layer of cementing material that is found between two adjacent cells. It is deposited by the joint activity of the adjacent cells.
The cell wall is interrupted by narrow pores which carry fine strands of cytoplasm interlinking the contents of cell. These pores are called plasmodesmata. They allow movement of substances between adjacent cells.