Write a short note on cell wall.

The cell wall is a rigid, protective outer layer found in plant cells, fungi, bacteria, and some protists, but absent in animal cells. In plants, the cell wall is primarily composed of cellulose, a complex polysaccharide made of glucose molecules arranged in strong, fibrous chains. This structure is located outside the plasma membrane and provides mechanical strength and support to the cell. The plant cell wall is not a single uniform layer but typically consists of three parts: the middle lamella (a pectin-rich layer that glues adjacent cells together), the primary cell wall (flexible and deposited during cell growth), and sometimes a secondary cell wall (thicker and more rigid, deposited after growth stops in certain specialized cells like those in wood).

The cell wall performs several critical functions beyond structural support. It determines and maintains the distinctive shape of plant cells and prevents them from bursting when water enters by osmosis, instead creating turgor pressure that keeps plant tissues rigid and upright. This is why plants wilt when dehydrated—their cells lose turgor pressure. The cell wall also acts as a defensive barrier against mechanical injury and pathogen invasion, though it contains plasmodesmata, which are channels that allow communication and transport between adjacent cells. Unlike the plasma membrane, the cell wall is fully permeable to water and small dissolved substances, so it does not regulate what enters the cell—that selectivity is the membrane's role.

The presence of the cell wall is a defining characteristic that distinguishes plant cells from animal cells and reflects fundamental differences in lifestyle. While animals have internal or external skeletons for support and can move to escape threats, plants must rely on the cumulative strength of billions of cell walls to stand upright and endure environmental stresses. The strength and durability of plant cell walls have made them invaluable to humans—wood for construction, cotton and linen fibers for textiles, and paper products all derive from the cellulose in plant cell walls, demonstrating how this cellular structure has shaped human civilization.