Write function of cell wall.

The cell wall provides mechanical strength and structural support to plant cells, fungi, and bacteria, maintaining a definite cell shape and preventing cells from bursting when water enters through osmosis. This rigid framework, composed of cellulose in plants, enables plants to grow upright against gravity and reach toward sunlight without requiring an internal skeleton. The wall's tensile strength comes from the arrangement of cellulose microfibrils embedded in a matrix of other polysaccharides and proteins, creating a structure stronger than concrete pound-for-pound.

Beyond structural support, the cell wall serves multiple protective functions. It acts as the first line of defense against mechanical injury, pathogen invasion, and extreme environmental conditions like drought or temperature fluctuations. The wall's porosity allows water, gases, and dissolved nutrients to pass through freely while blocking larger harmful particles and pathogens. In plant tissues, cell walls bind adjacent cells together, providing tissue-level strength and creating pathways (apoplast) for water and mineral transport. The wall can also be modified for specialized functions—thickened with lignin for support in woody tissues, impregnated with suberin for waterproofing in cork, or fortified with additional compounds during pathogen attack. This multifunctional structure is essential for terrestrial plant life and distinguishes plant cells from animal cells.