Cell wall

The name of Chlamydomonas is applied to certain microscopic green bodies, each of which consists of a protoplasmic central substance invested by a structureless sac. The latter contains cellulose, as in ordinary plants; and the chlorophyll which gives the green colour enables the Chlamydomonas to decompose carbonic acid and fix carbon, as they do. Two long cilia protrude through the cell-wall, and effect the rapid locomotion of this "monad," which, in all respects except its mobility, is characteristically a plant.

Source: wiktionary

The structural distinctiveness of the cell—in plants especially—is dues in large part to the presence of an outer layer, or coat, the cell wall, which in plants is usually firm, and often hard and thick; in animals, delicate or lacking.

Source: wiktionary

Terrestrial vascular plants contain considerable quantities of lignin, an amorphous phenylpropanoid structural polymer that binds cell wall components and cell walls together providing structural stability to plants.

Source: wiktionary

Think of the cell wall as a wicker basket in which a balloon has been inflated so that it exerts pressure from the inside. Such a basket is very rigid and resistant to mechanical damage.

Source: wiktionary