cell wall
/ SELL wawl /
An animal cell is wrapped only in a flexible membrane, like a water balloon. But plants, fungi, and bacteria add a rigid outer layer on top of that membrane, like putting the balloon inside a cardboard box. That rigid box is the cell wall: a tough structural shell outside the plasma membrane that gives the cell shape, strength, and protection.
The cell wall is a firm layer built mostly of long, strong fibers, but what those fibers are made of differs by organism: plants use cellulose (a glucose polymer), fungi use chitin, and bacteria use peptidoglycan (a mesh of sugars and short protein chains). Unlike the plasma membrane, the wall is porous and not very selective — water and small molecules pass through it freely. Its real job is mechanical: it resists the internal water pressure (turgor) that would otherwise burst the cell, and it sets the cell's fixed shape.
Because animal cells lack a wall, the difference shows up everywhere: plants stand upright without a skeleton, fungi build rigid mushrooms, and bacteria hold their rod or sphere shapes. The bacterial cell wall is also a prime drug target — penicillin works by blocking the construction of peptidoglycan, weakening the wall until the bacterium bursts, while leaving wall-free human cells unharmed.
Wood, cotton, and paper are all essentially the leftover cellulose cell walls of dead plant cells — proof of just how durable that material is.
Much of the plant material humans use is really just cell wall.
A cell wall is not the same as the plasma membrane and does not control what enters the cell; the membrane (inside the wall) does the selective gatekeeping.