The Cell Wall

Plant cells and bacteria sit in water that's often hypotonic, meaning it has fewer solutes than the cell's interior. Water follows the osmotic gradient and moves into the cell. An animal cell, bounded only by a flexible membrane, has no way to resist this inflow. It swells and swells until the membrane ruptures, a process called lysis. A plant or bacterial cell has a wall made of rigid material sitting just outside the membrane. That wall doesn't stop water from entering, but it stops the cell from expanding past a certain point.

As water pushes the membrane against the wall, the wall pushes back. This buildup of pressure is turgor pressure. Once turgor pressure rises enough to balance the osmotic pull of water, net water movement into the cell stops. The cell stays full and firm instead of bursting. This is also why watering a wilted plant perks it back up: water re-enters the cells, turgor pressure rebuilds, pushes membranes firmly against walls, and the stems and leaves regain their stiffness. A plant wilts when water loss drops turgor pressure, leaving cells floppy and cell membranes pulled away from the slack walls.

This connects directly to why cell walls matter for survival. Take away the wall and the protection disappears, even if the membrane itself is perfectly normal. That's the logic behind drugs like penicillin, which block bacteria from finishing new cell wall material as they grow and divide. Without a complete wall, the bacterium has nothing to resist the constant osmotic pressure of water rushing in, so it swells and bursts. The wall, not the membrane, is doing the structural work.