Part (a) is a schematic diagram showing the sequence of events that lead to the osmotic lysis of a bacterium. Initially the cell wall and the cell membrane beneath it are intact. As water enters by osmosis, the cell wall becomes defective. Eventually the cell contents and surrounding membrane expand through the defective cell wall, the membrane then ruptures and the cell contents spill out; that is, the cell lyses. In the light micrograph in part (b), the intact near-spherical cell appears orange–yellow on the black background; while the lysed cell has collapsed and lost most of its contents and so has a shrivelled shape and appears mostly black.