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Cytolysis is the breakdown or destruction of cells. A cell is the smallest, most basic unit of life, that is capable of existing by itself. In cytolysis, it is usually the cell membrane (outer cell surface) that is destroyed by rupturing. This occurs because too much water has moved into the cell and the cell can no longer contain it. This process occurs through osmosis. Osmosis is the movement of a solvent (a liquid substance such as water that can dissolve one or more substance) through a semi-permeable membrane from a solution that has a lower concentration of solutes to one that has a higher concentration of solutes. A solute is a substance that dissolves in a solvent.

Cytolysis is known to occur after a stroke. A stroke is a burst artery (a type of blood vessel that carries blood away from the heart) or a blockage of an artery in the brain. The reason cytolysis occurs after stroke is because the cell damage that results from it causes poor distribution of nutrients and poor removal of wastes. This alters cell metabolism, which is the chemical actions in cells that release energy from nutrients or use energy to create other substances. In turn, this causes too much water to enter the cell.
Cytolysis can only occur in cells that do not have a cell wall, which is a tough layer that surrounds some types of cells. The cell wall’s toughness prevents the cell from bursting.  If the cell wall has been destroyed, however then cytolysis can occur. The cell wall can be damaged with enzymes known as lysozymes. An enzyme is a type of protein that helps produce chemical reactions in the body.

To prevent cytolysis, some organisms have developed defense mechanisms to rapidly removes excess water from inside the cell. A reverse defense mechanism is for the body to move enough solutes outside of the cell. If this happens in sufficient quantity, not enough water will move inside the cell to destroy it.  Another defense mechanism is having cell membranes that do not allow water to move through them so easily. Cytolytic means pertaining to cytolysis. Cytolysis is also known as osmotic lysis. Cytolysis comes from the Greek word "kytos" meaning "a hollow cell," and the Greek word "lysis" meaning "loosening." Put the two words together and you have "loosening (of) a hollow cell."