Crenation
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Crenation is the contraction of cells within animals in a hypertonic solution, due to the loss of water through osmosis. The word is from the latin "crenatus" meaning scalloped or notched, and named for the scalloped-edged shape the cells take on when crenated.
Crenation occurs because in a hypertonic environment (that is, the cell has less solutes than the extracellular fluid does) osmosis (the diffusion of water) causes a net movement of the water outside of the cell, causing the cytoplasm to decrease in size. As a result, the cell also decreases in size.
The effects of crenation are especially visible in red blood cells, as they become distorted in shape rather than the usual disc-like shape with the dimple that the usual blood cell has. Rather, the red blood cell looks as if it has legs extending from a smaller central area, like a spiked ball.
Plasmolysis is the term which describes plant cells where the cytoplasm strinks from the cell wall in a hypertonic environment. Cytolysis is the term which describes the bursting of cells without cell walls in a hypotonic environment.
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