Cells and Their Organelles: 4) Tonicity

 Tonicity

Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water. 

A hypotonic solution refers to a solution that, when surrounding a cell, causes the cell to gain water. This is because there is more solute inside the cell than outside the cell. In Greek, 'hypo' means less. Since there are more solutes inside the cell than outside, water will move into the cell.

A hypertonic solution refers to a solution that, when surrounded a cell, causes the cell to lose water. This is because there is less solute inside the cell than outside the cell. In Greek, "hyper" means more. Since there are more solutes outside the cell than inside, water will move out of the cell.

An isotonic solution refers to a solution that, when surrounding a cell, doesn't induce any change in the water content of the solution or cell. This is because there are equal amounts of solute inside and outside the cell, so there will be no net movement of water.

Remember how animal cells do not have a cell wall like plant cells? This means that when the animal cell gets too much water inside the cell, there's no cell wall stopping it from bursting apart. Lysis is the process of a cell bursting open from too much water. If a cell is in a hypotonic solution, water will move into the cell very quickly. The cell will keep gaining and gaining water until it bursts apart. It will become lysed. (The past tense form of lysis is lysed).

In the case of the plant cell, which has a cell wall, an intake of water, no matter how much, will not burst the cell. The term for a plant cell which has a lot of water is turgid. Plant cells will generally prefer to be turgid and very full of water. So, a large intake of water is harmful for animal cells but not plant cells.

A plant cell that has what would have what is considered a "normal" amount of water for an animal cell would be considered flaccid because there is less water in the plant cell than it would generally like. 

An animal cell in a hypertonic solution will lose a lot of water and become shriveled. It loses so much water that the cell will shrink from all the water loss.

A plant cell in a hypertonic solution will lose a lot of water and become plasmolyzed (another term for shriveled, but specific to only plant cells). The membrane and cytoplasm loses so much water that it will shrink, and in the process, pull away from the cell wall, like so. 

That's it!