Osmosis

When substances (solutes) are dissolved in water (the solvent), they lower the water potential, making it a negative value. The more solutes that are dissolved, the more negative the value becomes. Water will always move across a membrane from an area with a higher (less negative) water potential to an area with a lower (more negative) water potential.

However, water moves in both directions across the membrane at any given time, but the net movement will be toward the area with the more negative water potential. This net movement continues until the water potential is equal on both sides of the membrane, known as equilibrium.

Osmosis.

The solution around a cell is described as isotonic if it has the same solute concentration as the cell (and therefore the same water potential), hypertonic if it has a higher solute concentration (more negative/lower water potential) than the cell, and hypotonic if it has a lower solute concentration (less negative/higher water potential) than the cell. When animal cells are placed in a hypertonic solution, they shrivel. When plant cells are placed in a hypertonic solution, they become flaccid. When animal cells are placed in a hypotonic solution, they swell and burst. When plant cells are placed in a hypotonic solution, they become turgid as the cell wall prevents bursting.

Plant cells rely on turgor pressure (the high pressure in cells or their vacuoles when water is forced to move in by osmosis) to provide structural support and keep them upright. Most animal cells require isotonic surroundings to maintain their shape and allow chemical reactions to occur under normal conditions.

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