Exosmosis is the movement of water out of a cell, across its semipermeable membrane, into a surrounding solution. This occurs when the cell is placed in a hypertonic solution—a solution with a higher solute concentration than the inside of the cell. The water moves from an area of high water concentration (inside the cell) to an area of low water concentration (outside the cell), aiming to equalize the concentrations on both sides of the membrane. This outward flow of water causes the cell to shrink or become flaccid.
Understanding the Process
- Semipermeable Membrane: The cell membrane acts as a selective barrier, allowing water to pass through but restricting the movement of larger solute molecules.
- Hypertonic Solution: A solution with a higher concentration of solutes (like salt or sugar) compared to the cell's interior.
- Water Potential: Water moves from an area of higher water potential (more water, fewer solutes) to an area of lower water potential (less water, more solutes).
Examples of Exosmosis
- Plant Cells: When a plant cell is placed in a highly salty solution, water leaves the cell causing the cell membrane to pull away from the cell wall, a process called plasmolysis.
- Animal Cells: Similarly, animal cells in a hypertonic environment will shrink and crenate (become shriveled) due to water loss through exosmosis.
Exosmosis vs. Endosmosis
It's important to contrast exosmosis with endosmosis, where water moves into the cell due to a hypotonic solution (lower solute concentration outside the cell). Endosmosis causes the cell to swell and potentially burst (lyse) in animal cells.
The provided references consistently define exosmosis as the outward movement of water from a cell across a semipermeable membrane, usually occurring in hypertonic environments. Several sources explicitly state that this process leads to cell shrinkage or flaccidity.
