2.3: Osmosis
- Page ID
- 124509
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Osmosis is diffusion of water across a semi-permeable membrane. Osmosis follows the same rules as diffusion but always refers to water as the principal solvent in cells.
Figure \(\PageIndex{1}\): Experimental setup for four cellular models used to measure the rate of osmosis.
Bag A simulates a cell that is in a hypertonic solution. Hypertonic refers to a solution with a higher concentration of solutes. When a cell is in a hypertonic solution, water will move out of the cell, as the water will chase the higher solute concentration in the hypertonic solution, as compared to the lower concentration of solutes inside the cell. Bags C and D simulate cells that are in a hypotonic solution. Hypotonic describes a solution with a lower concentration of solutes. When a cell is in a hypotonic solution, water will move into the cell, as the water will chase the higher solute concentration in the cell, as compared to the lower concentration of solutes in the hypotonic solution.
Bag B represents a cell’s internal solute concentration equals the concentration in the environment; that is this cell (bag B) is isotonic to its environment. Isotonic refers to two solutions that have equal concentrations of solutes. There is no net osmosis in isotonic conditions.
We can describe the solutions as compared to the cells, and we can describe the cells as compared to the solutions. For example, as previously mentioned, Bag A is surrounded by a hypertonic solution. We can also describe this as Bag A is hypotonic to the surrounding solution. Water will always move from a hypotonic solution towards a hypertonic solution. Remember the golden rule of osmosis: Water always chases high solute concentrations! Also remember that the solute (in our experiment, sugar) does not pass through the dialysis membrane – only water molecules do.