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Virtual Lab – Osmosis – Tonicity in Elodea Cells (Plant Cell…

Virtual Lab – Osmosis – Tonicity in Elodea Cells (Plant Cells) A specific type of diffusion that deals with the movement of water molecules across the membrane down its concentration gradient is called osmosis. Since water molecules are permeable to the cell membrane, its movement across the membrane depends on the relative concentration of solutes. Solutes are any substance that are dissolved in the solvent. The solvent is any liquid that is capable of dissolving substances. With these definitions in mind, we can formulate the following equation to demonstrate the composition of any solution. Solution (100%) = Solvent (H2O %) + Solute (%) The solvent in a living system is water and the direction of water movement is dependent upon the solute concentration across the membrane. If the membrane is permeable to water molecules but not to solutes, water molecules will diffuse from the solution with lower solute concentration (hypotonic solution) toward the solution with greater solute concentration (hypertonic solution). If solute concentrations are equal (isotonic solution), the rate of water molecules moving across the membrane will be the same. Plant cells response to changing tonicity in the environment. However, plant cells, unlike animal cells, have a semi-rigid cell wall on the outside of the plasma membrane. The cell wall retains the general cell shape even when the osmotic pressure has changed. When water rushes in, the plasma membrane swells, which increases the pressure of the cytoplasm against the cell wall. This is known as turgor pressure. The opposite response to turgor pressure happens when water moves out of the cell. The cytoplasm shrinks and peels away from the cell wall. This is known as plasmolysis. The internal solute concentration of the plant cell is approximately 1%. By changing the solute concentration of the environment, the plant cells will respond accordingly.