Osmosis.mp4

Osmosis is the movement of water molecules across a semi-permeable membrane from an area of higher concentration to an area of lower concentration. Tonicity measures the osmotic pressure of two solutions separated by a semi-permeable membrane and describes how cells respond to external solutions. Tonicity is only affected by solutes that cannot cross the membrane.

There are three classifications of tonicity: isotonic, hypertonic, and hypotonic. In a hypotonic solution, there is a greater solute concentration inside the cell than outside, causing water molecules to enter the cell. This makes the cell swell, potentially leading to lysis or rupture. A demonstration using dialysis tubing filled with a concentrated sugar solution and placed in tap water shows a significant weight gain due to water entering the cell.

In a hypertonic solution, there is a greater solute concentration outside the cell than inside, causing water to leave the cell and the cell to shrink, a process called crenation. A demonstration with dialysis tubing filled with tap water and placed in a concentrated sugar solution shows a significant loss of weight and a flaccid cell due to water leaving it.

In an isotonic solution, there is an equal concentration of solute both inside and outside the cell. Water molecules continuously move in and out, but the net concentration on both sides of the semi-permeable membrane remains the same. A demonstration with dialysis tubing filled with tap water and placed in tap water shows that the cell's weight remains constant.