3.2) Osmosis

3.2) Osmosis


Osmosis: is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentration solution) through a partially permeable membrane.


A dilute solution means it has lots of water molecules, and a high water potential.

A concentrated solution has few water molecules and low water potential.


  • Water moves in and out of cells by osmosis through the cell membrane.
  • It is important that the cells in an animal’s body are surrounded by a liquid which has the same concentration as the liquid inside the cells.
  • Tissue fluid: the liquid outside the cells.
  • Plants are supported by the pressure of water inside the cells pressing outwards on the cell wall.

 If a plant cell is placed in distilled water, water molecules will move from the distilled water to the cell, the cell swells up and becomes turgid but it will never burst because plant cells are surrounded by cell walls, which are made of cellulose and is elastic, it will stretch but never break. The vacuole is exerting turgor pressure on the elastic cell wall.


If we place a plant cell in a concentrated salt solution with low water potential, water will move from the cell to the solution causing the cell to become plasmolysed.


The importance of osmosis:

Plants gain water through osmosis in their roots from the soil. Without a water potential gradient,water will be loss from the roots. Plant cells contain vacuoles, which, if not full with water, will cause the cell to become flaccid. If all the cells in a leaf become flaccid, the whole leaf will become flaccid, hence causing the plant to wilt. Plant cells therefore need water to remain turgid and keep firm.


If an animal cell surrounded with a high water potential, osmosis will take place, and if the water is not expelled some way or another, the cell will burst (a process called haemolysis in red blood cells). This is because an animal cell doesn’t have a cell wall to keep it strong. If an animal cell is surrounded with low water potential, the water in the cytoplasm will diffuse outwards, causing the cell to shrink (crenated)