The figure above shows the guard cells with a stoma in between them. The guard cells are actually an example of specialized cells or modified cells. Modified in such a way that they can perform their function well. Their function is to allow the stoma to open or to close.
In what way are they specialized?
They are specialized in such a way that their cell wall around the stoma is much thicker. This allows the cells to bend on one side when they become turgid. Hence causing the stoma to open.
Opening:
Stoma opens when the guard cells are turgid. They become turgid when the water potential of the cells adjacent to the guard cells are higher than that in the cell sap of the guard cells. This result in water molecules from the adjacent cells to move into the guard cells by osmosis. When water molecules enter, the vacuoles increase in size causing osmotic pressure to build up and this in turn causes the guard cells to become turgid. When the guard cells become turgid, they bend inwards causing the stoma to open. The opening of the stoma is an advantage because it allows gaseous exchange to take place.
Closing:
When the water potential of the adjacent cells is lower than that in the cell sap of the guard cells, water molecules move out of the guard cells into the adjacent cells by osmosis. When this happens, the guard cells become plasmolysed which in turn causes the stoma to close.
In what way are they specialized?
They are specialized in such a way that their cell wall around the stoma is much thicker. This allows the cells to bend on one side when they become turgid. Hence causing the stoma to open.
Opening:
Stoma opens when the guard cells are turgid. They become turgid when the water potential of the cells adjacent to the guard cells are higher than that in the cell sap of the guard cells. This result in water molecules from the adjacent cells to move into the guard cells by osmosis. When water molecules enter, the vacuoles increase in size causing osmotic pressure to build up and this in turn causes the guard cells to become turgid. When the guard cells become turgid, they bend inwards causing the stoma to open. The opening of the stoma is an advantage because it allows gaseous exchange to take place.
Closing:
When the water potential of the adjacent cells is lower than that in the cell sap of the guard cells, water molecules move out of the guard cells into the adjacent cells by osmosis. When this happens, the guard cells become plasmolysed which in turn causes the stoma to close.