Pressure, Turgor Pressure, Water Potential.Pdf

Water movement in plant cells is complicated by the fact that plant cells may also be under pressure. As plant cells ﬁll up their central vacuoles with water, their plasma membranes push against the walls around the cells. The pressure on plant cells that results from the uptake of water is called turgor pressure. Under pressure

As water ﬂows into a plant cell and the cell becomes increasingly ﬁrm, or turgid, the increase in turgor pressure may slow the movement of water even if the water concentration hasn’t yet reached equilibrium. Under pressure

To understand the relationship between osmosis and turgor pressure, think of a plant cell as a water balloon inside a shoe box. In this case, the latex of the balloon represents the plasma membrane of the cell, and the box is the cell wall. The water balloon is ﬁlled with salty water to represent the cytoplasm of the cell. If you attach a hose to the water balloon so that pure water comes into contact with the cytoplasm, then water will ﬂow into the balloon. Under pressure

The water is moving by osmosis from an area of low solutes (pure water) to an area of high solutes (the salty solution inside the balloon). As water moves into the balloon, however, the balloon stretches and begins to push against the box. Eventually, the pressure from the box will prevent any more water from ﬂowing into the balloon, even if the solution inside the balloon hasn’t reached equilibrium with the solution from the hose. Under pressure

Whether or not water moves in or out of plant cells depends upon both turgor pressure and solute concentration. Botanists combine both of these factors to determine water potential of a cell. Water potential is represented by the symbol and is measured in megapascals (mPa), which is a unit of pressure (one atmosphere, which is the pressure that the atmosphere puts on the earth at sea level, equals 0.1 mPa). Under pressure

You can think of water potential as the tendency of water to leave one place in favor of another. To ﬁgure out the water potential of a cell or solution within a plant, you add together the effects of solute concentration and turgor pressure. Water always moves from an area of higher water potential to an area of lower water potential. Losing it

Water can move into plant cells, and it can move out of plant cells, too. If you put celery or carrot sticks into a salty cup of water, they’ll pretty quickly get limp and rubbery as they lose their turgor pressure. Losing it

Because of the dissolved salt, the salty water has a lower water potential than the plant cells. Water ﬂows out of the plant cells, and the cells no longer push up against their cell walls. Turgor pressure is lost, and the vegetables become limp. Losing it

If you looked at the cells in the vegetables under a microscope, you’d see plasmolyzed cells. When water leaves a walled cell through osmosis, the cytoplasm collapses, and the plasma membrane pulls away from the cell wall. The collapse of the cell is called plasmolysis.