More than 90% of most plant tissues is composed of water. Water is needed for most life processes to take place. The movement of water (a) from cell vacuole (b) to another cell’s vacuole (c) within the plant is by diffusion through the cell’s semipermeable plasma membrane (d), which encloses the cytoplasm (e). The flow is from a region of high concentration of water to a region of low concentration of water, a process that is called osmosis. The more substances (solutes) dissolved in the water, the lower the concentration (water potential) of water. For example, in a leaf cell that is producing sugar (a solute, f) from photosynthesis, water passes into the cell. An osmotic equilibrium is reached when water no longer enters the cell.

Turgor Pressure

As water (a) passes into a cell, it expands the plasma membrane (d), exerting pressure against the cell wall (g). This is called turgor pressure. In fully inflated cells, turgor pressure is one of the key “driving forces” in plant growth. In cells of developing fruits, turgor pressure is very important for fruit enlargement, and indicates the necessity of keeping plants well watered during this period. Turgor pressure maintains the size and shape of
plant parts such as leaves and flowers.

Plasmolysis

Water (a) is lost from a cell when there is a lower water concentration in adjacent cells that have more solutes present. Turgor pressure within the cell then decreases.
When there is no turgor pressure, the plasma membrane (d) shrinks from the cell wall (g), leaving through the cell wall. This process is termed plasmolysis and is
reversible when water is returned to the cytoplasm and vacuole through osmosis. Plasmolysis of cells results in wilting of plant shoots and is the opposite of fully turgid cells, where plants are fully “inflated” or crisp, like fresh celery.

Wilting

When excessive cell water (a) is lost through evapora￾tion to surrounding dry air, “wilting” occurs. In this condition cell turgor pressure is lost, the plasma membrane
(d) shrinks from the cell wall, and cell wall (g) contracts because there is no external solution to fill in between.
This condition can result in plants that are “permanently wilted” if the wilting persists over a period of 10 to 12 days. If wilted plants are watered before they reach the
permanent wilting condition, they regain turgor in several hours time.