Turgor is the internal pressure generated by fluid within a plant cell pushing its contents against the cell wall. This force can be compared to the air pressure inside a tire that keeps it firm and functional. When a plant has sufficient water, this internal force keeps the cells rigid, providing structural integrity to the entire organism.
The Mechanism of Turgor Pressure
Turgor pressure is a direct result of osmosis, where water moves across a semipermeable membrane from an area of higher water concentration to one of lower concentration. In plants, the cell membrane acts as this barrier, allowing water to pass through but restricting solutes. Inside a plant cell, the cytoplasm and a large sac called the central vacuole contain water with dissolved sugars, salts, and other molecules.
The high concentration of solutes inside the cell draws water inward. As water flows into the cell, it primarily fills the central vacuole, causing it to swell. This swelling pushes the cell’s contents, known as the protoplast, against the rigid cell wall.
Unlike an animal cell, which would burst from such an influx of water, a plant cell is encased in a strong cell wall primarily made of cellulose. This wall provides structural resistance, pushing back against the swelling protoplast. The force exerted by the protoplast against the cell wall is turgor pressure, which makes the cell firm, or turgid.
The Importance of Turgor in Plants
Turgor pressure provides structural support to non-woody parts of a plant, like leaves and young stems. This internal pressure in each cell creates overall rigidity, allowing leaves to be held out towards sunlight and keeping stems upright. Without this support, these plant parts would droop and the plant could not maintain its shape.
Cell expansion and plant growth are driven by turgor pressure. For a cell to enlarge, its wall must expand, and turgor provides the outward force necessary to stretch it. This process allows for irreversible expansion and is particularly active in growing root tips and young leaves.
Turgor pressure also facilitates regulatory functions and movements. The opening and closing of stomata, small pores on the leaf surface for gas exchange, are regulated by turgor changes in the surrounding guard cells. High pressure opens the stomata for carbon dioxide uptake, while low pressure closes them to conserve water. Some plants also use rapid turgor changes for movement, like the folding of a sensitive plant’s leaves when touched.
Loss of Turgor and Wilting
When a plant loses water faster than its roots can absorb it, the central vacuole shrinks, reducing the volume of the cell’s contents. As the vacuole retracts, it no longer pushes firmly against the cell wall, causing a drop in turgor pressure. A cell in this soft or limp condition is described as being flaccid.
The visible consequence of widespread flaccidity is wilting, the drooping of leaves and stems that can no longer be supported by internal pressure. Wilting indicates the plant is experiencing water stress, often caused by insufficient soil water or high rates of water loss through transpiration on a hot day.
In severe cases of water loss, the cell membrane can pull away from the cell wall, a process known as plasmolysis. While wilting is often reversible with rehydration, allowing cells to regain turgor, prolonged water loss can lead to cell death and permanent damage.