In biology, turgid refers to cells or tissues that are swollen from water uptake. Many cell types in many different organisms can become turgid due to water uptake. Some cells will lyse, or split open if they become too turgid. Other cells are meant to be turgid and have a dense and complexly woven extracellular matrix made of special fibrous molecules. In animals, turgid cells are protected by an extracellular matrix consisting of many different molecules. Cartilage and other connective tissues, for example, are made of many proteoglycan molecules. These molecules are mixtures of proteins and sugars that create a firm connection between adjacent cells. The extracellular matrix provides support for the cells and allows them to remain rigid in the face of external pressures on the joints, like running or jumping.
Plant cells, in contrast to animal cells, are almost always turgid due to the action of a large vacuole in each of their cells. The special membrane of this plant-specific organelle, the tonoplast, actively moves water into the vacuole, along with other molecules that need to be stored. This swells the vacuole, creating a pressure on the walls of the cell. This pressure is called turgor pressure. A cell with high turgor pressure is said to be turgid. The turgor pressure exerted by the vacuole pushes outward on the cellulose in the cell wall. The strong cellulose fibers are wound tightly around each other to create a strong cell wall. When the walls of many turgid cells push against each other, a plant can gain a rigid form. While animals use turgid cells only for special functions, the many turgid cells in a plant allow it to stand straight up. Further, by lowering the pressure on specific sides of the plant, the plant can move its leaves and stems to intake the maximum amount of sunlight. Many plants that must compete at getting sunlight are experts in manipulating the pressures of each of their cells in order to move their leaves. It is not yet completely understood how this process works.
Other types of life, like bacteria and fungi, also have cell walls that surround their plasma membranes. Fungi use their cell walls in much the same way as plants, to create a rigid structure that can withstand the elements. Bacteria, because they are single cells, do not use their cell wall to build multi-cellular structures. Instead, the cell wall functions to protect the bacteria from breaking open. Without the cell wall, if the bacteria was exposed to pure water, the water would make the cell too turgid as it rushed in, and the cell would lyse. Instead, as water rushes out and the cell becomes more turgid, the cell wall contains the pressure, and keeps the cell membrane from rupturing. The increased pressure inside the cell offsets the water potential, and the water flow into the cell is cut back. Thus, bacterial cells and other cells with cell walls are protected from becoming too turgid when placed in a weak solution.
As a common laboratory experiment, animal cells will become turgid if they are placed in an environment that is hypotonic in comparison to the contents of the cell. This means that the concentration of solutes in the environment is less than the concentration of solutes in the cell. To deal with too much water, animals must actively pump water out of their cells. If the cells cannot remove the water faster than it enters the cells, the cells will soon become too turgid and lyse. If plant cells are observed in the same situation, it will be seen that they swell up and become turgid, but their cell walls will keep them from lysing.