What is a vacuole? Definition:
vacuoles are extensible cell organelles, which occur mainly in plant and fungal cells and act as a storage medium for water (and nutrients dissolved therein). The term derives from the Latin "vacuus" (vacuum or empty space). Under the microscope, the vacuoles filled with water are usually very visible because of their relatively large size. In plant cells, vacuoles generally occupy more than 3/4 of the cell interior.
Vacuoles arise during cell growth; more specifically in the course of extension growth. The extension growth serves to increase the size of the plant. In contrast to the division growth, the extension growth is much more energy efficient.
Variants of the vacuole Vacuoles also occur in the form of contractile vacuoles in the known paramecia. Contractile vacuoles are contracting blisters that serve to remove water from the cell. For this, the contractile vacuoles increase and decrease rhythmically, taking up cellulosic fluid from within and releasing it to the outside of the cell. The influx of water is due to the higher osmotic pressure within the cell and is due to this. Within the cell, the salt concentration is thus higher than in the surrounding freshwater. Without the presence of contractile vacuoles, the osmotic pressure within the cell would be too high and the cell would burst.
Structure of the vacuole
Vacuoles have a similar structure to vesicles. Both organelles are only surrounded by a membrane. While vacuoles are always equipped with a single membrane called tonoplast, vesicles can also have a double membrane. The tonoplast can absorb water on demand through its semipermeable membrane and release it again for transpiration or for photosynthesis. As soon as the expandable vacuole becomes saturated with water, its circumference increases many times and is limited only by the cell wall. Vacuoles are therefore so large that they can be easily recognized under a microscope.
Function of the vacuole
The most important function is fulfilled by the vacuole by generating the so-called Turgordrucks. As the vacuole increases its volume in the cell, the pressure inside the cell also increases. Because particles always strive for a balance of the substance concentration (see osmosis and diffusion), water flows from the outside into the cell.
Vacuoles perform a variety of tasks. For example, they can serve as a substance store for proteins, such as legumes in the cotyledons, but also absorb organic compounds. They serve as temporary storage of toxic waste products from the metabolism of the plant. In the vacuoles, the toxins can be reliably isolated from the rest of the cell. Toxic plants store their toxins in the vacuoles. In this way, a plant can protect itself against predators without poisoning itself by the poison itself.