24. Vacuole- structure and function

In plant cells, the vacuole takes up a large amount of space, at times, it occupies more than 90% of the plant cell space. It is said that vacuoles are usually formed by the fusion of many membrane vesicles. Due to this reason, a vacuole does not have any specific size nor shape. However, when it comes to the vacuole structure, it is designed so as to complement its function. Many mature and grown plant cells usually have a single large vacuole. This vacuole is surrounded by a structure known as a tonoplast. This tonoplast is said to be a very active and dynamic membrane of this all important part of plant cell structure.

The vacuole in its central, hollow region contains a fluid known as cell sap. This fluid or cell sap contains different compounds, some of which are secretory and some are excretory in nature. Also, the vacuole in the middle, depending on the cell type and requirement, contains various concentrations of salts, sugars and different kinds of soluble pigments. The cell sap, which is a part of the central vacuole structure, also contains various enzymes that are even capable of digesting the cell itself. Although most mature plant cells contain a large single vacuole, when studying the cell biology in young plant cells, there are many vacuoles which slowly enlarge and eventually coalesce together. This eventually pushes the cytoplasm, nucleus and other such structures against the plasma membrane and the cell wall.

How does the Structure of Vacuole Help in its Functioning?

The vacuole structure is designed to aid this important cell organelle, which is one of the key plant and animal cell differences. The vacuole membrane, the tonoplast, helps to separate the contents of the vacuole from those floating in the cytoplasm. Thus, this membrane does not allow harmful substances present in the vacuole from entering and harming the rest of the cell. As the tonoplast is selectively permeable in nature, it also tends to maintain the pH and ionic concentration of the cell, by regulating what travels in and out of the vacuole. Also, due to the vacuole being so large, it pushes the contents of the cell to the borders, near the cell wall and cell membrane and thus, helps maintain the turgor pressure of the cell. Furthermore, the vacuole sap contains many different digestive enzymes that are capable of destroying the cell (which come handy when there is a need for apoptosis). The tonoplast also aids in, along with maintaining turgor pressure, supporting the structures of the leaves and flowers of the plant.

This was all about the vacuole structure and how this structure has been designed to aid in the vacuole function. There is usually a slightly acidic, that is, a low pH maintained inside the vacuole, because this helps in the functioning of the degradative enzymes present inside the vacuole. In some cases, the vacuole structure may differ slightly depending on the type of cell it is present in and its function, for example, in yeast cells, the vacuole is a very dynamic structure that can change its morphology. However, in general, the vacuole usually contains a tonoplast and the cell sap within it.