A feature of plant cells is the presence in their protoplasts of special fluid reservoirs - vacuoles with cell sap. Since its contents are chemically different from the composition of the hyaloplasm, a membrane boundary called the tonoplast passes between them. This shell surrounding the vacuole performs many functions: from maintaining the shape of the organoid itself to regulating the state of the entire cell.
The term is based on two Greek words: tonos (tension) and plastos (fashioned).
Definition of a concept
In short, the tonoplast is a vacuole membrane that separates its contents from the protoplast of the plant cell. According to the features of topography, this structure is referred to as endomembranes. In mature cells, in which there is one large (central) vacuole, the tonoplast becomes the inner border of the protoplast (the plasmalemma serves as the outer one). Thus, the cytoplasm is located between two membranes.
In other words, the tonoplast is a barrier between the two most important sections of a plant cell: protoplast and cell juice, the interaction between which regulates its vital activity.
General characteristics and significance of tonoplast
The contents of the vacuole plays a huge role for the plant cell. Here, various compounds necessary for the functioning of the plant (proteins, salts, pigments, minerals, nutrients), and sometimes degradation products, can accumulate. Vacuolar fluid forms a special intracellular medium with a concentrated content of various compounds.
The structure and functions of the tonoplast are somewhat similar to the plasmalemma. However, if the latter serves as the boundary between the cell and the environment, then the vacuolar membrane is responsible for the material exchange between the cytoplasm and the cell sap. Due to this interaction, the following are regulated:
- chemical composition of hyaloplasm and vacuole;
- processes of storage or, conversely, the release of nutrients and other substances;
- the concentration of ions in the protoplast;
- osmotic characteristics;
- turgor.
Often, it is due to the central vacuole that turgor pressure arises due to the large amount of water entering it. This effect provides elasticity and shape of the plant cell.
Since all the functions of a vacuole are associated with the entry and exit of various substances from it, we can say that tonoplast is a key structure of this organoid, since all transport systems are localized in it.
Tonoplast structure
The structure of the vacuolar membrane was studied using infrared spectroscopy. The latter showed that tonoplast is a lipid bilayer in which various proteins are integrated. That is, in general terms, the structure resembles a typical plasmalemma, however, at a finer level, these membranes have many differences.
Tonoplastic lipids are characterized by an ordered arrangement with a predominance of polar molecules, which provides high elasticity and flowing properties. The membrane contains alpha tocopherol, which causes antioxidant activity.
In the photo below: 1 - mesoplasm; 2 - tonoplast; 3 - vacuole.
Proteins integrated into the tonoplast have varying degrees of immersion. The connection between them and lipid molecules is rather weak. In the spatial structure of the proteins of the vacuolar membrane, a high content of alpha-helical motifs was noted (up to 56%).
The tonoplast surface is penetrated by pores and molecular transport systems that provide selective penetration of substances from the protoplast into the vacuole and vice versa. The carrier channels are formed by various proteins integrated into the lipid layer, including porins.
Tonoplast Functions
Tonoplast performs the following functions:
- isolating - delimits the contents of the vacuole from the protoplast and vice versa;
- protective - ensures the integrity of the organoid, determines the safety of the protoplast (mixing the contents of vacuole with hyaloplasm would disrupt the functioning of the cell);
- osmotic - due to the regulation of ion transport, certain gradients of the concentration of substances are established on both sides of the membrane;
- transmembrane - provides the selective transfer of various compounds between vacuolar contents and protoplast.
In fact, it is tonoplast that controls the chemical composition of the vacuole cell juice and the use of its contents for cellular needs. Of course, the transport channels of the membrane do not work autonomously, but are associated with the biochemical regulatory systems of protoplast.