A feature of plant cells is the presence in their protoplasts of special liquid reservoirs - vacuoles with cell sap. Since its contents are chemically different from the composition of the hyaloplasm, a membrane boundary passes between them, called the tonoplast. 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 (sculpted).
Definition of concept
In short, the tonoplast is the membrane of the vacuole that separates its contents from the protoplast of the plant cell. According to the features of the topography, this structure is referred to as endomembrane. 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 between two membranes.
In other words, the tonoplast is a barrier between the two most important compartments of a plant cell: the protoplast and cell sap, the interaction between which regulates its vital activity.
General characteristics and significance of the tonoplast
The content of the vacuole plays a huge role for the plant cell. Various compounds necessary for the functioning of the plant (proteins, s alts, pigments, minerals, nutrients), and sometimes degradation products, can accumulate here. The vacuolar fluid forms a special intracellular environment 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 of the interaction of the cell with the external environment, then the vacuolar membrane is responsible for the material exchange between the cytoplasm and cell sap. Due to this interaction are regulated:
- chemical composition of hyaloplasm and vacuole;
- processes of storage or, conversely, the release of nutrients and other substances;
- concentration of ions in the protoplast;
- osmotic characteristics;
- turgor.
It is often due to the central vacuole that turgor pressure occurs, which is created due to the inflow of a large amount of water into it. This effect ensures the elasticity and shape of the plant cell.
Since all the functions of the vacuole are associated with the entry and exit of various substances from it, we can say that the tonoplast is the key structure of this organoid, since it is inall transport systems are localized there.
Tonoplast structure
The structure of the vacuolar membrane was studied using infrared spectroscopy. The latter showed that the tonoplast is a lipid bilayer into which various proteins are integrated. That is, in general terms, the structure resembles a typical plasmalemma, but at a more subtle level, these membranes have many differences.
Tonoplast lipids are characterized by an ordered arrangement with a predominance of polar molecules, which provides high elasticity and fluid properties. The membrane contains alpha-tocopherol, which determines the antioxidant activity.
In the photo below: 1 - mesoplasm; 2 - tonoplast; 3 - vacuole.
Proteins integrated into the tonoplast have different degrees of immersion. The bond between them and lipid molecules is rather weak. A high content of alpha-helical motifs (up to 56%) was noted in the spatial structure of vacuolar membrane proteins (up to 56%).
The surface of the tonoplast is permeated with pores and molecular transport systems that provide selective penetration of substances from the protoplast into the vacuole and back. 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 safetyprotoplast (mixing the contents of the vacuole with hyaloplasm would disrupt the functioning of the cell);
- osmotic - due to the regulation of ion transport, certain concentration gradients of substances are established on both sides of the membrane;
- transmembrane - provides selective transfer of various compounds between the vacuolar content and the protoplast.
In fact, it is the tonoplast that controls the chemical composition of the cell sap of the vacuole and the use of its contents for cellular needs. Of course, the transport channels of the membrane do not work autonomously, but are connected with the biochemical regulatory systems of the protoplast.
