Plant Vacuoles

Plant Vacuoles,

VACUOLES

A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells.

Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been engulfed.
Vacuoles are formed by the fusion of multiple membrane vesicles and are effectively just larger forms of these.
The organelle has no basic shape or size; its structure varies according to the requirements of the cell.

Contractile vacuoles (“stars”) were first observed by Spallanzani (1776) in protozoa, although mistaken for respiratory organs.

Dujardin (1841) named these “stars” as vacuoles. In 1842,
Schleiden applied the term for plant cells, to distinguish the structure with cell sap from the rest of the protoplasm.
In 1885, de Vries named the vacuole membrane as tonoplast.

Most mature plant cells have one large vacuole that typically occupies more than 30% of the cell’s volume, and that can occupy as much as 80% of the volume for certain cell types and conditions.

Strands of cytoplasm often run through the vacuole.
A vacuole is surrounded by a membrane called the tonoplast (word origin: Gk tón(os) + -o-, meaning “stretching”, “tension”, “tone” + comb. form repr. Gk plastós formed, molded) and filled with cell sap.
Also called the vacuolar membrane, the tonoplast is the cytoplasmic membrane surrounding a vacuole, separating the vacuolar contents from the cell’s cytoplasm.
As a membrane, it is mainly involved in regulating the movements of ions around the cell, and isolating materials that might be harmful or a threat to the cell.
Transport of protons from the cytosol to the vacuole stabilizes cytoplasmic pH, while making the vacuolar interior more acidic creating a proton motive force which the cell can use to transport nutrients into or out of the vacuole.
The low pH of the vacuole also allows degradative enzymes to act. Although single large vacuoles are most common, the size and number of vacuoles may vary in different tissues and stages of development.
For example, developing cells in the meristems contain small provacuoles and cells of the vascular cambium have many small vacuoles in the winter and one large one in the summer.

Aside from storage, the main role of the central vacuole is to maintain turgor pressure against the cell wall. Proteins found in the tonoplast (aquaporins) control the flow of water into and out of the vacuole through active transport, pumping potassium (K+) ions into and out of the vacuolar interior. Due to osmosis, water will diffuse into the vacuole, placing pressure on the cell wall. If water loss leads to a significant decline in turgor pressure, the cell will plasmolyze.
Turgor pressure exerted by vacuoles is also required for cellular elongation: as the cell wall is partially degraded by the action of expansins, the less rigid wall is expanded by the pressure coming from within the vacuole.

Turgor pressure exerted by the vacuole is also essential in supporting plants in an upright position.
Another function of a central vacuole is that it pushes all contents of the cell’s cytoplasm against the cellular membrane, and thus keeps the chloroplasts closer to light.
Most plants store chemicals in the vacuole that react with chemicals in the cytosol. If the cell is broken, for example by a herbivore, then the two chemicals can react forming toxic chemicals.
In garlic, alliin and the enzyme alliinase are normally separated but form allicin if the vacuole is broken.
A similar reaction is responsible for the production of syn-propanethial-S-oxide when onions are cut.