A vacuole is
a membrane-bound cell organelle. In animal cells, vacuoles are generally small and help sequester waste products. In plant cells, vacuoles help maintain water balance. Sometimes a single vacuole can take up most of the interior space of the plant cell.
The function and significance of vacuoles varies greatly according to the type of cell in which they are present, having much greater prominence in the cells of plants, fungi and certain protists than those of animals and bacteria. In general, the functions of the vacuole include:
- Isolating materials that might be harmful or a threat to the cell
- Containing waste products
- Containing water in plant cells
- Maintaining internal hydrostatic pressure or turgor within the cell
- Maintaining an acidic internal pH
- Containing small molecules
- Exporting unwanted substances from the cell
- Allows plants to support structures such as leaves and flowers due to the pressure of the central vacuole
- By increasing in size, allows the germinating plant or its organs (such as leaves) to grow very quickly and using up mostly just water.[9]
- In seeds, stored proteins needed for germination are kept in 'protein bodies', which are modified vacuoles.[10]
Vacuoles also play a major role in
autophagy, maintaining a balance between
biogenesis (production) and degradation (or turnover), of many substances and cell structures in certain organisms. They also aid in the
lysis and recycling of misfolded proteins that have begun to build up within the cell. Thomas Boller
[11] and others proposed that the vacuole participates in the destruction of invading
bacteria and
Robert B. Mellor proposed organ-specific forms have a role in 'housing' symbiotic bacteria. In protists,
[12] vacuoles have the additional function of storing food which has been absorbed by the organism and assisting in the digestive and waste management process for the cell.
[13]
In animal cells, vacuoles perform mostly subordinate roles, assisting in larger processes of
exocytosis and
endocytosis.
Animal vacuoles are smaller than their plant counterparts but also usually greater in number.
[14] There are also animal cells that do not have any vacuoles.
[15]
Exocytosis is the extrusion process of proteins and lipids from the cell. These materials are absorbed into secretory granules within the
Golgi apparatus before being transported to the cell membrane and secreted into the extracellular environment. In this capacity, vacuoles are simply storage vesicles which allow for the containment, transport and disposal of selected proteins and lipids to the extracellular environment of the cell.
Endocytosis is the reverse of exocytosis and can occur in a variety of forms.
Phagocytosis ("cell eating") is the process by which bacteria, dead tissue, or other bits of material visible under the microscope are engulfed by cells. The material makes contact with the cell membrane, which then invaginates. The
invagination is pinched off, leaving the engulfed material in the membrane-enclosed vacuole and the cell membrane intact.
Pinocytosis ("cell drinking") is essentially the same process, the difference being that the substances ingested are in solution and not visible under the microscope.
[16] Phagocytosis and pinocytosis are both undertaken in association with
lysosomes which complete the breakdown of the material which has been engulfed.
[17]
Salmonella is able to survive and reproduce in the vacuoles of several
mammal species after being engulfed.
[18]
The vacuole probably evolved several times independently, even within the
Viridiplantae.
[14]
Vacuole types
Gas vacuoles
Gas vesicles, also known as gas vacuoles, are nanocompartments which are freely permeable to gas,
[19] and occur mainly in Cyanobacteria, but are also found in other bacteria species and some archaea.
[20] Gas vesicles allow the bacteria to control their buoyancy. They are formed when small biconical structures grow to form spindles. The vesicle walls are composed of a hydrophobic gas vesicle protein A (GvpA) which form a cylindrical hollow, proteinaceous structure that fills with gas.
[20][21] Small variances in the amino acid sequence produce changes in morphology of the gas vesicle, for example, GvpC, is a larger protein.
[22]
Central vacuoles

The
anthocyanin-storing vacuoles of
Rhoeo spathacea, a
spiderwort, in cells that have plasmolyzed
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.
[23] 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.
[24]
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.
[25] 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.[
citation needed]
Vacuoles in fungal cells perform similar functions to those in plants and there can be more than one vacuole per cell. In
yeast cells the vacuole (
Vac7) is a
dynamic structure that can rapidly modify its
morphology. They are involved in many processes including the
homeostasis of cell pH and the concentration of ions,
osmoregulation, storing
amino acids and
polyphosphate and degradative processes. Toxic ions, such as
strontium (Sr2+
),
cobalt(II) (Co2+
), and
lead(II) (Pb2+
) are transported into the vacuole to isolate them from the rest of the cell.
[26]