Vacuole

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Template:Cleanup-date Vacuoles are large membrane-bound compartments within some eukaryotic cells where they serve a variety of different functions in different types of organisms, mainly storage. The cavity that is the vacuole is considered nonprotoplasmic and the contents classified as ergastic according to some authors (Esau, 1965). Vacuoles are especially conspicuous in most plant cells.

Vacuoles are typically filled with a liquid called cell sap, the composition of which can vary (even between vacuoles in the same cell), but is principally water. Water tends to move along concentration gradients into vacuoles.

In general, vacuole functions include:

capturing food materials
removing unwanted structural debris surrounding the cell
sequestering materials that might be toxic to the cell
containment of waste products
maintaining internal hydrostatic pressure or turgor within the cell
maintaining an acidic internal pH
storing small molecules
exporting unwanted substances from the cell
enabling the cell to elongate rapidly or otherwise alter relative cell size.

Vacuoles also play a major role in autophagy. There, vacuoles maintain a balance between biogenesis (production) and degradation, or the turnover, of cell structures. They also aid the destruction of problematic and unnecessary bacteria and proteins that have begun to aggregate within the cell. Autophagy is especially prominent in insects that undergo complete metamorphosis; for example, larval tissue is recycled to become appendages in an adult insect.

1 Protists
2 Plants
3 Budding yeast cells
4 References

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Protists

Some protists and macrophages use food vacuoles in phagocytosis—the intake of large molecules, particles, or even other cells, by the cell for digestion.

A contractile vacuole is used to pump excess water out of the cell to reduce osmotic pressure and keep the cell from bursting, which is referred to as cytolysis or osmotic lysis. Contractile vacuoles are found in some freshwater protozoa, such as paramecium.

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Plants

Image:Rhoeo Discolor epidermis.jpg Image:Rhoeo Discolor - Plasmolysis.jpg Most mature plant cells have a central vacuole, which often takes up more than 80% of the cell interior. It is surrounded by a membrane, called the tonoplast. This vacuole houses large amounts of water, enzymes, inorganic ions (like K⁺ and Cl^-), salts (such as calcium), and other substances, like toxic byproducts which are hence removed from the cytosol away from cell metabolism. The toxic byproducts located in the vacoule also help to protect from predators. By letting protons in, it helps in keeping the cell's pH stable, while making its interior more acidic which is necessary for the degradative enzymes. Even though having a single, big vacuole is the most common case, the size and number of vacuoles may vary in different tissues and stages of development. Cambium cells, for example, have many small vacuoles in winter, and a large one in summer.

The central vacuole's main role (besides being a storage center) is to keep a turgor pressure against the cell wall. Due to osmosis, water will diffuse into the vacuole, placing the pressure on the cell wall. If it fails at this (by losing water) the cell will plasmolyse. This is why the central vacuole is so comparatively large in the plant cell. Proteins found in the tonoplast control the flow of water into and out of the vacuole by active transport, pumping ions of potassium (K⁺) into and out of the interior liquid. This pressure is also helpful for cellular elongation: when the cell wall is degraded (not entirely) by the action of auxins, this now less rigid wall is expanded by the pressure coming from within the cell. Yet another function is that it pushes all the cell's cytoplasm against the cellular membrane, and thus keeps the chloroplasts closer to light. The vacuoles also help some plant cells to reach considerable size.

The vacuole also stores the pigments in flowers.

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Budding yeast cells

In budding yeast cells, vacuoles are the storage compartments of amino acids and the detoxification compartment, which are described in previous topics. Recently, vacuoles of budding yeast cells have been studied. As a result, another function of vacuoles was discovered. Under conditions of starvation, proteins are degraded in vacuoles; this is called autophagy. First, cytoplasms, mitochondria, and small organelles are covered with multiplex plasma membranes called autophagosomes. Next, the autophagosomes fuse the vacuoles. Finally, the cytoplasms and the organelles are degraded.

In a vacuole of budding yeast, a black particle sometimes appears. It is called a dancing body. The dancing body moves actively in the vacuole and appears and disappears within 10 minutes to several hours. In previous research, it was suggested but not proven that the main component of the dancing body is polyphosphate acid. But the main component has been determined to be crystallized sodium polyphosphate and its function has been studied. It is thought that its function is to supply and store phosphates in budding yeast cells.

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References

Esau, K. 1965. Plant Anatomy, 2nd Edition. John Wiley & Sons. 767 pp.

Organelles of the cell
Acrosome \| Cell wall \| Cell membrane \| Chloroplast \| Cilium/Flagellum \| Centrosome \| Cytoplasm \| Endoplasmic reticulum \| Golgi apparatus \| Lysosome \| Melanosome \| Mitochondrion \| Myofibril \| Nucleus \| Parenthesome \| Peroxisome \| Plastid \| Ribosome \| Vacuole \| Vesicle