Vacuole
Vacuoles are membrane-bounded compartments within some eukaryotic cells that can serve a variety of secretory, excretory, and storage functions. Vacuoles and their contents are considered to be distinct from the cytoplasm, and are classified as ergastic according to some authors (Esau, 1965). Vacuoles are especially conspicuous in most plant cells.
In general, vacuole functions include:
- 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, maintaining a balance between biogenesis (production) and degradation (or turnover), of many substances and cell structures. They also aid in destruction of invading bacteria or of misfolded 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.
Protists
Some protists and macrophages use food vacuoles as a stage in phagocytosis—the intake of large molecules, particles, or even other cells, by the cell for digestion. They are also called "storage sacs."
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.
Plants
Most mature plant cells have a single 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 a liquid called cell sap, composed of water, enzymes, inorganic ions (like K+ and Cl-), salts (such as calcium), and other substances, including toxic byproducts removed from the cytosol to avoid interference with cell metabolism. Toxins located in the vacuole may also help to protect some plants from predators. Transport of protons from cytosol to vacuole aids in keeping cytoplasmic pH stable, while making the vacuolar interior more acidic, allowing degradative enzymes to act. Although having a large central vacuole is the most common case, the size and number of vacuoles may vary in different tissues and stages of development. Cells of the vascular cambium, for example, have many small vacuoles in winter, and one large one in 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 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 plasmolyse. Turgor pressure exerted by vacuoles is also helpful for cellular elongation: as the cell wall is partially degraded by the action of auxins, the less rigid wall is expanded by the pressure coming from within the vacuole. Vacuoles can help some plant cells to reach considerable size. 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.
The vacuole also stores the pigments in flowers.
Budding yeast
In budding yeast cells, vacuoles act as storage compartments of amino acids and detoxification compartments. 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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- Esau, K. 1965. Plant Anatomy, 2nd Edition. John Wiley & Sons. 767 pp.