Vacuole

  Vacuoles are found in the cytoplasm of most plant cells and some animal cells. Vacuoles are membrane-bound 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.^[1] Vacuoles are especially conspicuous in most plant cells.

In general, vacuole functions include also

• Removing unwanted structural debris
• Isolating materials that might be harmful or a threat to the cell
• Containing waste products
• Maintaining internal hydrostatic pressure or turgor within the cell
• Maintaining an acidic internal pH
• Containing small molecules
• Exporting unwanted substances from the cell
• Enabling the cell to change shape

Vacuoles also play a major role in autophagy, maintaining an 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 build up within the cell. The vacuole is a major part in the plant and animal cell.

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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.

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, black particles sometimes appear, 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.

Plants

Most mature plant cells have one or several vacuoles that typically occupy more than 30% of the cell's volume, and that can occupy as much as 90% of the volume for certain cell types and conditions.^[2] A vacuole 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 metabolism. Toxins present 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 and fruits.

Animals

Vacuoles in animals are a part of the processes of exocytosis and endocytosis. Exocytosis is the extrusion process of proteins from the Golgi apparatus initially enter secretory granules, where processing of prohormones to the mature hormones occurs before exocytosis, and also allows the animal cell to rid waste products. Endocytosis is the reverse of exocytosis. There are various types. 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 ^[3]

Hydropic (vacuolar) changes are of importance of identifying various pathologies, such as the reversible cell swelling in renal tubules caused by hypoperfusion of the kidneys during open heart surgery.

References

• (2003) Lange Medical Books/McGraw-Hill, Medical Publishing Division, New York