A vacuole is a membrane-bound organelle which is present in most eukaryotic cells.1 Vacuoles are essentially enclosed compartments which are filled with fluid such as water or various enzymes, though in certain cases they may contain solids which have been engulfed. The majority of vacuoles are formed through the fusion of multiple membrane vesicles.2 The organelle has no basic shape or size, instead varying its structure according to the needs of the cell.
In general, vacuole functions include:
Isolating materials that might be harmful or a threat to the cell
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. Vacuoles store food and other materials needed by a cell. They also aid in destruction of invading bacteria or of misfolded proteins that have begun to build up within the cell. The function and importance 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.
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.3 A vacuole is surrounded by a membrane called the tonoplast.
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 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 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.
Animals
In animal cells, vacuoles perform mostly subordinate roles, assisting in larger 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. 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.
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.4 Phagocytosis and Pinocytosis are both undertaken in association with lysosomes which complete the breakdown of the material which has been engulfed.5
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
^ Venes, Donald (2001). Taber's Cyclopedic Medical Dictionary (Twentieth Edition), (F.A. Davis Company, Philadelphia), p. 2287.
^ Brooker, Robert J, et al. (2007). Biology (First Edition), (McGraw-Hill, New York), p. 79.
^ Alberts, Bruce, Johnson, Alexander, Lewis, Julian, Raff, Martin, Roberts, Keith, and Walter, Peter (2002). Molecular Biology of the Cell (Fourth Edition), (Garland Science, New York), p. 740.
^ William F. Ganong, MD (2003). REVIEW OF MEDICAL PHYSIOLOGY - 21st Ed..
^ Reggiori F (2006). "Membrane Origin for Autophagy". Current Topics in Developmental Biology74: 1-30. doi:10.1016/S0070-2153(06)74001-7.
(2003) Lange Medical Books/McGraw-Hill, Medical Publishing Division, New York
