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Plant cell structure
Animal cell structure

A vacuole is a membrane bound organelle which is present in all plant and fungal cells and some protist, animal[1] and bacterial cells.[2] Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been "eaten up" by the vacuole. The vacuole can absorb materials within the cell because it has a permeable (it has holes to let stuff through) membrane.[3] Vacuoles are formed by the fusion of multiple membrane vesicles and are effectively just larger forms of these.[4] The organelle has no basic shape or size, its structure varies according to the needs of 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 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
  • 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

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 destruction of invading bacteria or of misfolded proteins that have begun to build up within the cell. In protists, vacuoles have the additional function of storing food which has been absorbed by the organism, and assist in the digestive and waste management process for the cell.[5]

Contents

Bacteria

Large central vacuoles are found in three genera of filamentous sulfur bacteria, the Thioploca, Beggiatoa and Thiomargarita. The cytoplasm is extremely reduced in these genera and the vacuole can occupy between 40-98% of the cell.[6] The vacuoles contain high concentrations of nitrate ions and is therefore thought to be a storage organelle.[2]

Plants

Most mature plant cells have one large central 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.[7] 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). 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.

Transport of protons from the cytosol to the vacuole stabilises 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 central 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 plasmolyse. 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.[8]

Some 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.

Fungi

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 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.[9]

Animals

In animal cells, vacuoles perform mostly subordinate roles, assisting in larger processes of exocytosis and endocytosis.

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.

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.[10] Phagocytosis and Pinocytosis are both undertaken in association with lysosomes which complete the breakdown of the material which has been engulfed.[11]

Salmonella is able to survive and reproduce in the vacuoles of several mammal species after being engulfed.[12]

References

  1. ^ Venes, Donald (2001). Taber's Cyclopedic Medical Dictionary (Twentieth Edition), (F.A. Davis Company, Philadelphia), p. 2287.
  2. ^ a b http://www.springerlink.com/content/b23m0r26770x8206/ Microbiology Monographs Volume 1/2006 Vacuoles Heide N. Schulz-Vogt
  3. ^ "Central Vacuole." DNA Fingerprinting. 2010. DNA-Fingerprinting.info 8 Mar. 2010. http://www.dna-fingerprinting.info/central-vacuole.htm
  4. ^ Brooker, Robert J, et al. (2007). Biology (First Edition), (McGraw-Hill, New York), p. 79.
  5. ^ Jezbera Jan, Karel Hornak, Karel Simek (2005). "Food selection by bacterivorous protists: insight from the analysis of the food vacuole by means of fluorescence in situ hybridization". FEMS Microbiology Ecology 52: 351–363. doi:10.1016/j.femsec.2004.12.001. 
  6. ^ Kalanetra KM, Huston SL, Nelson DC (December 2004). "Novel, attached, sulfur-oxidizing bacteria at shallow hydrothermal vents possess vacuoles not involved in respiratory nitrate accumulation". Appl. Environ. Microbiol. 70 (12): 7487–96. doi:10.1128/AEM.70.12.7487-7496.2004. PMID 15574952. 
  7. ^ Alberts, Bruce, Johnson, Alexander, Lewis, Julian, Raff, Martin, Roberts, Keith, and Walter, Peter (2008). Molecular Biology of the Cell (Fifth Edition), (Garland Science, New York), p. 781.
  8. ^ p.13 and 14 Taiz and Zeiger Plant Physiology 3rd Edition SINAUER 2002
  9. ^ http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=372777 Microbiol Rev. 1990 September; 54(3): 266–292. The fungal vacuole: composition, function, and biogenesis. D J Klionsky, P K Herman, and S D Emr
  10. ^ William F. Ganong, MD (2003). REVIEW OF MEDICAL PHYSIOLOGY - 21st Ed.. 
  11. ^ Reggiori F (2006). "Membrane Origin for Autophagy". Current Topics in Developmental Biology 74: 1–30. doi:10.1016/S0070-2153(06)74001-7. 
  12. ^ Leigh A. Knodler, Olivia Steele-Mortimer. Taking Possession: Biogenesis of the Salmonella-Containing Vacuole Traffic Volume 4 Number 9 12 Aug 2003 Pages 587-599 DOI: [1]
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Simple English

File:Biological
Schematic of typical animal cell, showing subcellular components. Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles

A Vacuole is a membranes-bound organelle. They are a kind of vesicle. Vacuoles are closed sacs, made of plasma membranes with inorganic or organic molecules inside, such as enzymes. They have no set shape or size, and the cell can change them as it wants. They are in most eukaryotic cells and do many things. They can store waste. Vacuoles and their contents are considered to be distinct from the cytoplasm, and are classified as ergastic according to some people.[1] The solution that fills the vacuole is called cell sap.

What a vacuole does and how important it is depends on what kind of cell they are in. They are much more important in plant and fungus cells than in animal cells. Some common jobs of a vacuole are:

  • Keeping bad things separate from the rest of the cell
  • Holding waste products
  • Holding water in plant cells
  • Keeping the internal hydrostatic pressure or turgor steady in a cell
  • Keeping an acidic pH on the inside of a cell
  • Holding small molecules
  • Getting rid of things the cell does not want
  • Allows plants to hold themselves upright with hydrostatic pressure
  • In seeds, proteins that seeds use to sprout are put in 'protein bodies'. Protein bodies are just vacuoles that are a little bit different from normal.[2]

Vacuoles are also important in autophagy, keeping a balance between making and getting rid of many things in cells and organisms. They also help with destroying and recycling broken proteins that build up in cells. Thomas Boller [1]and others think that vacuoles help attack bacteria and Robert B Mellor think that some kinds of vacuoles act as a house for symbiotic bacteria. In protists, vacuoles also store and help digest food that the protist ate.[3]

References

  1. Esau, K. (1965). Plant Anatomy, 2nd Edition. John Wiley & Sons. 767 pp.
  2. Matile, Phillipe (1993) Chapter 18: Vacuoles, discovery of lysosomal origin in Discoveries in Plant Biology: v. 1 (World Scientific Publishing Co Pte Ltd)
  3. Jezbera Jan, Karel Hornak, Karel Simek (2005). [Expression error: Unexpected < operator "Food selection by bacterivorous protists: insight from the analysis of the food vacuole by means of fluorescence in situ hybridization"]. FEMS Microbiology Ecology 52 (3): 351–363. doi:10.1016/j.femsec.2004.12.001. PMID 16329920. 
  • (2003) Lange Medical Books/McGraw-Hill, Medical Publishing Division, New York

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