The following sections of this BookRags Literature Study Guide is offprint from Gale's For Students Series: Presenting Analysis, Context, and Criticism on Commonly Studied Works: Introduction, Author Biography, Plot Summary, Characters, Themes, Style, Historical Context, Critical Overview, Criticism and Critical Essays, Media Adaptations, Topics for Further Study, Compare & Contrast, What Do I Read Next?, For Further Study, and Sources.
(c)1998-2002; (c)2002 by Gale. Gale is an imprint of The Gale Group, Inc., a division of Thomson Learning, Inc. Gale and Design and Thomson Learning are trademarks used herein under license.
The following sections, if they exist, are offprint from Beacham's Encyclopedia of Popular Fiction: "Social Concerns", "Thematic Overview", "Techniques", "Literary Precedents", "Key Questions", "Related Titles", "Adaptations", "Related Web Sites". (c)1994-2005, by Walton Beacham.
The following sections, if they exist, are offprint from Beacham's Guide to Literature for Young Adults: "About the Author", "Overview", "Setting", "Literary Qualities", "Social Sensitivity", "Topics for Discussion", "Ideas for Reports and Papers". (c)1994-2005, by Walton Beacham.
All other sections in this Literature Study Guide are owned and copyrighted by BookRags, Inc.
The term vacuole in biology refers to a membrane bound space, filled with liquid, often located in the center of a cell. Vacuoles are especially prominent in plant cells where they may occupy 95% or more of the cell volume. The vacuole was long considered to be merely a site in which plants disposed of waste material. This view began to change in the 1960s when researchers discovered that vacuoles often serve as protein storage centers. It was found that during embryo formation and seed development, proteins accumulate in the vacuoles of some cells in the cotyledons, or seed leaves, of dicotyledons (flowering plants with two cotyledons per embryo). When the seed germinates, enzymes called proteases are synthesized in the cytoplasm and moved to the vacuole where they digest the stored protein, releasing amino acids needed by the growing plant. It has also been discovered that some vacuoles store carbohydrates for later use as an energy source for the plant. In more mature plant cells, the content of the vacuole, called cell sap, contain a water solution of salts and sugars at sufficiently high concentrations to attract the entry of water into the vacuole, via osmosis. This creates turgor pressure that presses the cell membrane against the cell wall, creating the force needed to expand the volume of growing cells. Turgor pressure also contributes to the rigidity of mature plant tissue.
Some protozoans contain an organelle called a contractile vacuole that enlarges with the influx of water and then discharges its contents to the exterior of the organism when it reaches a certain volume. This cycle of filling and discharging may occur as often as every 30 seconds. The contractile vacuole functions in osmotic regulation by preventing excess water from entering the organism and swelling it beyond a viable size.