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.
Structural proteins fulfill a variety of roles within and outside of cells. Structural proteins within the cell give it shape, and allow it to withstand stress from the environment or from its own motions. Structural proteins are also used during mitosis and cytokinesis, the processes by which a cell reproduces itself. Structural proteins in bone and cartilage provide strength and support for muscles to work against, and structural proteins in skin, hair, and nails provide a tough outer covering to the body.
Within cells, several proteins help create and maintain the cell's dynamic structure. Together, these proteins form the cytoskeleton. Individual molecules of actin link together to form filaments. These actin filaments are often used to form cables or meshwork to counteract stresses. In the amoeba, actin filaments are rapidly disassembled and reassembled, allowing this single-celled organism to rapidly change its shape as it moves. Alpha and beta tubulin link together to form microtubules, the longest structural fibers within the cell. Microtubules are used to transport cell organelles, and during cell division, pull chromosomes apart into the newly formed daughter cells. Microtubules also form the structural scaffolding for cilia and the tails of sperm.
Collagen is secreted by cells of cartilage and bone to form a dense, resilient matrix. Collagen is formed from three helices of protein wound around one another. The triple helix is extensively cross-linked with others, allowing it to be both flexible and tough. The cells of skin, hair, and nails are packed with keratin. Keratin is naturally quite flexible, but can be strengthened and stiffened by increasing the number of disulfide cross-bridges that link different chains. Elastin is even more flexible due to its very loose network of cross-links, and the weak hydrogen bonds holding different strands together. Elastin gives arteries the ability to expand in response to stretch, and then contract again, an action aided by surrounding muscle tissue.