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Structural View of Biology

Infrastructure and Communication Cells require a complex infrastructure of molecules for support and communication. Our bodies contain about 10 trillion cells, which must cooperate for the good of the entire body. This requires a complex infrastructure to organize them into tissues and organs, and a complex network of signals to coordinate their action.

Huge proteins are built to support cells, tissues and organs. These include several types of filaments that form a cytoskeleton inside cells, and complex scaffolding of proteins that form structures outside cells, and adhesive proteins that connect the structures inside with the structures outside.

Scroll to a Molecule of the Month Feature in this subcategory:

  • Actin


    The complex ultrastructure of cells--their shape and internal structure--and the many motions of cells are largely supported by filaments of actin. A tangle of cross-linked actin filaments fills the cytoplasm of animal, plant and fungal cells, forming a "cytoskeleton" that gives the cell shape and form and provides a scaffold for organization. Tightly bundled actin filaments provide a sturdy backbone to extrude structures from the cell surface, such as the pseudopods used by amoebas for crawling and the finger-like microvilli of intestinal cells, which extend into the digestive tract and absorb nutrients. As we saw last month, actin also forms the ladder on which myosin climbs, providing the infrastructure for muscle contraction and creating the motion that we experience in our daily lives. Actin is plentiful throughout the body as it performs these basic structural tasks: it may comprise 5 percent of the protein in a typical cell, or up to one fifth of the protein in special cases, such as muscle cells.

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  • Cadherin


    Your body is composed of trillions of cells, all working together to keep you alive. As you might imagine, this requires a massive infrastructure to hold everything together. This infrastructure is built at many levels. Huge structures, like bones and tendons, are built to support and move the entire body. Many of the spaces between cells are supported by connective tissue, which is built from a collection of sturdy molecular cables and sheets. Finally, an intimate, molecule-sized infrastructure is used to adhere cells directly to their neighbors.

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  • Clathrin


    With its intricate meshwork of protein braids and alluring symmetry, clathrin is sure to seize your attention. It was named in the 1960s for its clathrate (lattice of bars) appearance in electron micrographs, and to this day, this beautiful molecule invokes intensive study. Like many proteins, clathrin represents a perfect case of form following function; it performs critical roles in shaping rounded vesicles for intracellular trafficking.

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    Discussed Structures
  • Collagen


    About one quarter of all of the protein in your body is collagen. Collagen is a major structural protein, forming molecular cables that strengthen the tendons and vast, resilient sheets that support the skin and internal organs. Bones and teeth are made by adding mineral crystals to collagen. Collagen provides structure to our bodies, protecting and supporting the softer tissues and connecting them with the skeleton. But, in spite of its critical function in the body, collagen is a relatively simple protein.

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  • Integrin


    Our bodies are composed of approximately ten trillion cells, which poses challenging problems for structure and communication. All of these cells must be connected strongly together, to allow us to stand and walk. The infrastructure holding us together, however, must also be malleable enough to allow repairs, to allow us to heal from wounds. These many cells must also communicate with each other, ensuring that each plays its own proper part. Many different molecules in our bodies are involved in this complex infrastructure of support and communication, and integrins play a central role.

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  • Microtubules


    Microtubules are the railways of the cell. They are huge, sturdy filaments that extend through the cytoplasm, providing support and providing tracks for the motion of two types of protein motors: kinesin and dynein. These motors pull many types of cargo through the cell, ranging from small vesicles to entire mitochondria. They also play a starring role in the process of cell division, separating the duplicated chromosomes into two daughter cells.

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  • Titin


    Titin is the largest protein chain in your body, with more than 34,000 amino acids. This titanic protein acts like a big rubber band in our muscles. It is attached at one end to the Z-disk (shown here at the top in blue), which organizes the thin actin filaments. The other end of titin is attached to the M-line (shown in red at the bottom), which organizes the thicker myosin filaments. In between, titin (shown in yellow) has a number of elastic elements that stretch and contract, holding the whole contractile apparatus in the proper shape as the muscle flexes.

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    Discussed Structures
  • Vaults


    Our cells are filled with compartments, each performing a specific function. Some of these compartments, such as mitochondria and lysozomes, are very large and enclose many different molecular machines. Other intracellular compartments are smaller, such as the transport vesicles that shuttle proteins from site to site inside the cell. Most of these compartments, including mitochondria, lysozomes and transport vesicles, are surrounded by membranes. However, in special cases, cells build smaller compartments surrounded by a protein shell. In our own cells, vaults are a spectacular example of these protein-enclosed compartments.

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    Discussed Structures

Please see our usage polices for citation and reprint information. Copies of the illustrations used in these features are available for download as high resolution TIFF images. Please note that the structures used to illustrate each installment are chosen at the discretion of the authors; the features are not intended to represent a historical record. The process behind the creation of this feature is described by the author.