Molecular motors are protein assemblages that perform mechanical motions for various purposes in living organisms. They derive energy from ATP and operate in a liquid environment influenced by Brownian motion. Research has produced synthetic molecular engines powered by light and heat energy. Myosin, kinesin, and dynein are the main transport motors, while actin is present in muscle contractions. The flagellum of bacteria is the most powerful natural molecular motor discovered.
Molecular motors are assemblages of proteins within the cellular environment of living organisms that, through complex folding and chemical processes, can perform mechanical motions for various purposes, such as transporting materials or electrical charges within the cytoplasm of a cell or replicate DNA and other compounds. Molecular motor proteins are also critical for muscle contractions and actions such as the movement of bacteria through a type of swimming motion driven by a propeller. Most natural molecular motors derive chemical energy for motion from the same basic process that organisms use to produce energy for life support, from the breakdown and synthesis of the compound adenosine triphosphate (ATP).
While at a basic level molecular motors perform many of the same functions as electromechanical motors at the macroscopic human scale, they operate in a very different kind of environment. Most molecular motor activity takes place in a liquid environment driven by thermal forces and directly influenced by the random motion of nearby molecules, known as Brownian motion. This organic environment, coupled with the complex nature of protein folding and the chemical reactions upon which a molecular engine relies to function, has made understanding their behavior a task that has taken decades of research.
Research in atomic and molecular-scale nanotechnology has focused on acquiring biological materials and producing molecular motors that resemble the motors with which everyday engineering is familiar. A striking example of this was an engine built by a team of scientists at Boston College of Massachusetts in the United States in 1999 which consisted of 78 atoms and took four years to build. The engine had a rotating spindle that would take several hours to complete one revolution and was designed to rotate in only one direction. The molecular motor relied on the synthesis of ATP as an energy source and was used as a research platform to understand the fundamentals of the transition of chemical energy into mechanical motion. Similar research has since been completed by Dutch and Japanese scientists using carbon to produce synthetic molecular engines powered by light and heat energy, and recent attempts since 2008 have developed a method to create an engine that produces a continuous level of rotation torque.
Biologically, molecular motors have a diverse list of functions and structures. The main motors of transport are powered by the proteins myosin, kinesin and dynein, and actin is the main protein present in the muscle contractions seen in species as diverse as algae to humans. Research into how these proteins work has become so detailed since 2011 that it is now known that, for every ATP molecule that consumes a 50-nanometer-long kinesin molecule, it is able to move the chemical cargo a distance of 8 nanometers inside a cell. Kinesin is also known to be 50% efficient at converting chemical energy into mechanical energy and capable of producing 15 times more horsepower for its size than a standard gasoline engine.
Myosin is known to be the smallest of the molecular motors, but it is essential for muscle contractions, and a form of ATP called ATP synthase is also a molecular motor used to build up adenosine diphosphate (ADP) for energy storage as ATP. Perhaps the most remarkable natural molecular motor discovered as of 2011, however, is the one that powers the movement of bacteria. A hair-like projection on the back of a bacterium called a flagellum rotates with a propeller-driven motion that, when scaled to the human level of everyday engines, would be 45 times more powerful than the average gasoline engine.
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