Molecular motors are assemblies of proteins within the cellular environment of living organisms that, through complex folding and chemical processes, can perform mechanical motion for various purposes, such as to transport materials or electrical charges within the cytoplasm of a cell or replicate DNA and other compounds. Molecular motor proteins are also fundamental to muscle contractions and actions such as the movement of bacteria through a type of propeller-driven swimming motion. Most natural molecular motors derive chemical energy for motion from the same basic process that organisms use to produce energy for life support — by the breakdown and synthesis of the compound adenosine triphosphate (ATP).
Though on a basic level molecular motors perform many of the same functions as electro-mechanical motors at the macroscopic human scale, they operate in a much different type of environment. Most molecular motor activity takes place in a liquid environment that is driven by thermal forces and directly affected by the random motion of nearby molecules, known as Brownian motion. This organic environment, along with the complex nature of protein folding and chemical reactions that a molecular motor relies on to function, has made gaining an understanding of their behavior one that has taken decades of research.
Research in nanotechnology at the atomic and molecular scale has focused on taking biological materials and manufacturing molecular motors that resemble the motors with which everyday engineering is familiar. A prominent example of this was a motor constructed by a team of scientists at the Boston College of Massachusetts in the US in 1999 that consisted of 78 atoms, and took four years of work to construct. The motor had a rotating spindle that would take several hours to make one revolution and was designed to rotate in only one direction. The molecular motor relied on ATP synthesis as its energy source and was used as a research platform to understand the fundamentals of transitioning chemical energy into mechanical motion. Similar research has since been completed by Dutch and Japanese scientists using carbon to produce synthetic molecular motors powered by light and heat energy, and recent attempts as of 2008 have developed a method for creating a motor that produces a continuous level of rotational torque.
Biologically, molecular motors have a diverse list of functions and structures. The major transport motors are powered by the proteins myosin, kinesin, and dynein, and actin is the major protein present in muscle contractions seen in species as diverse as algae to humans. Research into how these proteins function has become so detailed as of 2011 that it is now known that, for every molecule of ATP that a 50-nanometer-long molecule of kinesin consumes, it is able to move chemical cargo a distance of 8 nanometers within a cell. Kinesin is also known to be 50% efficient in converting chemical energy to mechanical energy and capable of producing 15 times more power for its size than a standard gasoline engine could.
Myosin is known to be the smallest of molecular motors, yet it is essential to 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 bacteria called a flagellum spins with a propeller driven motion which, if scaled up to the human level of everyday motors, would be 45 times more powerful than the average gasoline engine.