A major aim of nanotechnology is to develop molecular-scale machinery that can carry out the programmed synthesis (or rearrangement) of single molecules (or even atoms), or other similar nanoscale tasks. The term (nano)assembler refers to a nanomachine that can build nanoscale structures, molecule by molecule or atom by atom. And the term (nano)replicator refers to a nanomachine able to build copies of itself when provided with raw materials and energy. This, of course, sounds remarkably like a living cell. Indeed, the organelles of living cells may be regarded as nanomachines and have provided both inspiration and components for nanotechnologists.
To operate, nanomachines will need energy, which will be provided by “molecular motors.” At present such devices are still in development. It has been suggested that biological structures might be used for this purpose. Examples include the ATP synthase, the flagellar motor of bacterial cells, various enzymes that move along DNA or RNA, and assorted motor proteins of eukaryotic cells. Several of these systems are presently being investigated in the hope of making usable nanodevices that can be coupled to nanomachines to provide energy and/or moving parts.
The ATP synthase is a rotary motor whose natural role is to generate ATP. It is embedded in the mitochondrial membrane and uses energy from the proton motive force. The ATP synthase takes three steps to complete each rotation, and at each step it makes an ATP. For use in nanotechnology, the F1 subunit would be detached from the membrane and run in reverse (i.e., it would be given ATP as fuel and, from a biological perspective, rotate backwards).
Kinesin and dynein are motor proteins that use ATP as energy to move along the microtubules of eukaryotic cells. They therefore act as linear step motors (Fig. 7.21). Their natural role is to transport material. Kinesin moves cargo from the center to the periphery of the cell, whereas dynein carries cargo from the periphery to the center. Kinesin takes steps of 8 nanometers and can move at 100 steps per second (approximately 3mm/hour!). Each step consumes one ATP for energy. The microtubules they use as tracks are protein cylinders with an outside diameter of 30 nm.
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