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What is the difference between cilia and flagella in a cell?

Flagella usually occur one per organism whereas cilia are present as many, many per cell. Biochemically both are used for movement and are driven by 'molecular engines'. Cilia use a molecule called kinesin which has an ATPase activity, i.e., it uses the energy in ATP for its 'engine'. This energy is used to let kinesin bind to microtubules, long molecules present in nearly every eucaryotic cell, used for movement. Binding kinesin to microtubules and the generation of force , results in movement. Flagella are extracellular, attached to the cell at a structure which contains its 'molecular engine'. This engine is NOT powered by ATP, but rather by the "proton-motive force" across the plasma membrane. This is the force generated by the difference in proton concentrations on the two sides of the membrane. This results in a pH gradient (more H+ on one side than the other.)


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