Monday, October 19, 2009

kinesin to and fro

10.19.09. With enough external force, kinesin (see previous post) can be forced to take backwards steps as well as forward steps. In fact, the fraction of forward to backwards steps can be measured as a function of the external force applied by the optical trap (the plot shown is from Shao et al, PNAS, 2006). Forward and backward motion fit nicely with students' understanding of positive and negative numbers, and we imagined kinesin stepping along a number line. Students represented individual kinesin paths both visually as trajectories along a number line and numerically with an equation showing addition of positive and negative integers. Comparing the absolute value of all forward steps to that of all backward steps provides a quick way to determine the sign of the kinesin's net displacement.