1. The problem statement, all variables and given/known data 1. Because cytoplasmic vesicles are seen to move in both directions in the axon of a nerve cell, can you conclude that some of the microtubules are orientated with their plus-end facing the axon terminus and others orientated with the opposite polarity? Why or why not? 2. Relevant equations 3. The attempt at a solution The positive end is designated for the addition of actin subunits, where the ATP-binding cleft of the terminal actin subunit contacts the neighboring subunit (barbed). The negative end is favored for dissociation, where the cleft is exposed to the surrounding solution. Since the cleft is exposed, ATP will bind, hydrolysis of the bound nucleotide reduces the affinity of the subunit for neighboring subunit, and therefore the negative end is favored for dissociation.