By Newton's 2nd law, m*a=sum_of_forces where m is the mass and a the acceleration. Here there are two forces in opposed directions.
Thus 5*a=40-8=32 therefore a=32/5=6.4m^s/2
(if it's frictionless the length doesn't even matter :) )
It would have the same kinetic energy down as the potential energy up. That is,

or

(the mass doesn't even matter). The result is

, so only the height matters really. It is almost 9 (it is

).
if im not mistaken plant cells go through a stage called photosynthesis meaning the cells can create it's own energy (food) off of water, Co2 and sunlight.
The electrons are the only part of the atom that forms the bond