This question brought up a good point about un-mixing via Accio. However, were there any rules set about the size of an object that can be summoned via that charm? Could something on the cellular or molecular level be summoned? From the books the smallest thing I can think of would be hair that was used to disguise Harry at the wedding; also in The Goblet of Fire, a fly was summoned as well.
In my opinion, some spells require a variety of actions that will enable it to work. It usually falls under (or a combination of):
- Magical ability
- Movement/Aim of wand
- Saying the incantation
In the case of Accio, it requires not only magical ability, but also Focus/Concentration/Willpower, and saying the incantation. In some cases aiming at the target also works to narrow down targeting of the spell and make it easier (aiming at the beaded bag to pull out tent from Deathly Hallows).
"Concentrate, Harry, concentrate. . . ." "What d'you think I'm trying to do?" said Harry angrily. "A great big dragon keeps popping up in my head for some reason...Okay, try again. . . ." - GoF
Now because a big scary dragon kept popping up in his mind and disrupting his spell, I would be assuming he is picturing and focussing on the broom (much like you have to picture your destination when apparating). Therefore, Harry would have to visualise a molecule or atom, or something on a cellular level. As it would be difficult to remember exactly what an atom looks like for a wizard (and considering at best they won't study science post age 11), I doubt it would be possible to summon anything on a molecular level.
Furthermore, the boy in the village would probably have been in Freds line of sight, meaning it would be easier to picture and concentrate and aim as he could look at the boy to see what he wants, instead of focussing on it when he couldn't see it.
Also, it seems Accio seems to work on a 1 to 1 basis, meaning saying Accio hairstrand would bring one hairstrand and not all the strands of hair. (In GoF, Molly continuously said Accio to summon the Ton Tongue Toffees, picturing them as opposed to saying the actual name of the item she was summoning). So if you were to say "Accio Mitochondria", it would bring only 1 mitochondria, which would be invisible to the naked eye anyway, and show no sign of change on the plant, making it seem like nothing happened. Perhaps a Priori Incantatem would show otherwise.