Will addition of plasmid can modify the osmolarity of an intracellular solution for in vivo? Eg: Osm of intra (1x) is 290mm. will it increase, decrease or null the osm of the final solution?
small plasmids will do so only little, but if a strain has (got) several megaplasmids, the internal osmolarity will be effected. But this does not mean that the strain will suffer. However, the expression of plasmid-borne genes can influence osmolarity.
It would be possible to have a plasmid carrying a gene that perturbs the osmolarity. But if you are just asking does a random plasmid affect it, then the answer is no. The added nucleic acid contributed by the plasmid to the total intracellular nucleic acid is very small.
usual plasmids are nanometric particles so they can not change the Osmolarity but they may borne genes that can change physiological conditions of cell like osmolarity after expression