if i understand cell physiology correctly, plasma has a negligible positive charge. nearly all objects a human interacts with on a daily basis are electrically neutral. anything that is even slightly charged gives you a "static shock" as the charge gradient is dissipated.
bulk electroneutrality must be maintained - this means that all regions of the human body are electrically neutral at the cellular level. the electrostatic force is extremely strong - even a slight difference in charge produces a powerful force. if there were a significant charge difference between the intracellular fluid and the plasma, the cell membrane would quickly be ripped apart by ions moving down their charge gradients.
a relatively small charge separation (so small as to be negligible) can support a large concentration gradient