The charges on either side of the plasma membrane are defined by the distribution of ions across the membrane. These ions include sodium, potassium, chloride, and other small molecules. The inside of the cell is typically more negative than the outside due to the presence of anionic proteins and other negatively-charged molecules, while the outside of the cell is typically more positive due to the abundance of cations.
anionic proteins,
sodium, potassium, chloride,