The cytosol is the aqueous portion of the cytoplasm that lies enclosed within the semi-permeable cell membrane. It forms the liquid component of the cytoplasm, however it is not a homogeneous solution. As much as 70% of the cytosol is water. In addition, it also contains a variety of small molecules, macromolecules, and dissolved ions including Ca2+, Na+, K+, Mg2+, Cl-, and bicarbonate. The cytosol is not uniformly spread throughout the cell.
The cytosol serves several functions within the cell. It plays a role in osmoregulation and signal transduction between the cell membrane and the nucleus and organelles. It serves as a site where many chemical reactions and metabolic processes take place and transports metabolites from the production site to other parts of the cell. It also helps to generate action potentials such as in nerve, endocrine, and muscle cells.