The part of the protoplasm where there is relatively free exchange of inorganic ions between the protoplasm and extracellular fluids. This occurs because the outer membrane of the protoplasm is permeable to most inorganic ions. The membrane surrounding the cell vacuole is not so permeable and though ions may enter by *active transport
they cannot pass out again; certain salts thus tend to accumulate in the vacuole. The composition of the free space and the vacuole may therefore be quite different.