Cytoplasm is a thick, gel-like substance that fills the interior of the cell and is enclosed by the cell membrane. In eukaryotic cells, which have a distinct nucleus bound by a nuclear membrane, the cytoplasm occupies the space between the plasma membrane and the nuclear membrane, completely surrounding the nuclear membrane. The liquid inside of organelles is considered to be part of the cytoplasm as well. In prokaryotic cells, which do not have a membrane-bound nucleus, the cytoplasm fills the entire space within the plasma membrane.