Cell-associated or volume found outside of the plasma membrane of cell.
Often described either also or instead as the , periplasms are perhaps most familiarly associated with Gram-negative bacteria, as found between the inner membrane and outer membrane. Similar, physiologically defined or demarcated volumes are found in association also with fungi as well as some Gram-positive bacteria.
In Gram negative bacteria, the periplasm can constitute a relatively large fraction of cell volume, e.g., 20-40% . It also is a location of important digestive enzymes, used for breaking down nutrients to forms that can be into the cell.
The periplasm, in other words, can be viewed to at least a degree as a site of a kind of "intracellular" or intra-"body" digestion of food, i.e., as equivalent to a phagolysosome, gastrovascular cavity, or gastrointestinal tract.
of gram-positive bacteria are usually secreted into the external medium. The retention of similar enzymes by the outer membrane of gram-negative bacteria has been suggested to be an adapation  to a relatively dilute . Even apart from the involved in transport… the periplasm clearly constitutes an important organelle.
See also exoenzymes.