One of the most distinguishing features of prokaryotic organisms is the presence of peptidoglycan in their cell walls. Unlike eukaryotic cells, which may contain cellulose or chitin in their cell walls (if they have them), prokaryotic cells are characterized by this unique structure.
Peptidoglycan is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane. This structure provides strength and rigidity to the cell wall, allowing prokaryotes to maintain their shape and protect themselves from osmotic pressure and environmental stresses. Interestingly, this structure is targeted by certain antibiotics, like penicillin, which interfere with the synthesis of peptidoglycan, ultimately leading to the death of the bacterial cell.
In summary, the unique cellular structure that is exclusive to prokaryotic organisms is the peptidoglycan layer in their cell walls, which plays a crucial role in their integrity and survival.