<P> A prokaryotic cell has three architectural regions: </P> <Ul> <Li> Enclosing the cell is the cell envelope--generally consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsule . Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea) which only possess the cell membrane layer . The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter . The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces . It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment . Some eukaryotic cells (plant cells and fungal cells) also have a cell wall . </Li> <Li> Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of inclusions . The genetic material is freely found in the cytoplasm . Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular . Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease . Though not forming a nucleus, the DNA is condensed in a nucleoid . Plasmids encode additional genes, such as antibiotic resistance genes . </Li> <Li> On the outside, flagella and pili project from the cell's surface . These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells . </Li> </Ul> <Li> Enclosing the cell is the cell envelope--generally consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsule . Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea) which only possess the cell membrane layer . The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter . The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces . It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment . Some eukaryotic cells (plant cells and fungal cells) also have a cell wall . </Li> <Li> Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of inclusions . The genetic material is freely found in the cytoplasm . Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular . Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease . Though not forming a nucleus, the DNA is condensed in a nucleoid . Plasmids encode additional genes, such as antibiotic resistance genes . </Li>

Why are cells called structural and functional unit of life