Most of the organelles present in eukaryotic (eu-, “true”, karyon, “nucleus”)
cells are lacking in prokaryotic cells. Prokaryotic cells have no nuclear envelope
(pro-, “before”, karyon, “nucleus”), but they also
lack other membranous structures such as an endoplasmic reticulum, a Golgi
apparatus, lysosomes, peroxisomes, and mitochondria (although many of the functions
of mitochondria are carried out in bacteria by the cell membrane). Most photosynthetic
bacteria, however, do have membranous structures containing light-absorbing
pigments. These membranes, however, are usually formed as inpocketings of the
cell membrane rather than being independent membrane-enclosed plastids.
For a long time it was thought that prokaryotic cells had no chromosomes.
With the advent of the electron microscope, however, it became possible
in each prokaryotic cell a region containing a single, large, circular DNA
molecule. Although this molecule is not closely associated with proteins,
as DNA is in eukaryotic cells, it is considered a chromosome.
Some bacterial cells have hairlike organelles used in swimming, and these
have traditionally been called flagella. In most prokaryotic species these
do not have microtubules and their structure and movement are completely
different from that of eukaryotic flagella). (The prokaryotic flagella
rotates like a
minipropeller rather than bending, as seen in eukaryotic cilia and flagella.)
Finally, the cell walls of most bacteria are made up of the polysaccharide
murein (a polymer of amino sugars), which is found only in prokaryotes.
Table 5.1 summarizes some of the most important differences between
prokaryotic and eukaryotic cells.
Table 5.1. A comparison of typical
prokaryotic and eukaryotic
cells, and certain eukaryotic organelles.
(click on image for larger version)
a Certain parasites and anaerobic
organisms lack mitochondria, having lost them either because of high-energy
compounds are available directly
from the host cell, or because they cannot use the oxygen-dependent enzymatic
pathways unique to mitochondria. One kingdom of primitive eukaryotes lacks
mitochondria, ER, and Golgi.
Scale Drawings of E. Coli and
various cellular components: