EUKARYOTES &PROKARYOTES
EUKARYOTES &PROKARYOTES
Cells have evolved into two fundamentally different types,
eukaryotic and prokaryotic, which can be distinguished based
on their structure and the complexity of their organization.
Fungi, protozoa, and helminths are eukaryotic, whereas bacteria
are prokaryotic.
(1) The eukaryotic cell has a true nucleus with multiple
chromosomes surrounded by a nuclear membrane and uses
a mitotic apparatus to ensure equal allocation of the chromo-
somes to progeny cells.
(2) The nucleoid of a prokaryotic cell typically consists of a
single circular molecule of loosely organized DNA and lacks a
nuclear membrane and mitotic apparatus (Table 1-3).
In addition to the different types of nuclei, the two classes of
cells are distinguished by several other characteristics:
(1) Eukaryotic cells contain organelles, such as mitochon-
dria and lysosomes, and larger (80S) ribosomes, whereas pro-
karyotes contain no organelles and smaller (70S) ribosomes.
(2) Most prokaryotes have a rigid external cell wall that con-
tains peptidoglycan, a polymer of amino acids and sugars, as its unique structural component. Eukaryotes, on the other hand,
do not contain peptidoglycan. Either they are bound by a flexi-
ble cell membrane, or, in the case of fungi, they have a rigid cell
wall with chitin, a homopolymer of N-acetylglucosamine, typ
cally forming the framework.
(3) The eukaryoticc cell membrane contains sterols, whereas
no prokaryote, except the wall-less Mycoplasma, has sterols in
its membranes.
Motility is another characteristic by which these organisms
can be distinguished. Most protozoa and some bacteria are
motile, whereas fungi and viruses are nonmotile. The protozoa
are a heterogeneous group that possesses three different organs of
locomotion: flagella, cilia, and pseudopods. The motile bacteria
move only by means of flagella.
TERMINOLOGY
Bacteria, fungi, protozoa, and helminths are named according
to the binomial Linnean system that uses genus and species.
For example, regarding the name of the well-known bacteria
Escherichia coli, Escherichia is the genus and coli is the species
name.
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