Biomedical Engineering Reference
In-Depth Information
2.
Undulipodia. Prokaryotes and eukaryotes have flagella, rods that protrude
from the organism; their back-and-forth motion propels cells forward
through water. Aside from a superficial resemblance, the flagella of
eukaryotes have no relationship to the flagella of prokaryotes. Eukaryotic
flagella are orders of magnitude larger than the prokaryotic flagella,
contain hundreds of proteins not present in the flagella of prokaryotes,
have a completely different internal structure, anchor to a different
cellular location, and do not descend phylogenetically from prokaryotic
flagella [68]. Biologists provided the eukaryotic flagellum with its
own name: undulipodium. Perhaps they chose a term with a few too
many syllables. Most biologists continue to use the misleading term “fla-
gellum” (plural, “flagella”) to apply to prokaryotes and to eukaryotes.
Regardless, every existing eukaryote descended from an organism with
a undulipodium. In humans, cilia on the surface of mucosal lining cells
are modified undulipodia. Every human spermatocyte has a tail with one
posterior undulipodium.
As far as anyone knows, the very first eukaryote came fully equipped with a
nucleus, one or more undulipodium, and one or more mitochondria. Based
on similarities between the eukaryotic nucleus and archaean cells, in terms
of the structure and organization of DNA, RNA, and ribosomes, it has been
hypothesized that the eukaryotic nucleus was derived from an archaean
organism. Conversely, similarities between mitochondria and eubacteria of
Class Rickettsia suggest that the eukaryotic mitochondrium was derived
from an ancestor of a modern Rickettsia. A single eukaryotic cell may con-
tain thousands of mitochondria, indicating the huge size differential between
prokaryotes and eukaryotes. Regarding the origin of the undulipodium, it is a
profound mystery.
For well over a century, biologists had a very simple way of organizing
the eukaryotes [12]. Basically, the one-celled eukaryotes were called protists.
The multi-celled eukaryotes were assigned to the kingdom of plants or the
kingdom of animals, often referred to as flora and fauna, respectively. The
fungi were considered a subclass of plants. To this day, university-based
mycologists (specialists in fungi) are typically employed in the Botany
Department (not the Fungus Department). The protists, also known as
protoctista, were considered the ancestral organisms for the multicellular
organisms. Biologists applied the term “protozoa” to protist species that
were the ancestral forms of animals (from “proto,” the first and “zoa,”
animals; see Glossary item, Protozoa).
In the past decade, taxonomists have abandoned the venerable Class
Protoctista. The former Class Protoctista was a grab-bag of unrelated classes
that did not fit under Class Animalia, Class Fungi, or Class Plantae.
Furthermore, as the phylogenetic lineage of the various eukaryotes became
better understood, it became clear that the multicellular classes had much
