156 ❯ STEP 4. Review the Knowledge You Need to Score High
We can see why scientists examining these two organelles would think that they may
have originated from prokaryotes. They share many characteristics: (1) they are the same
size as eubacteria, (2) they also reproduce in the same way as prokaryotes (binary fission),
and (3), if their ribosomes are sliced open and studied, they are found to more closely
resemble those of a prokaryote than those of a eukaryote. They are prokaryotic groupies
living in a eukaryotic world.
The eukaryotic organism that scientists believe most closely resembles prokaryotes is
thearchezoa,which does not have mitochondria. One phylum grouped with the archezoa
is the diplomonads.A good example of a diplomonad you should remember is Giardia—
an infectious agent you would do well to avoid. Giardiais a parasitic organism that takes
hold in your intestines and essentially denies your body the ability to absorb any fat. This
infection makes for very uncomfortable and unpleasant GI (gastrointestinal) issues and usu-
ally results from the ingestion of contaminated water.Kingdom Protista
The evolution of protists from prokaryotes gave rise to the characteristics that make eukary-
otes different from their prokaryotic predecessors. Protists were around a long time before
fungi, plants, or animals graced our planet with their presence. Most protists use aerobic
metabolism. Since this is a chapter on classification, it would be silly, if not too kind of us,
to not mention how these different protists are organized. They are usually grouped into
three major categories:- Animal-like protists:heterotrophic protists, also called protozoa
- Funguslike protists:protists that resemble fungi; also called absorptive protists
- Plantlike protists:photosynthetic protists, also called algae
Protists are usually unicellular or colonial. This is why they are notconsidered plants, ani-
mals, or fungi. All protists are capable of asexual reproduction. Some reproduce only asex-
ually, and others can reproduce sexually as well. This variability in the life cycles found
among various members of the protist kingdom is just one reason why they are considered
to be one of the most diverse kingdoms in existence.
Animal-Like Protists (Protozoa)
This division includes protists that ingestfoods—as do animals. As with the rest of this
chapter, you do not need to become an expert on protozoans and know everything about
every member. But the following is a list that contains basic information about some names
that may help you on the multiple-choice section of the test. We will italicize the most
important things to remember about each of them.- Rhizopoda.Theseunicellularandasexualorganisms are also known as amoebas.They
get from place to place through the use of pseudopods,which are extensions from their
cells. Every living creature has to eat, and they do so through phagocytosis. - Foraminifera.Thesemarineprotists live attached to structures such as rocks and algae.
Their name is derived from the word foramenbecause of the presence of calcium car-
bonate (CaCO 3 ) shells full of holes. Some of these protists obtain nutrients through
photosynthesisperformed by symbiotic algae living in their shells. - Actinopoda.These organisms move by pseudopodiaand make up part of plankton, the
organisms that drift near the surface of bodies of water. The two divisions of actinopoda
include heliozoans and radiozoans. Just recognize the names; do not worry about any-
thing more than that.