familiar organisms are classified according to this system. Biology does not have a central
authority for determining how to organize animals into phyla. The classifications gain cur-
rency by general usage and adoption in textbooks.
Species may be further subdivided into subspecies, strain (especially in prokaryotes),
or cultivar (in crop plants).
Formerly, the eukaryotic domain was considered to consist of only two kingdoms,
plants and animals. Now it has been further subdivided to include fungi and protists. Sev-
eral of these kingdoms are charact erized based on their source of carbon. Autotrophs
obtain their carbon from the inorganic form, in particular carbon dioxide. Heterotrophs
are organisms that must obtain their carbon in the form of organic chemicals such as
sugars, fats, or proteins.
The fungi kingdom is defined to include the plantlike, mostly multicellular hetero-
trophic organisms lacking the green pigment chlorophyll. Fungal cells are surrounded
by a rigid capsule called a cell wall. Fungal cell walls are made of a polymer called
chitin. Fungi have a major ecological role in breaking down dead organisms and
making their nutrients available to other organisms. Four divisions have been recog-
nized: the ascomycetes include yeast, mildew, and the prized edibles morels and
truffles; basidiomycetes include most of the common mushrooms; deuteromycetes
include the mold that produces penicillin; zygomycetes include common black bread
mold.
Plants are multicellular photoautotrophs that reproduce sexually to form embryos (an
early stage of multicellular development). Photoautotrophs are autotrophs that get their
energy from light (as opposed to chemical sources of energy). Plant cells usually cont ain
chloroplasts, which are organelles containing the chlorophyll. The cells of plants are sur-
rounded by a cell wall composed largely of cellulose fibers. The major ecological role is
the capture of light energy from the sun to fix atmospheric carbon dioxide and produce
oxygen in the process called photosynthesis. This provides food for other organisms and
oxygen for their respiration, the biochemical oxidation of organic matter to produce
energy that is the opposite reaction to photosynthesis. The plant kingdom consists of
12 divisions: the bryophytes (mosses and relatives), four seedles s vascular plants divi-
sions (e.g., ferns), gymnosperms (including conifers), and anthophyta (also referred to
as angiosperms, the flowering plants).
Like the fungi, animals, are multicellular heterotrophic eukaryotes. In addition, they
reproduce primarily by sexual reproduction, are motile in some part of their life cycle,
and their cells lack a cell wall. There are 33 animal phyla, including several marine organ-
isms, such as sponges, corals, and jellyfish; several phyla containing wormlike organisms;
mollusks; arthropods (which include crustaceans and insects); and the chordates. The lat-
ter include the vertebrates, which are the organisms with an internal skeleton. Thus, we
humans are chordates. All of the other animal phyla, plus a small group within the chor-
dates, are called invertebrates.
The remaining kingdom in domain eukarya is that of the protists. This kingdom some-
what arbitrarily combines single-celled eukaryotes that would otherwise be part of either
the fungi, plan t, or animal kingdoms. Thus, they are classified into three groups: animal-
like protists, or protozoans, which have four phyla, including those with paramecium and
the amoeba; plantlike protists, with six divisions, euglena, dinoflagellates, diatoms, red
algae, brown algae, and green algae; and funguslike protists, with three divisions, includ-
ing slime molds. Each of these groups function ecologically in ways that are similar to
their analogous multicellular kingdom.
TA XONOMY 29