The Diversity of The World of Life

Eukaryotes (Domain Eukaryota)

Miscellaneous Protists

(various species of eukaryotes of unclear relationships)

Representatives

Various Amoebae:  Various Radiolarians, Heliozoons, Forams, Slime Molds, etc. (various species)

Various Flagellates:  Euglenids (Euglena), Trichonympha (Termite Gut Symbionts), Various Parasites (including Trypanosoma, Leishmania, Giardia, & Trichomonas), etc. (various species)

Various Other Protists (various species)

Biology

ENVIRONMENTS

Protists, in general, live in salt- or fresh-water, in soil, or in the bodies of other animals.

Most slime molds live in the shade, as on rocks, leaves, or rotting tree trunks.

Most euglenids live in freshwater.

DESCRIPTION

The term "Protists" is something of a catch-all for eukaryotes other than true plants, fungi, or animals.  Although some protists (such as certain red, brown, or green algae) are multicellular and macroscopic, most protists are unicellular and microscopic, traditionally classed as either "protozoans" (primitive animals) or "algae" (primitive plants).

The "organelles" (membrane-bound bodies within the "cytoplasm") of a typical, unicellular protist perform the various life-sustaining functions that are performed by the "organs" of multicellular organisms:  Although protists are "primitive" in the sense of being an ancient line of life, their cells are often far more complex than those of us more "advanced" forms of life.

Typically in amoebas, the "ectoplasm" (the dense outer layer of cytoplasm) gives direction to the creeping movement of the cell.

Often bearing external filaments, the cells of some protists, such as certain amoebas, are shelled, often beautifully so.  These shells sometimes cement living cells together as colonies, and the shells of dead protists often accumulate on the ocean floor as a thick ooze.

Slime molds are essentially macroscopic amoebas.  One type of slime mold is composed of a "plasmodium" -- a slimy, shapeless mass of naked cytoplasm with many nuclei (there are no cell walls, cell membranes, or other divisions).  The other type of slime mold is composed of individual, amoeboid cells that occasionally coalesce together (in response to a chemical signal they send out).

Euglenids are unicellular, with a unique "pellicle" -- a protein-rich, spiral-ridged coat -- but no cell wall and with one or two propeller-like "flagella".

FEEDING HABITS

Most protists feed on "plankton" (microscopic organisms adrift in water), typically by engulfing their prey.

Slime molds are "heterotrophic" (not producing their own food molecules).  Like other, microscopic amoebae, slime molds engulf their foodstuffs, which are typically dead organisms or non-living products of other living organisms (Most slime molds are "saprophytic").

Most euglenids do not feed but, rather, produce their own food, photosynthetically, capturing light-energy by means of pigments (green chlorophylls a and b, as in higher plants) embedded in their "chloroplasts".  In the absence of light or if cell division has outpaced chloroplast replication, some euglenids can survive quite well as heterotrophs, consuming organic matter in the environment; and some colorless euglenid species are solely predatory.  Such "phytoflagellates" as euglenids have been claimed by both botanists and zoologists (or by neither).

A euglenid ingests its food by means of a special "feeding apparatus" (a mouth- and gut-like invagination of the outer cell membrane, of various degrees of complexity).  Note that euglenids were probably originally predatory and only later photosynthetic -- a prehistoric euglenid presumably ingested but did not digest a green algal cell (with photosynthetic membranes inside of its cell membrane), which (surrounded by an envelope of membrane from the ingesting euglenid) thus became the (triple-membraned) euglenid-chloroplast.

MOTION

As is typical amongst animals, most of the food-energy consumed by heterotrophic protists ends-up being lost as heat, radiated off into the environment; however, like most animals, most protists do convert some of their food-energy into movement.

"Pseudopods" (typical amoebas) move by means of "pseudopodia" (arm- or tentacle-like extensions of the cell).

A slime mold moves about in a creeping, typically amoeboid manner.

"Zooflagellates" (flagellates traditionally classed as animals) move by means of long, whip- or propeller-like "flagella" filaments and sometimes also by means of undulating, membranous "fins".

A euglenid typically moves about by means of its one or two propeller-like flagella, but many may also use their pellicle coat to creep along the bottom (in a unique, "wriggling" process called "metaboly").

DIGESTION

Water and other materials are typically exchanged through a cell membrane via simple diffusion or via "passive" or "active" transport (both forms of transport employing proteins embedded within the membrane, the active form also requiring the biochemical expenditure of energy).

RESPIRATION

Gases dissolved in environmental water are typically exchanged through a cell membrane via simple diffusion.

CIRCULATION

The materials within a eukaryotic cell are continually mixed by the active process of "cytoplasmic streaming".

The materials within the plasmodium of a slime mold are moved by simple diffusion, by active cytoplasmic streaming (readily observable within the large mass), and perhaps by the amoeboid movement of the entire body.

EXCRETION

Within the cell of a euglenid, or of most other freshwater species of protists, one or two "contractile vacuoles" (water-expelling "vesicles", or sacs) help prevent the cell from soaking-up too much water and bursting ("osmolysing"):  Like most substances, water tends to flow from where it is more concentrated (in the typically freshwater environment) to where it is less concentrated (within the cell itself, whose cytoplasm is filled with water-soluble substances).

Water and other materials are typically exchanged through a cell membrane via simple diffusion or via "passive" or "active" transport (both forms of transport employing proteins embedded within the membrane, the active form also requiring the biochemical expenditure of energy).

COORDINATION

The activities, growth, and development of protists are under "genetic control" (that is, under the control of the DNA "chemical blueprint" for the cell), with feedback from such environmental stimuli as heat, light, or gravity.

In euglenids, there is an "eyespot", sensing light by means of a yellow "carotene" pigment (However, because this is a single-celled organism, there is no nervous system, as in even the simplest eyes of animals).

The growth and development of colonial forms of protists, such as cellular slime molds, is influenced not only by genetics but also by "hormones" (in effect, chemical messengers).

REPRODUCTION

Most protists can reproduce "asexually" (that is, each by itself).  Typically, as a protist grows, its organelles are duplicated; and after reaching a maximum size (beyond which there is not enough contact between the contents of the cell and its environment for effective exchange of materials, both nutrients and wastes), the cell replicates its "chromosomes" (DNA bodies, within the nucleus) and divides by "fission" (pinching itself in two).  This typical form of cell division (in which the cells produced have the same number of chromosomes as the parent cells) is called "mitosis" (Compare "meiosis", in sexual reproduction, below).

Flagellates, such as euglenids, reproduce by "longtitudinal fission" (splitting in half lengthwise) -- compare ciliates.

In various species of shelled protists, the shell may be divided; the shell may be retained by one of the offspring and re-grown by the other; or the shell may at some point prohibit cell-division altogether (See the Asexual Reproduction of diatoms).

Such blood-sucking insects as mosquitoes or tsetse flies may carry the infective life stages of certain "pathogenic" (disease-producing) protists between their various "hosts".

Although euglenids have not been observed to do so, many protists can reproduce "sexually"; that is, a pair of parent cells each contribute copies of half of their genetic material to an offspring -- sexual reproduction, in general, spreads beneficial traits and "dilutes" harmful traits throughout a population.  In preparation for sexual reproduction, cells divide by "meiosis", which cuts in half the number of chromosomes in the resultant cells, typically called male and female "gametes" (sex cells); the eventual union of gametes produces a "zygote" (a sexually fertilized egg cell), with both sets of chromosomes.

Slime molds occasionally reproduce asexually, by simple fission (dividing in two).  Slime molds also reproduce sexually, in much the same way as fungi, with which they were traditionally classified (and only recently distinguished, as by genetic research).  Typically, a plasmodium will produce "sporangia", bodies in which "spores" form, by meiosis (each spore is thus "haploid", with just half as many chromosomes as in the parent nuclei).  The spores have cell walls (somewhat like those of fungi), which presumably provide protection during periods of unfavorable environmental conditions.  Eventually, amoeboid or flagellated bodies of naked cytoplasm emerge from the spores.  When a pair of these haploid bodies eventually fuse, they restore the "diploid" condition (with both sets of chromosomes) in a "zygote" (a fertilized "egg", so to speak), which either alone or fusing with others grows into new plasmodium.

Eukaryotes (Domain Eukaryota)

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