Chp. 26 Notes
The Protista Kingdom
Most protists are unicellular, microscopic organisms, but a few are complex and
multicellular. These are the most diverse (different) organisms according to life cycles,
since they reproduce asexually and sexually. This kingdom contains ancestral life-forms
that gave rise to three kingdoms of multicellular organisms--the fungi, the plants and
animals.
As we talked about the theory of endosymbiosis, we should now be able to
understand that early in the development of our planet, scientist feel that prokaryotic
organisms were the only forms of life on our planet. As time went on, these organisms
were invaded by parasites which eventually lost their ability to live outside the host cell
and became organelles of the host. This brought about the Eukaryotic cells. Protist
organisms exibit the most basic characteristics of these eukaryotic cells as well as some
complex characteristics involved in organisms within the fungi, plant and animal
kingdoms. Several features evolved as did the eukaryotic cell. These characteristics that
evolved in protist organisms are;
1) Multicellularity- The prokaryotic bacteria are the only groups of organisms which
are soley unicellular. Most protist are unicellular but there quit a few groups of
multicellular organisms in this kingdom as well.
2) Sexual Reproduction- All prokaryotic organism asexually reproduced which meant
that they reproduced an exact copy of the parent organism unless a mutation occured in
the offspring. With protist organisms sexual reproduction came about as a form of
necessity due to environmental stresses ( ie. lack of nutrition, temp. change, predation.
etc.) Sexual reproduction brings about genetic variation without mutation needing to
occur due to there being two parent organisms producing offspring.
3) Complex development of the Cilia and Flagella- is another characteristic which
evolved in the protist group. In bacteria the flagella were just a single protein filament
(basically solid in structure) but in the protist, they had cilia and flagella develop with the
9 & 2 principle. 9 pairs of microtubules surrounding 2 microtubules. A microtubule is a
hollow protein filament. These hollow filaments allowed for easier movement and lighter
weight, which allowed them greater range of motion in the movement of these structures.
Differences between Cilia and Flagella
a) cilia are short hairlike projections which are used for
movement of either the organism or materials around or through an organism. There are
usually hundreds of these per cell.
b) Flagella are long whiplike structures used primarily for
movement ( locomotion). There are usually one or two pair.
4) Eyespots- another characteristic to emerge in this group is the eyespot which is
sensative to the quantity or quality of light. Some protist are heterotrophic but must reside
in a moist environment. With light comes heat and desication( drying out) so they must
stay away from the light. Others are photosynthetic and aquatic and need the sun to
maintain photosynthesis, so they need to go toward light sources.
