11. What are some intracellular specialized structures
possessed by protists and what are their functions? What
structures in multicellular animals are they analogous to?
• The protist kingdom includes protozoans and algae. (Two groups of fungi
with similar characteristics to protozoans, myxomycetes and oomycetes,
have been classified as protists.)
• Unicellular protozoans and algae are unicellular eukaryotes. The
pluricellular algae are eukaryotes of simple structure too. It is believed
that protists are phylogenetic ancestors of living beings of the other
eukariotic kingdoms (fungi, animals and plants).
• The basic difference between protozoans and algae is the fact that
protozoans are heterotrophs while algae are photosynthetic autotrophs.
• Protozoans are unicellular beings with some similar characteristics to
• In comparison to pluricellular organisms protozoans are more proximal to
the animal kingdom than to plants: they are heterotrophs, they have a
rudimentary locomotion system (amoeboid movements, cilia, flagella),
they do not have cell wall, some species present structures that resemble
structures of a primitive digestive system, with cytostome (mouth) and
cytopyge (anus), specialized in digestion and excretion.
• The evolutionary hypothesis that animal cells have come from
differentiation of protozoans is strong.
• Protozoans are eukaryotic cells so they have organelles and structures
common to this kind of cell: endoplasmic reticula, Golgi apparatus,
digestive vesicles, ribosomes, mitochondria, nucleus with genetic
material, karyotheca, etc. All these elements are found dispersed
throughout the cytoplasm. Protozoans do not have cell walls.
• Protozoans from the mastigophora group (like trichomonas) have flagella
and others, others from the ciliated group (like paramecium) have cilia.
• All protozoans, as eukaryotes, have nucleus. Some species, like the
paramecium, have two nuclei: the macronucleus and the micronucleus. 12. What are the two major cytoskeletal filaments in
• Eukaryotic cells contain three main kinds of cytoskeletal
filaments, which are microfilaments, intermediate filaments, and
microtubules. The cytoskeleton provides the cell with structure
and shape, and by excluding macromolecules from some of the
cytosol it adds to the level of macromolecular crowding in this
compartment. Cytoskeletal elements interact extensively and
intimately with cellular membranes.
Microfilaments (actin filaments)
• These are the thinnest filaments of the cytoskeleton. They are
composed of linear polymers of actin subunits, and generate
force by elongation at one end of the filament coupled with
shrinkage at the other, causing net movement of the intervening
strand. They also act as tracks for the movement of myosin
molecules that attach to the microfilament and "walk" along
them. Actin structures are controlled by the Rho family of small
GTP-binding proteins such as Rho itself for contractile acto-
myosin filaments ("stress fibers"), Rac for lamellipodia and Cdc42
• These filaments, around 10 nanometers in diameter, are more
stable (strongly bound) than actin filaments, and heterogeneous
constituents of the cytoskeleton. Like acti