Download Animal-Like Protista

Animal-Like Protista
Introduction
• Protists are eukaryotes and include unicellular (e.g., amoeba) and
multicellular forms (e.g., algae)
Eukaryotic cell
Filamentous algae
• The word protista (Gr. protos, very first, ktistos, to establish) implies
great antiquity
• Protista represent a polyphyletic group
• Two interesting scenarios regarding the history of life on earth
emerged during the evolution of protists: the origin of the eukaryotic
cell and the subsequent emergence of multicellular eukaryotes
The Emergence of the Eukaryotic Cell
• The small size and simpler construction of the prokaryotic cell has
many advantages but also imposes a number of limitations:
• Number of metabolic activities that can occur at any one
time is smaller
• Smaller size of the prokaryotic genome limits the number of
genes which code for enzymes controlling activities
• Natural selection resulted in increasing complexity in some groups
of prokaryotes; two major trends were apparent:
1. Toward multicellular forms
such as cyanobacteria;
different cell types with
specialized functions
Anabaena
2. The compartmentalization of different functions within cells;
the first eukaryotes resulted from this solution
The Evolution of Eukaryotes
• The evolution of the compartmentalized nature of eukaryotic cells
may have resulted from two processes
1. Specialization of plasma membrane invaginations.
The Evolution of Eukaryotes cont.
2. Endosymbiotic associations of prokaryotes may have
resulted in the appearance of some organelles. Mitochondria,
chloroplasts, and some other organelles evolved from
prokaryotes living within other prokaryotic cells
The Endosymbiotic Theory
•The endosymbiotic theory was developed by Lynn Margulis
• It proposes that certain prokaryotic species, called
endosymbionts lived within larger prokaryotes
• Chloroplasts are believed to have descended from endosymbiotic
photosynthesizng prokaryotes, such as cyanobacteria, living in
larger cells
• Mitochondria are postulated to be descendents of prokaryotic
areobic heterotrophs.
• Perhaps they gained entry as parasites or undigested prey of larger
prokaryotes.
• The association may then have progressed from parasitism or
predation to mutualism.
• And, as the host and endosymbiont became more interdependent,
they integrated into a single organism
Evidence for the Endosymbiotic Theory
• Chloroplasts and mitochondria are of the appropriate size to be
descendents of eubacteria
• Their inner membranes contain several enzymes and transport systems
similar to those of prokaryotic plasma membranes
• They replicate by splitting processes similar to binary fission present in
prokaryotes
• They have DNA which is circular and not associated with histones or
other proteins, as in prokaryotes
• They contain their own tRNA, ribosomes and other components for
DNA transcription and translation into proteins
• Chloroplasts have ribosomes more similar to prokaryotic ribosomes
(with regards to size, biochemical characters, etc) than to eukaryotic
ribosomes; mitochondrial ribosomes vary, but are also more similar to
prokaryotic ribosomes
• rRNA of chloroplasts is more similar in basic sequence to that of certain
photosynthetic eubacteria than to rRNA in eukaryotic cytoplasm; also,
chlorplast rRNA is transcribed from genes in the chloropast while
eukaryotic rRNA is transcribed from nuclear DNA
Archezoans and Early Evolution of Eukaryotes
• An ancient lineage of eukaryotes branched away from the eukaryotic tree
very early, perhaps as long as 2 billion years ago
• The group is referred to as the archezoa and contains only a few phyla
• Lack mitochondria and plastids, and have relatively simple
cytoskeletons
• Their ribosomes have some characteristics more closely aligned with
prokaryotes than with eukaryote
• Giardia intestinalis is a modern representative of an archezoan
• If organisms like Giardia diverged from the eukaryotic lineage before the
process of nuclear fusion and meiosis evolved, their dual nuclei may be a
clue to the past
• This coupled with the absence of mitochondria in this group and other
archezoan is consistent with an origin occurring before the endosymbiotic
relations that gave rise to the mitochondria in aerobic species
Overview of the ‘Tree of Life’
Introduction to Protozoan Protists
• Protozoans (Gr. proto = first; zoa = animal) are the
single-celled animal-like members of the kingdom Protista
• They are clearly eukaryotes, e,g., distinct nuclei,
membrane bound organelles, etc.; unlike animals, never
develop from a blastula
• Remarkably diverse in terms of size, morphology, mode
of nutrition, locomotory mechanism, and reproductive
biology
• Protozoans are regarded as being a polyphyletic group
General Characteristics of Protozoan Protists
• Entire organism is bounded by the
plasmalemma (cell membrane)
• The cytoplasm is often
differentiated into a clear, outer
gelatinous region, the ectoplasm,
and an inner, more fluid region
fluid or sol state, the endoplasm
• Many organelles are typical of
most multicellular metazoan cells
• However, many protozoans
contain organelles not generally
found among the metazoa, e.g.,
contractile vacuoles and trichocysts
Cilia and Flagella
• Locomotor appendages that protrude from the protozoan cell
• Cilia are shorter and more numerous, whereas, flagella are longer
and less less numerous
• Cilia and flagella are similar structurally; microtubules are arranged
in a ring of 9 microtubule doublets surrounding a central pair of
microtubles (9+2 arrangement); microtubules are covered by an
extension of the plasma membrane; they are anchored to the cell by a
basal body
• Cilia and flagella differ in their beating patterns
Pseudopodia
• When organisms like amoeba are feeding and moving, they form
temporary cell extensions called pseudopodia
• The most familiar form are called lobopodia: contain ectoplasm and
endoplasm; used for locomotion and engulfing food
• When a lobopodium forms, an extension of the ectoplasm called the
hyaline cap appears and endoplasm flows into this cap
• As the endoplasm moves into the cap it fountains out and it changes
from the fluid state to the gel state (endoplasm to ectoplasm)
• Pseudopodium anchors to the substrate and the cell is drawn forward
Nutrition and Digestion
• Ingested food particles generally become surrounded by a membrane,
forming a distinct food vacuole; digestion is entirely intracellular
• Vacuoles move about in the fluid cytoplasm and the contents are broken
down by enzymes
• The contents of the vacuoles can change, e.g., go from acidic to basic
• This is important because digestion for these organisms requires exposing
the food to a series of enzymes, each of which has a specific role that
operates under a narrow range of pH
• Controlled changes of pH that occur within the food vacuoles allow for
the sequential disassembly of foods
• Once solubilized, nutrients move across the vacuole wall and into the
endoplasm of the cell
• Indigestible solid wastes are commonly discharged to the outside through
an opening in the plasma membrane
Excretion and Osmoregulation
• Contractile vacuoles are organelles involved in expelling water
from the cytoplasm
• Fluid is collected from the cytoplasm by a system of membranous
vesicles and tubules called spongiome tubules
• The collected fluid is transferred to a contractile vacuole and is
subsequently discharged to the outside through a pore in the cell
membrane
• Vacuoles are most commonly found among freshwater species
Spongiome
tubules
Vacuole
pore
ampulla
Reproduction
• Asexual reproduction is commonly encountered among protozoans
• Some reproduce asexually through fission, a controlled mitotic
replication of chromosomes and splitting of the parent into two or more
parts
• Binary fission - protozoan splits into two individuals
• Multiple fission many nuclear divisions precede the rapid
differentiation of the cytoplasm into many distinct individuals
• Budding a portion of the parent breaks off and differentiates into a
new individual
fission
Reproduction cont.
• Many protozoans possess the capacity for regeneration
• For example, encystment and excystment exhibited by freshwater
and parasitic species
• During encystment, substantial dedifferentiation of the organism
occurs, forming a cyst: compact, expels excess water, forms a
gelatinous covering is secreted
• The cyst can withstand long periods of exposure to what would
otherwise be intolerable conditions of acidity, thermal stress, dryness,
etc.
• Once conditions improve excystment ensues with the regeneration
of all former internal and external structures
Classification
Phylum Sarcomastigophora
• Move by means of flagella and/or pseudopodia; possess a single type
of nucleus.
Subphylum Mastigophora
Locomotion is by means of one of more flagella.
Class Phytomastigophorea or phytoflagellates; autotrophic
forms containing chlorophyll; one or two flagella,
e.g., dinoflagellates; Euglena; Volvox
Class Zoomastigophorea or zooflagellates: heterotrophic forms,
e.g., trypanosmes that parasitize humans and cause “sleeping
sickness”; tsetse flies serve as vectors
Blood
cell
Subphylum Sarcodina
• Mostly marine, but some inhabit freshwater and soil; some are parasitic
• Use pseudopodia for feeding and locomotion.
• Feed by a process known as phagocytosis.
• A number of species of sarcodines possess a protective outer shell or test,
e.g. the radiolarians (silica) and foraminiferans (calcium carbonate)
• Both the radiolarians and the forams feed by extending their pseuopodia
through openings in the shell.
Foraminiferan
Amoeba
Radiolarian
Phylum Ciliophora
• Exclusive to freshwater
• Cilia or ciliary organelles present in at least one stage of the life
cycle.
• The ciliates are unique in that they possess 2 kinds of nuclei: a
large macronucleus and one or more smaller micronuclei.
• The macronucleus controls the normal metabolism of the cell,
while the micronuclei are concerned with sexual reproduction.
Ciliophoran Reproduction
• Asexually via binary fission; sexually via conjugation.
• 2 individual align and partially fuse; all but one micronucleus in
each cell disintegrates.
• The partners swap one micronucleus; this micronucleus then fuses
to another micronucleus, forming a diploid organism with genetic
material from the 2 individuals.
conjugation
Phylum Apicomplexa
• This is an exclusively
parasitic group of protozoans
that lack locomotory
organelles, except during
certain reproductive stages.
• They possess a
characteristic set or organelles
called the apical complex,
which aids in penetrating host
cells.
• Includes parasites that cause
malaria (e.g., Plasmodium) to
humans; mosquitoes serve as
vectors
apical complex