2 Animal Like Protists

8/8/2019 2 Animal Like Protists

http://slidepdf.com/reader/full/2-animal-like-protists 1/22

Animal-Like Protista



Introduction Protists are eukaryotes and include unicellular (e.g., amoeba) and

multicellular forms (e.g., 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

Eukaryotic cell Filamentous algae



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 onetime 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:

Anabaena

2. The compartmentalization of different functions within cells;

the first eukaryotes resulted from this solution

1. Toward multicellular forms

such as cyanobacteria;

different cell types with

specialized functions



The Evolution of Eukaryotes

The evolution of the compartmentalized nature of eukaryotic cellsmay have resulted from two processes

1. Specialization of plasma membrane invaginations.



2. Endosymbiotic associations of prokaryotes may haveresulted in the appearance of some organelles. Mitochondria,

chloroplasts, and some other organelles evolved from

prokaryotes living within other prokaryotic cells

The Evolution of Eukaryotes cont.



The EndosymbioticTheory

The endosymbiotic theory was developed by Lynn Margulis

It proposes that certain prokaryotic species, calledendosymbionts 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 bedescendents of eubacteria

Their inner membranes contain several enzymes and transport systemssimilar 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 eukaryoticribosomes; 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 µTree of Life¶



Introduction to Protozoan Protists

Protozoans (Gr. proto = first; zoa = animal) are thesingle-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 andendoplasm; 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 thatoperates 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 membranousvesicles 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

pore

ampulla

Spongiome

tubules

Vacuole



R eproduction

Asexual reproduction is commonly encountered among protozoans

Some reproduce asexually through fission, a controlled mitoticreplication 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



R eproduction 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 wouldotherwise be intolerable conditions of acidity, thermal stress, dryness,

etc.

Once conditions improve excystment ensues with the regeneration

of all former internal and external structures



ClassificationPhylum 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; Eug lena; Volvox

Class Zoomastigophorea or zooflagellates: heterotrophic forms,

e.g., trypanosmes that parasitize humans and cause ³sleeping

sickness´; tsetse flies serve as vectorsBlood

cell





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.



CiliophoranR eproduction

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 protozoansthat 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 causemalaria (e.g., P lasmodium) to

humans; mosquitoes serve as

vectorsapical complex

Documents

2 Animal Like Protists