Protists Chapter 28
Eukarya is now divided into 4 supergroups, Excavata, SAR
Clade, Archaeplastida and Unikonta.
1. It replaces the earlier 5-kingdom classification of Monera – all prokaryotes, Protista – early
eukaryotes and 3 multicellular kingdoms Plants, Fungi and Animals.
2. Kingdom monera is replaced by 2 new domains Bacteria and Archaea.
3. The classification of domain Eukarya is going on. Molecular data has broken the boundaries
between protista and 3 multicellular kingdoms.
4. Eukarya is likely to get more than 20 kingdoms but none of them is protista. Protists are a
polyphyletic group and does not exist as a kingdom anymore.
Protists were first eukaryotes to evolve.
1. Protists include unicellular eukaryotes and their simple multicellular relatives. The latter are
filamentous or colonial.
2. Mitosis, Meiosis and sexual reproduction arose for the first time in this kingdom. All the
organelles of plants, fungi and animals arose in this kingdom.
3. Protists also possess many unique organelles not found anywhere else in the 3 domains.
4. Like the 3 higher kingdoms the Protists are photosynthetic (like plants) or heterotrophic
absorptive (like fungi) or heterotrophic Ingestive (like animals).
Diversity of plastids produced by endosymbiosis
Primary Endosymbiosis: Ancestral eukaryote cell picked a cyanobacterium by endocytosis.
Cyanobacterium had 2 membranes and 3 membrane formed due to endocytosis and became
plastids. One of these 3 membranes degenerated in both red and green descendants.
Secondary Endosymbiosis: A second endosymbiosis took place later.
Dinoflagellates and Stramenopiles picked a red alga by endocytosis and became plastids.
Euglenoids and Chlorarachniophytes picked a green alga by endocytosis and it evolved as their
plastids.
Protists: of past are separated into 7 new groups that also have plants, fungi and animals.
1. Excavata
2. Stramenopiles
3. Alveolata
4. Rhizaria
5. Archaeplastida, also has plants
6. Ameobozoans
7. Opisthokonts, also has fungi and animals
Excavata: is a clade, some have excavated groove on side of body, reduced or modified mitochondria
and unique flagella. Taxons include Diplomonads, Parabasalids and Eugleonozoans (kinetoplastids and
euglenoids).
a) Diplomonads: reduced mitochondria, called mitosomes which cannot use oxygen, therefore,
diplomonads are anaerobic. Giardia intestinalis lives in intestines of mammals. It has 2 similar nuclei
and several flagella. It causes abdominal cramps and diarrhea in humans.
b) Parabaslids have reduced mitochondria called hydrogenosomes and produce some ATP
anaerobically and generate hydrogen. Trichomonas vaginalis causes sexually transmitted infections
of vagina and urethra in males.
c) Euglenozoans are a diverse clade having heterotrophs, autotrophs and parasites. All have a spiral or
crystal rod of unknown function in flagella.
a. Kinetoplastids have a single mitochondrion with organized DNA mass called kinetoplast.
Trypanosoma species cause sleeping sickness, fatal if untreated. Tsetse fly bites and spreads
the sleeping sickness in Africa. Trypanosoma has undulating membrane to locomote in
thicker plasma.
b. Euglenoids has a pocket at one end and 1or 2 flagella emerge from it. Flagella have a
crystalline rod besides 9+2 microtubules found in all eukaryotic flagella. Euglena has
eyespot to shield photodetector present on long flagellum. It enables Euglena to swim
towards source of light. Pellicle covers the body. It has proteins strips below cell membrane.
Besides swimming, Euglena can glide by changing its shape. Mixotrophic Nutrition: Euglena
has several plastids covered with 3 membranes but can be heterotrophic and absorb
organic nutrients from surroundings. Euglena divides by longitudinal fission and lacks sexual
reproduction.
Stramenopiles have diatoms, golden algae, brown algae and oomycetes. Most have one long hairy and
one short smooth flagellum.
d) Diatoms (100,000 species) are most abundant unicellular forms in oceans. The body is covered with
an intricately designed 2-half silica shell. Adults are Diploid and lack cilia or flagella. Main
reproduction is by binary fission. Sexual reproduction is by flagellated gametes. The remains of
shells produce Diatomaceous earth mined for filtering and abrasive materials. It acts as Biological
Carbon Pump to move CO2 from air to sea floor. Diatoms are most important producers in
biosphere.
e) Golden Algae have yellow and brown carotenoids and all cells have 2 flagella at one end. Most
golden algae are autotrophs but some are mixotrophic. Most golden algae are planktons in marine
or fresh water. Dinobryon is a a colonial golden alga of fresh water.
Brown Algae - are usually the large sea weeds called Kelps.
1. In addition to Chl a, these have fucoxanthin a brown pigment.
2. Laminaria is a large kelp in Coastal areas of California and Sea palm (Postelsia) live off west coast
of Canada and Unites States.
3. Body has root like holdfast, stem like stipe and leaf like blades.
4. Blades bear sporangia. Cells in sporangia undergo meiosis and form zoospores with 2 flagella.
Zoospores germinate to form multicellular male and female gametophytes.
5. Male gametophytes produce sperms also with 2 flagella.
6. Female gametophytes develop eggs.
7. Fertilization of egg and sperm forms zygote which divides to form the sporophyte.
8. Alternation of generations involves a dominant diploid sporophyte generation and multicellular
haploid gametophyte generation.
9. Algin a gelatinous material added to ice creams and cream cheese is extracted from brown
algae.
Oomyces include water molds, white rusts and the downy mildews.
1. These have cell walls of cellulose. These have fungus like septate hyphae.
2. Asexual reproduction by flagellated zoospores and sexual reproduction involves flagellated
sperms.
3. Phytophthora infestans causes potato late blight and was responsible for Irish Famine in 19th
century killing a million people.
Alveoates: have membrane bound sacs below cell membrane.
Dinoflagellates – Pyrrophyta are unicellular. For example, Pfiesteria.
1. Dinoflagellates have cellulose plates in alveoli beneath cell membrane.
2. They have 2 flagella placed at right angle to each other. These are important phytoplanktons in
oceans.
3. They are also responsible for causing Red Tides, example Gonyaulax. The red tide is caused due
to sudden growth (algal blooms) due to availability of Nitrogen or Phosphorus brought as
runaway water in farming areas by rivers.
Apicomplexans are unicellular parasites of animals.
1. The taxon includes malaria causing Plasmodium species.
2. No flagella present.
3. All have apical complex at one end of cell to penetrate host cells.
4. It is transmitted by females of mosquito, Anopheles species. Malarial parasite completes sexual
phase in Anopheles and asexual reproduction in human liver and red blood cells.
5. Sporozoites, asexual stages of malarial parasite attack liver cells and later erythrocytes and brain
cells.
6. Malaria causes 700,000 deaths per year out of millions affected each year.
Ciliates have cilia and covered with pellicle. Common example is Paramecium species.
1. It has a Meganucleus and a Micronucleus.
2. They have 2 contractile vacuoles surrounded by feeding canals to expel excess water entered
due to endosmosis.
3. Oral groove guides food like bacteria and form a food vacuole.
4. Food vacuoles stream in a circular path inside the body. Lysosomes join food vacuole and help
to digest food.
5. These reproduce asexually by transverse cell division.
6. Sexual reproduction is by Conjugation for which 2 paramecia join temporarily and exchange
nuclei and separate. 2 nuclei from different paramecia fuse to result in fertilization.
Rhizarians
Rhizarians include a diverse group of unicellular organisms with thread like pseudopodia.
Forams: are foraminifera having body covered with calcareous porous shells called tests with many
chambers outside cell membrane. Forams are unicellular heterotrophs and some may grow to
several centimeters. Forams form sediments and have been discovered from fossils. Globigerina.
Radiolarians are unicellular and have internal delicate skeleton form of silica. Pseudopodia radiate
from the central body.
Archaeplastida - Red Algae
Archaeplastida are autotrophs and include red algae, green algae and land plants. Plastids in red
and green algae arose by primary endosymbiosis by engulfing a cyanobacterium.
Red Algae - These are mainly filamentous or leafy. Besides Chlorophyll a, these have red and blue
pigments to give various colors. Red algae are delicate sea weeds.
Red algae are important sources of gelling agents Agar and Carageenan.
Porphyra is edible red alga used as a sushi-wrap.
Archaeplastida - Green Algae
Green Algae include Chlorophytes and Charophytes. These are most common in fresh water.
These have Chlorophyll a and b, store food as starch and have cellulose in cell walls. All
characteristics are common with plants.
Asexual reproduction can be by flagellated spores called zoospores or spores without flagella.
Sexual reproduction is by flagellated gametes. Many algae, like plants, form a non-flagellated egg
and a flagellated sperm. This sexual reproduction is called Oogamy.
Algae lack multicellular sex organs.
They have various forms: unicellular-Chlamydomonas; filamentous-Spirogyra; leafy - Ulva or colonial
- Volvox
with daughter colonies inside.
Land plants originated from charophytes.
Amoebozoans
Amoebozoans are amoeba that have lobe- or tube-shaped, rather than threadlike, pseudopodia
They include slime molds, tubulinids, and entamoebas
Slime Molds
Slime Molds are naked protists which feed by phagocytosis.
Plasmodial Slime Molds have multinucleate protoplasmic masses called plasmodium. These
reproduce asexually by spores formed in walled sporangia in a cluster called fruiting body. Spores
develop into amoeboid or flagellated gametes. Common example is Physarum species.
Cellular Slime Molds live as solitary amoeboid cells. Under unfavorable conditions these can form
aggregations called pseudoplasmodium. When favorable conditions return sporangia in a fruiting
body produce spores. The spores give rise to single cells to complete the cycle.
Amoebas- Tubulinids and Entameobas
Tubulinids form tube like pseudopodia and are covered with cell membrane. They change, all the
time, their body-shape. They feed by phagocytosis. Freshwater forms have contractile vacuoles to
expel excess water. Common example is Amoeba proteus.
Entamoebas include 6 species living in humans
Entameoba histolytica causes amoebic dysentery-blood in stools.
E. gingivitis aggravates gingivitis (peridontal decay) caused by bacteria
Opisthokonts
Opisthokonts have posterior flagella. These include Nucleariids, fungi, choanoflagellates and
animals
Nucleariids are closest to fungi.
Choanoflagellates are simple multicellular protists and closest line to animals.
Animals arose from an ancestor close to choanoflagellates.
Symbiotic Protists
Some protists live as symbionts and benefit their hosts
Dinoflagellates nourish coral polyps that build reefs
Wood-digesting protists inhabit the gut of termites, Trichonympha