Chapter 26: Protozoa
26-1 Overview of Protozoa
26-2 Protozoan Diversity
26-1 Overview of Protozoa
I. Protists
• A diversity of protozoa, algae, slime molds, and water molds that lack
tissue differentiation and are eukaryotes. (A-L, P-L, F-L)
II. Characteristics of Protozoa
• Approximately 65,000 microscopic species have been identified (half
extinct), can have inhabited the marine, freshwater, and terrestrial
habitats.
(1) Protozoa
• Unicellular and heterotrophic (most species), can be either free-living
or parasitic, requiring multiple hosts.
Critical Thinking
(1) Some protozoan parasites are very difficult to grow in test tubes
because they lose the ability to produce certain key enzymes or growth
factors when outside their host. How could this host-parasite relationship
be similar to that described in the Endosymbiosis Hypothesis? How could it
be different?
(2) Food Vacuoles
• Membrane-bound chambers containing digestive enzymes that break down
small food molecules ingested by heterotrophic protozoans.
(3) Zooplankton
• Population of organisms that is made up by numerous protozoan species; a
critical trophic level in aquatic ecosystems.
(A) Reproduction
• All protozoa are capable of binary fission (asexual), where one divides
into two identical offspring; however some are capable of sexual
reproduction as well.
(1) Multiple Fission (asexual)
• A form of asexual reproduction where a number (>2) of identical
offspring are yielded.
(2) Conjugation (sexual)
• Individuals from opposite “mating-strains” pair up and exchange genetic
material (slightly more complex than conjugation in bacteria).
Critical Thinking
(2) The process of conjugation is complex, requiring an expenditure of
energy and resources. How can you relate the high biological cost of
conjugation to the adaptive advantage of exchanging genetic material?
(B) Classification
• Protozoa belong to Kingdom Protista and are further classified into
four phyla, distinguished by form of locomotion:
(1) Phylum Sarcodina
(2) Phylum Ciliophora
(3) Phylum Zoomastigina
(4) Phylum Sporozoa
(C) Adaptations
• Many species of protozoa have physiological mechanisms for monitoring
conditions/noxious chemicals in their environment.
(1) Eyespot
• Localized region of pigment that can detect changes in the amount of
sunlight (dark or light).
(2) Cyst
• A dormant form characterized by a hardened external covering in
which metabolic activity ceases.
NOTE: Stressors include drought, nutrient deficiency, decreased oxygen
levels, or pH/temperature changes. Emergence occurs when conditions
become favorable once again.
Critical Thinking
(3) Many parasitic protozoa, such as Entamoeba histolytica, are able to
form cysts whenever they pass out of a host. Why would cyst formation
represent an advantage to a parasitic protozoan?
(D) Evolution (~1.5 b.y.a.)
• Descended from the earliest eukaryotic cells, followed prokaryotes
(bacteria) possibly due to endosymbiosis.
26-2 Protozoan Diversity
I. Phylum Sarcodina (40,000 species of sarcodines)
• Include amoebas, which inhabit fresh water, saltwater, and soil, and are
characterized by cytoplasmic extensions called pseudopodia and flexible
cell membranes.
(1) Pseudopodia
• Large, rounded cytoplasmic extensions that function in movement.
(2) Endoplasm
• Inner portion of the cytoplasm that pushes against the ectoplasm.
(3) Ectoplasm
• Outer layer of cytoplasm that is pushed outward to create a blunt,
armlike extension.
Critical Thinking
(4) Many species of protozoa feed by means of pseudopodia. What do
you believe could be the stimulus that might cause these cytoplasmic
extensions to form?
(4) Ameboid Movement
• The simultaneous extension and contraction of the pseudopodia to initiate
movement (powered by cytoplasmic streaming).
(5) Cytoplasmic Streaming
• Internal flowing of a cell’s cytoplasm; includes ameboid movement
(6) Contractile Vacuole
• Organelle that expels fluid from the cell in order to maintain homeostasis
(i.e., freshwater protists are usually hypertonic relative to their habitat).
(A) Ecological Role
• Calcium-rich layer of sediment deposited at the bottoms of oceans
from the tests of dead foraminifera.
(1) Test
• A protective but porous CaCO3 shell (slender pseudopodia extend
through these openings) that covers the delicate cell membrane of some
sarcodines, including foraminifera.
(2) Foraminifera
• Ancient group of shelled sarcodines found primarily in oceans with
chambered tests.
(3) Radiolarians
• Oldest of foraminifera, inhabit shallow waters and usually have a radial
arrangement of spines that extend through their test.
(B) Human Diseases
• Although the majority of amoebas are free-living, some parasitize the
large intestines of humans or other animals.
(1) Amebic Dysentery (Entamoeba histolytica)
• Amoeba enter through the consumption of contaminated food or water
and as the amoeba grow, enzymes are secreted that agitate the intestinal
lining resulting in painful ulcers.
II. Phylum Ciliophora (8,000 species of ciliates)
• Swim by cilia and are abundant in ponds and slow-moving streams
containing bacteria, algae, and decaying organic matter. (e.g., Paramecium
Caudatum)
(A) Internal Structure
• Ciliates have the most elaborate organelles of any protozoa and are
multinucleate.
(1) Pellicle & Oral Groove
• A clear, elastic layer of protein surrounding the cell membrane (pellicle);
the pellicle contains a funnel-like tube lined with cilia (oral groove)
(2) Mouth Pore & Gullet
• Beating cilia sweep in food particles into the mouth pore, of which
opens into the gullet where food vacuoles are formed and circulated.
(3) Anal Pore
• Vacuoles of indigestible materials are moved to the anal pore where
they are expelled into the surroundings.
(4) Macronucleus (larger)
• Contains multiple copies of DNA and is responsible for metabolic,
developmental, and asexual reproductive functions.
(5) Micronucleus (smaller)
• Participates in the exchange of genetic material between individual
ciliates during conjugation.
(B) Reproduction (asexual and sexual)
• Asexually via binary fission and sexually via conjugation.
NOTE: Following conjugation, each paramecium divides, producing a total
of four genetically identical paramecia. Because genetic material is
exchanged between the two original paramecia, the four offspring
paramecia are genetically different from either original paramecium.
Critical Thinking
(5) Many biologists suggest that paramecia are more complex than
amoebas. What adaptations in paramecia would justify this claim?
III. Phylum Zoomastigina (2,500 species of zooflagellates)
• Characterized by the presence of one or more flagella, constructed by
microtubules and used for locomotion (e.g., Giardia lamblia)
(A) Human Disease
• Although most are free-living, feeding on smaller organisms, many of the
zooflagellates belonging to the genus Trypanosoma are parasites (using
bloodsucking insects as carriers from host to host)
(1) Trypanosomiasis (African Sleeping Sickness)
• Transmitted by a bite of the tsetse fly, resulting in fever, neurological
degradation, and ultimately coma.
(2) Chagas’ Disease (South America)
• Transmitted by a bite of the “kissing bug,” resulting in fever and severe
cardiac damage.
(3) Giardiasis (U.S.A.)
• Contaminated water with Giardia lamblia that produces severe diarrhea
and intestinal cramps.
IV. Phylum Sporozoa (6,000 species of sporozoans)
• Characterized by adult forms with no locomotion; most are parasitic with
complex life cycles in which a spore (infective form with a resistant coat)
is developed.
(1) Toxoplasmosis (Toxoplasma gondii)
• A disease resulting from a sporozoan found in birds, rodents, and
domestic cats that can be dangerous to a developing fetus or newborn.
(A) Plasmodium (four species infect humans, all involving a mosquito)
• Genus of sporozoans that is responsible for causing malaria and more
human deaths than any other genus in recorded history.
(1) Malaria
• Infected victims die of anemia, kidney failure, or brain damage; afflicts
500 million (killing as many as 2.7 million) annually.
(2) Sporozites (liver-stage parasite)
• When an infected mosquito bites a person, Plasmodium sporozites enter
the bloodstream and travel to the liver cells where they divide repeatedly
forming new spores called “merozoites.”
(3) Merozoites (erythrocyte-stage parasite)
• Emerge and infect red blood cells in order to reproduce asexually; the
destruction of RBCs and the release of toxins produce the symptoms.
(4) Gametocytes (specialized cells developed from merozoites)
• Ingested when a female mosquito (Anopheles) bites an infected person,
combining zygotes which hatch into new sporozites that migrate to the
salivary glands of the mosquito.
Critical Thinking
(5) Observing the micrograph below depicting the sporozoan Plasmodium,
what evolutionary advantage does Plasmodium gain by forming gametocytes?
Critical Thinking
(6) Biologists are working to develop a vaccine against malaria, but since
malaria has several life stages, biologists must decide which life stage to
develop a vaccine against. Some biologists are trying to develop a vaccine
against gametocytes as a way of controlling malaria. If they are
successful in developing this vaccine, how will it help people living in areas
where malaria regularly grows?
Revisiting Cell Structure and Function
• Unicellular organisms contain structures that perform specialized
functions, including feeding and locomotion.
Assessing Prior Knowledge
• Describe at least five cellular organelles and their functions.
• Predict what will happen if a protozoan such as an amoeba is placed in a
distilled water solution.
• Define host and parasite.