Chapter 26: Protozoa

26-1 Overview of Protozoa

26-2 Protozoan Diversity

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)

26-1 Overview of Protozoa

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.

(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?

Critical Thinking

(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).

(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?

Critical Thinking

(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.

(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?

Critical Thinking

(D) Evolution (~1.5 b.y.a.)

• Descended from the earliest eukaryotic cells, followed prokaryotes (bacteria) possibly due to endosymbiosis.

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.

26-2 Protozoan Diversity

(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.

(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?

Critical Thinking

(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.

(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?

Critical Thinking

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.