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Biology LibreTexts

Reading: Protists

  • Page ID
    7677
  • The goal of this exercise is to learn about the protists. We will study major groups and for most of the groups, we will study representative genera.

    Procedure

    For each specimen:

    1. Read the information thoroughly.
    2. Create notes. Your notes will be most helpful if they include a drawing, a description, and significant information such as life cycle, commercial value, ecological significance, and unusual characteristics.

    Euglenozoans

    Kinetoplastids

    Kinetoplastids are flagellated and unicellular. They have a dark staining region of mitochondria called a kinetoplast.

    Some kinetoplastids are symbiotic (close) relationships with other organisms. Trypanosomes are Kinetoplastids that cause African sleeping sickness. They are transmitted to their human hosts by the bite of a tsetse fly. Trypanosoma causes African sleeping sickness.

    Euglena

    Euglena are unicellular. Many Euglenids feed by phagocytosis. Many species of Euglenids are photosynthetic but can become heterotrophic when sunlight is unavailable (mixotrophs).

    Euglena use flagella for moving. The outer covering called a pellicle, is flexible and assists in moving. Some have an eyespot with a photoreceptor is capable of detecting the presence of light. Reproduction is asexual.

    Diatoms

    Diatoms are the most numerous unicellular algae in the oceans and as such are an important source of food and oxygen. They are also important in freshwater environments. They capture 20 to 25% of solar energy captured by living organisms. The cell walls of diatoms contain silica (a component of glass) and are formed in 2 halves like a pillbox. Their remains form diatomaceous earth. It is used for filtering agents, and abrasives such as scouring powders. Diatoms are a major component of phytoplankton in freshwater and marine environments.

    Brown Algae

    Brown algae are autotrophs (photosynthetic). Their characteristic brown color is due to carotenoid pigments. They are multicellular and range in size from small to very large. Some are 50 m to 100 m long. They are often found along rocky shores in temperate climates. The body (thallus) contains holdfasts for attachment, blades, and a stem-like structure that holds the blades is called a stipe. Many species have floats that function in floatation. Some have gas-filled floats. Mucilaginous (slimy) material in the cell walls retards drying in exposed individuals when the tide goes out. Most species have a life cycle with alternation of generations.

    Fucus

    Fucus is a common “seaweed” found along the rocky coast. Some species of Fucus have diploid adults.

    Photo of fucus. Receptacles are at the ends of the fucus.

    Figure 1. Gametes are produced in the receptacles.

    Macrocystis and Nereocystis

    Macrocystis and nereocystis are deep-water kelps.

    Sargassam

    Sargassam sometimes breaks off to form floating masses. Other marine organisms congregate around these masses.

    Laminaria

    Laminaria is a brown alga that is usually found attached just below the intertidal zone. It has a life cycle with alternation of generations.

    Zoospores become male and female gametophytes. The gametophytes produce sperm and eggs respectively. The eggs undergo fertilization and become a zygote. The Sporophyte contains sporangia. the sporophyte undergoes meiosis and produces zoospores. The cycle continues from generation to generation.

    Figure 2. Alternation of Generations

    Dinoflagellates

    Protective cellulose plates cover dinoflagellates and two flagella enable them to move. One of the flagella lies in a transverse groove that causes cell to spin as it moves.

    Most are found in marine or freshwater environments and many are photosynthetic. They are important components of phytoplankton and thus are important in aquatic food chains. This group also includes many heterotrophic and many mixotrophic species.

    Some species are responsible for red tides that kill fish and shellfish (Gymnodinium, Gonyaulax, Pfiesteria). Some live as symbiants within some invertebrates. For example, some corals grow faster with dinoflagellates living within their cells. Some species are capable of bioluminescence (they produce light).

    Both sexual and asexual reproduction occur. Sexual reproduction produces cysts which are resistant to unfavorable environmental conditions. Cysts are dormant and become active when environmental conditions improve.

    Ciliates

    The genus Vorticella belongs in this group.

    Paramecium

    Figure 3. Paramecium caudatum X 100

    Figure 3. Paramecium caudatum X 100

    The pellicle (outer covering) of paramecium is covered with hundreds of cilia. They have numerous organelles including a gullet (oral groove) and an anal pore. Ciliates have a large macronucleus and a smaller micronucleus.

    The micronucleus is involved in sexual and asexual reproduction. Other nuclear activities are handled by the macronucleus. The macronucleus is polyploid (approximately 860 N in Paramecium aurelia) and the micronucleus is diploid.

    Conjugation in Paramecium

    Figure 4. Paramecium X 200

    During reproduction, the macronucleus disintegrates. Later, a micronucleus will develop into a macronucleus. Most reproduction is asexual (mitosis). Sexual reproduction is by conjugation.

    The micronucleus will divide by meiosis; 3 of the 4 resulting nuclei will disintegrate as will the macronucleus. The remaining haploid nucleus will divide by mitosis producing an individual with two haploid nuclei. Two conjugating individuals will each exchange one of the nuclei. The two haploid nuclei will then fuse producing a diploid nucleus.
    Two cells, each with a macronucleus and a micronucleus: They are compatible mating types. They come together and partially fuse. The micronucleus of each cell undergoes meiosis. Cytonekesis does not occur. Each micronuclues is haploid.
    Three micronuclei disintegrate in each cell. Mitosis produced another micronucleus in each cell. the cels exchange micronuclei.
    The two cells separate. The following process occurs separately in each cell. The two micronuclei fuse, producing a diploid cell. Mitosis occurs three times without cytokinesis. There are now 8 micronuclei. The macronucleus disintegrates. 4 micronuclei become macronuclei. Cytokinesis occurs twice, producing 4 cells, each with a macronucleus and a micronucleus.

    Red Algae

    Red algae are mostly multicellular and are found mainly in warmer, tropical oceans. Their red color is due to an accessory photosynthetic pigment called phycoerythrin. The accessory pigments of red algae are able to absorb blue and green light. This allows some species to survive in deep waters where blue and green light predominates.

    Some species are filamentous but most have a complex pattern of branching. Some coralline forms deposit calcium carbonate in their cell walls, which contributes to the development of coral reefs.

    Green Algae

    Four common forms of green algae are single-celled, colonial, filamentous, and multicellular. Green algae are thought to be ancestors of the first plants. Both kinds of organisms have the following characteristics in common:

    1. They have a cell wall that contains cellulose.
    2. They have chlorophyll a and b.
    3. They store their food as starch inside the chloroplast.

    Most species are freshwater but there are many marine species. Some live in damp soil.

    Chlamydomonas

    Chlamydomonas is a single-celled organism with two flagella. Although this organism is a single cell, the life cycle is similar to that with haploid adults.

    Adults, who reproduce asexually produce positive and negative gametes. The The gametes undergo fertilization and become a zygospore. The zygospore undergoes meiosis and produces an adult that asexually reproduces. The cycle continues from generation to generation.

    Figure 5. Chlamydomonas’ life cycle

    It reproduces asexually (by mitosis) when conditions are favorable. Sexual reproduction occurs when conditions become unfavorable. The zygote forms a thick-walled zygospore that is resistant to environmental extremes and divides by meiosis when environmental conditions become favorable.

    Most species of Chlamydomonas are isogamous (both gametes are the same size; they are isogametes), some are oogamous (gametes are two sizes; the larger gametes are eggs, the smaller ones are sperm).

    Volvox

    Volvox is a colonial green algae. The cells are arranged in a gelatinous sphere with two flagella directed to the outside. They divide asexually to produce a daughter colony.

    Some cells are specialized to produce sperm and eggs for sexual reproduction. Specialization of cells as seen in the reproductive cells is a characteristic of multicellular organisms. Volvox is considered to be a colony because it appears to be intermediate between a group of individual cells and a multicellular organism.

    Spirogyra

    Spirogyra is a filamentous form. It has a ribbonlike spiral-shaped chloroplast. The life cycle has haploid adults.

    Sexual reproduction occurs by conjugation. Conjugation refers to the process where gametes are transferred from one individual to another by a connection between the two.

    The zygote is resistant and overwinters. In the spring, it divides by meiosis to produce haploid filaments.

    Ulva

    Ulva is multicellular with a leaflike body that is two cells thick but up to one meter long. The life cycle is alternation of generations. Both the haploid and the diploid generations look alike (isomorphic).

    Tubulinids

    Microscope image of Amoeba proteus

    Figure 7. Amoeba proteus X 100

    Tubulinids move by cytoplasmic extensions called pseudopodia. They feed by phagocytizing(engulfing) their prey. Tubulinids are found in soil, marine, and freshwater environments. Amoeba proteus (figure 7) is found in freshwater.

    Amoeba movement and phagocytosis (video)


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