AP Biology
Notes: Protista

Introduction to Protisa:

·         Protists are eukaryotes and thus are much more complex than the prokaryotes.

·         For about 2 billion years, eukaryotes consisted of mostly microscopic organisms known by the informal name “protists.”

·         Systematists have split the former kingdom Protista into as many as 20 separate kingdoms.

·         Protists are so diverse that few general characteristics can be cited without exceptions.

·         Most of the 60,000 known protists are unicellular, but some are colonial and others multicellular.

·         The eukaryotic flagella are not homologous to those of prokaryotes.

• The eukaryotic flagella are extensions of the cytoplasm with a support of the 9 + 2 microtubule system.
• Cilia are shorter and more numerous than flagella.
• Cilia and flagella move the cell with rhythmic power strokes, analogous to the oars of a boat.

·         Many protists form resistant cells (cysts) that can survive harsh conditions.

·         Protists are found almost anywhere there is water.

• This includes oceans, ponds, and lakes, but also damp soil, leaf litter, and other moist terrestrial habitats.
• In aquatic habitats, protists may be bottom-dwellers attached to rocks and other anchorages or may creep through sand and silt.
• Protists are also important parts of plankton, communities of organisms that drift passively or swim weakly in the water.
• Phytoplankton (including planktonic eukaryotic algae and prokaryotic cyanobacteria) are the bases of most marine and freshwater food chains.

·         Many protists are symbionts that inhabit the body fluids, tissues, or cells of hosts.

·         These symbiotic relationships span the continuum from mutualism to parasitism.

• Some parasitic protists are important pathogens of animals, including those that cause potentially fatal diseases in humans.

·         Protists are the most nutritionally diverse of all eukaryotes,

• Most protists are aerobic, with mitochondria for cellular respiration.
• Some protists are photoautotrophs with chloroplasts.
• Still others are heterotrophs that absorb organic molecules or ingest larger food particles.
• A few are mixotrophs, combining photosynthesis and heterotrophic nutrition.

Example: Euglena, a single celled mixotrophic protist, can use chloroplasts to undergo
 photosynthesis if light is available or live as a heterotroph by absorbing organic nutrients
 from the environment.

• The same group may include photosynthetic species, heterotrophic species, and mixotrophs.

Protists can be divided into three ecological categories:

·         Protozoa—ingestive, animal-like protests

·         Absorptive, fungus-like protests

·         Algae—photosynthetic, plant-like protists.

Protista Diversity:

1. Diplomonadida and Parabasala:

·         A few protists, including the diplomonds and the parabasalids, lack                
mitochondria.

·         These protests probably lost their mitochondria during their evolution.

·         The diplomonads have multiple flagella, two separate nuclei, a simple
cytoskeleton, and no mitochondria or plastids.

·         One example is Giardia lamblia, a parasite that infects the human intestine.

• The most common method of acquiring Giardia is by drinking water
  contaminated with feces containing the
   parasite in a dormant cyst stage.

                                                                                    ·         The parabasalids include trichomonads.

                     The best known species, Trichomonas vaginalis,
           in habits the vagina of human females.
           It can infect the vaginal lining if the normal acidity
           of the vagina is disturbed.

           The male urethra may also be infected, but without symptoms.

         Sexual transmission can spread the infection.

2. Euglenozoa: ( photosynthetic and heterotrophic flagellates)

·      Use flagella for locomotion.

·         The euglenoids (Euglenophyta) are characterized by an anterior
pocket from which one or two flagella emerge.

• They also have a unique glucose polymer, paramylon, as a
   storage molecule.
• While Euglena is chiefly autotrophic, other euglenoids are
  mixotrophic or heterotrophic.

·         The kinetoplastids (Kinetoplastida) have a single large mitochondrion
associated  with a unique organelle, the kinetoplast.

• The kinetoplast houses extranuclear DNA.

·         Kinetoplastids are symbiotic and include pathogenic parasites.

                  example, Trypanosoma causes African sleeping sickness.

3. Alveolata:

·         The Alveolata combines flagellated protists (dinoflagellates), parasites (apicomplexans), and
ciliated protists (the ciliates).

·       The alveolates are unicellular protists with subsurface cavities (alveoli)

·         Their function is not known, but they may help stabilize the cell surface and regulate water and ion content.

 Dinoflagellates
·    Are abundant components of the phytoplankton that are
     suspended near the water surface.

• Dinoflagellates and other phytoplankton form the foundation
  of most marine and many freshwater food chains.
• Other species of dinoflagellates are heterotrophic.
• Most dinoflagellates are unicellular, but some are colonial.

        ·         Each dinoflagellate species has a characteristic shape, often reinforced
        by internal  plates of cellulose.

        ·         Two flagella sit in perpendicular grooves in the “armor” and produce a spinning movement.

        ·         Dinoflagellate blooms, characterized by explosive population growth, cause red tides in coastal waters.

•   The blooms are brownish-red or pinkish-orange because of the predominant pigments in the plastids.

•   Toxins produced by some red-tide organisms have produced massive invertebrate and fish kills.

•    These toxins can be deadly to humans as we

                            Example, Pfiesteria piscicida,

• This organism produces a toxin that stuns fish.

• The dinoflagellate can then feed on the body fluids of its prey.

• In the past decade, the frequency of Pfiesteria blooms and fish kills have increased in the
   Mid-Atlantic states of the U.S.

• One hypothesis for this change is an increase in the pollution of coastal waters with fertilizers,
  especially nitrates and phosphates.

Apicomplexans (parasites of animals and some cause serious human diseases).

• The parasites disseminate as tiny infectious cells
   (sporozoites) with a complex of organelles specialized for
  penetrating host cells and tissues at the apex of the
  sporozoite cell.
• Most apicomplexans have intricate life cycles with both
   sexual and asexual stages and often require
  two or more different host species for completion.

·         Plasmodium, the parasite that causes malaria, spends part of its life in mosquitoes and part in humans.

·         The incidence of malaria was greatly diminished in the 1960s by the use of insecticides against the
Anopheles mosquitoes, which spread the disease, and by drugs that killed the parasites in humans.

• However, resistant varieties of the mosquitoes and the Plasmodium species have caused a malarial resurgence.

• About 300 million people are infected with malaria in the tropics, and up to 2 million die each year

• It spends most of its time inside human liver and blood cells, and continually changes its surface proteins, thereby changing its “face” to the human immune system.

 Ciliophora (ciliates)

·         Most ciliates live as solitary cells in freshwater.

·         Their cilia are associated with a submembrane system of microtubules that may coordinate movement.

• Some ciliates are completely covered by rows of cilia, whereas others have cilia clustered into fewer rows or tufts.
• The specific arrangement of cilia adapts the ciliates for their diverse lifestyles.

·         Some species have leg-like structures constructed from many cilia bonded together, while others have tightly packed cilia that function as a locomotor membranelle.

·         In a Paramecium, cilia along the oral groove draw in food
 that is engulfed by phagocytosis.

·         Like other freshwater protists, the hyperosmotic Paramecium
 expels accumulated water from the contractile vacuole.

·         Ciliates have two types of nuclei, a large macronucleus and usually
 several tiny micronuclei.

• Ciliates generally reproduce asexually by binary fission of the macronucleus,
  rather than mitotic division.
• The micronuclei (with between 1 and 80 copies) are required for sexual
  processes that generate genetic variation.

·         The sexual shuffling of genes occurs during conjugation, during which micronuclei that have undergone meiosis are exchanged.

• In ciliates, sexual mechanisms of meiosis and syngamy are separate from reproduction.

4. Stramenopila: (water molds and heterokont algae)

·         The Stramenopila includes both heterotrophic and photosynthetic
protists.

• The name of this group is derived from the presence of numerous fine,
  hairlike projections on the flagella.
• In most cases a “hairy” flagellum is paired with a smooth flagellum.
• In most stramenopile groups, the only flagellated stages are motile
  reproductive cells.
• Unlike fungi,  they have biflagellated cells.
• These filamentous bodies have extensive surface area, enhancing absorption of nutrients.

        ·  Water molds are important decomposers, mainly in fresh water.

• They form cottony masses on dead algae and animals.
• Some water molds are parasitic, growing on the skin and gills of injured fish.

·         White rusts and downy mildews are parasites of terrestrial plants.

·         The photosynthetic stramenopile taxa are known collectively as the heterokont algae.

Heterokont algae(diatoms, golden algae, and brown algae).

Diatoms (Bacillariophyta) have unique glasslike walls composed of hydrated
        silica embedded in an organic matrix.

• The wall is divided into two parts that overlap like a shoe box and lid.

        ·         Most of the year, diatoms reproduce asexually by mitosis with each daughter
        cell receiving half of the cell wall and regenerating a new second half.

        ·         Some species form cysts as resistant stages.

        ·         Sexual stages are not common, but sperm may be amoeboid or flagellated, depending on species.

        ·         Diatoms are abundant members of both freshwater and marine plankton.

• Massive accumulations of fossilized diatoms are major constituents of diatomaceous earth.
  
  

Golden algae (Chrysophyta)

    ·         Some species are mixotrophic and many live among freshwater and marine plankton.

    ·         While most are unicellular, some are colonial.

    ·         At high densities, they can form resistant cysts that remain viable for decades.

 Brown algae (Phaeophyta)

        ·    Are the largest and most complex algae.

• Most brown algae are multicellular.
• Most species are marine.

·         Brown algae are especially common along temperate coasts in areas of
cool water and adequate nutrients.

·         They owe their characteristic brown or olive color to
accessory pigments in the plastids.

    The largest marine algae, including brown, red, and green algae, are known collectively as seaweeds.

         ·         Seaweeds inhabit the intertidal and subtidal zones of coastal waters.

• This environment is characterized by extreme physical conditions, including wave forces and exposure to sun and drying conditions at low tide.

·         Seaweeds have a complex multicellular anatomy, with some differentiated tissues and organs that resemble those in plants.

• These analogous features include the thallus or body of the seaweed.
• The thallus typically consists of a rootlike holdfast and a stemlike stipe, which supports leaf-like photosynthetic blades.

·         Some brown algae have floats to raise the blades toward the surface.

• Giant brown algae, known as kelps, form forests in deeper water.
• The stipes of these plants may be 60 m long.

        ·         Many seaweeds are eaten by coastal people, including Laminaria (“kombu” in Japan)
        in soup and Porphyra (Japanese “nori”) for sushi wraps.

·         A variety of gel-forming substances are extracted in commercial operations.

• Algin from brown algae and agar and carageenan from red algae are used as thickeners
   in food, lubricants in oil drilling, or culture media in microbiology.

 Algae have life cycles with alternating multicellular haploid and diploid generations

·         The multicellular brown, red, and green algae show complex life cycles with alternation of multicellular haploid and multicellular diploid forms.

• A similar alternation of generations evolved convergently in the life cycle of plants.

·         The life cycle of the brown alga Laminaria is an example of alternation of generations.

• The diploid individual, the sporophyte, produces haploid spores (zoospores) by meiosis.
• The haploid individual, the gametophyte, produces gametes by mitosis that fuse to form a diploid zygote.

·         In Laminaria, the sporophyte and gametophyte are structurally different, or heteromorphic.

·         In other algae, the alternating generations look alike (isomorphic), but they differ in the number of chromosomes.

Rhodophyta: (Red algae)

·         Unlike other eukaryotic algae, red algae have no flagellated stages in their life cycle.

·         The red coloration visible in many members is due to the accessory pigment phycoerythrin.

• Coloration varies among species and depends on the depth which they inhabit

·         Some species lack pigmentation and are parasites on other red algae.

·         Red algae  are the most common seaweeds in the warm coastal waters of tropical oceans.

• Others live in freshwater, still others in soils.

·         Some red algae inhabit deeper waters than other photosynthetic eukaryotes.

• Their photosynthetic pigments, especially phycobilins, allow some species to absorb those wavelengths (blues and greens) that penetrate down to deep water.
• One red algal species has been discovered off the Bahamas at a depth of over 260m.

Chlorophyta: (Green algae)

• These are similar in ultrastructure and pigment composition to those of plants.
• The common ancestor of green algae and plants probably had chloroplasts derived from cyanobacteria by primary endosymbiosis.

        ·         Most of the 7,000 species of chlorophytes live in freshwater.

• Other species are marine, inhabit damp soil or snow, or live symbiotically within other eukaryotes.
• Some chlorophytes live symbiotically with fungi to form lichens, a mutualistic collective.

·         Chlorophytes range in complexity, including:

• Biflagellated unicells that resemble gametes
• Colonial species and filamentous forms.
• Multicellular forms large enough to qualify as seaweeds.

·         Large size and complexity in chlorophytes has evolved by three different mechanisms:

·                      2) The repeated division of nuclei without cytoplasmic division to form  multinucleate
                   filaments (Caulerpa).