Study Questions

1. What characteristics distinguish the protists?

2. What is the endosymbiont hypothesis?  What genetic evidence supports this hypothesis? READ THIS BELOW - KNOW IT

6.  Name three human protist parasites and describe their phylogenetic relationships.

7.  Describe alternation of generation, using a diagram.

 B. Domain Eubacteria

1. Distinguishing characteristics:
         - peptidoglycan in cell wall
         - unbranched lipids in phospholipid bilayer

2. Groups to know:
         - Proteobacteria: metabolically diverse, including chemoheterotrophic and photoautotrpohic bacteria that use H2S (without or with light, respectively). Also some nityrogen fixing bacteria.

         - Cyanobacteria: autotrophic; probably responsible for the "oxygen revolution" about 2 billion years ago.

         - Others (Spirochetes, Chlamydias, and Firmicules): contain pathogenic and decomposing bacteria.

C. Domain Archaea

1. Characteristics
         - odd phospholipids
         - more similar genetically to eukarya than to eubacteria
         - exploit extreme environments

2. Groups to Know:
            - Halophiles: live in very salty environments.  They are photosynthetic, but harvest energy from light in an unusual way.  The light energy is used to "pump" H+ across the membrane.  When it floods back across in response to its concentration gradient, the charge differential (an exchange of energy) is used to add P + ADP---> ATP.
            - Methanogens: Anaerobes that harvest energy from CO2 and H2O, producing methane as a waste product.  They inhabit the guts of ruminants (cows) and other animals (including humans), and also the bottom of marshes.

- Thermacidophiles: live at high temperatures (up to 85C) or under acidic conditions.  They harvest energy from sulphur compounds expeled at sulphur spring and geothermal vents.

IV. Domain Eukarya

A. Kingdom Protista

    1. Characteristics:

        a. The protists are a diverse group of organisms; most are single celled, but often the multicellular brown algae (kelps) and red algae are lumped in because they have very little tissue specialization - they are colonial.

        b. There are animal-like heterotrophs like Ameoba and Paramecia, plant-like autotrophs like algae, and heterotrophic, saprophytic fungus-like protists like slime molds. This variety, and similarity to truly multicellular life forms, suggests that animals, plants, and fungi have protist ancestors.

        c. A major innovation of eukaryotic life was sexual reproduction, and the extraordinary variation that it produces.

        d. obviously, as eukaryotes, they have nuclei and membrane-bound organelles
 

    2. Groups (see phylogeny on pg. 550):  Protists were long classified by morphological and locomotory characteristics into groups like the amoeboids, flagelaates, cilates, etc.  However, it turns out that genetic analysis has revealed that these morphologies occur in a variety of genetically different groups - so morphology is not a good index of genetic relatedness among protists.  Of course, with only a single cell, there is not much morphology to have, anyway.

        a. Diplomonads: have lost their mitochondria - very primitive - Giardia - a human parasite that lives in fresh water

        b. Euglenozoans: Include the photosynthetic euglenoids with anterior flagella, and the kinetoplastids which include Trypanosoma.

        c. Alveolates: have a cavity below their plasma membrane (alveoli). This group incldues the dinoflagellates (photosynthetic marine organisms - important primary producers which also cause red tides and fish kills (Pfisteria)).  This group also includes the apicomplexans, which can have an ameoboid body form.  THis group includes Toxoplasma and Plasmodium (malaria). The last group of alveolates are the ciliates, which move with small paddle-like cilia.  Paramecium is a representative.  They have macro and micro nuclei and can produce new genotypes by trading micronuclei during conjugation.

        d.Stramenopiles: typically have two flagella of unequal length.  This group includes the yellow and brown algae, such as diatoms and kelp, respectively.  THey may also have alternation of generation, like green algae.

        e. Red algae:  Non-flagellated gametes and a particular storage starch.
 

        f. Choanoflagellates: Have a collar of membraneous fingers (microvilli) that surround the flagellum.  The colonial choanoflagellates are strikingly similar to simple sponges, the most primitive and simple animals.  As such, the animals may be descendants of choanoflagellate-like ancestors.

        g. Green "algae" with the same chlorophyll as plants. These organisms are primarily aquatic and are single celled or colonial with little tissue differentiation.  Given their similarity to plants, plants probably evolved from chlorophyte ancestors. Life Cycle:  Haploid (1n) cells divide by mitosis until environment becomes stressful.  Then, when they dive the daughter cells are gametes.  Gametes meet and fuse into a zygote (2n).  The zygote forms a thick-wall; protective stage to 'overwinter'.  When conditions improve, meiosis occurs and haploid daughter cells are produced. See figure 28.26 on page 561.
 

    3. The Origin of Protists
 

        a. In the 1970's, Lynne Margulis hypothesized that mitochondria and chloroplasts in eukaryotic cells were descendants of free-living bacteria, based on observations of membranes and the presence of DNA in mito's and chloro's.

        b. She hypothesized that the evolution of eukaryotes, with their  complex organelles, occurred by some large cells absorbing mitochondria-like bacteria and chloroplast-like bacteria without digesting them.

        c. The host cells could then harvest the ATP and glucose produced by the "bacteria", and the bacteria had a ready supply of nutrients absorbed by the host cell.  Both parties benefitted from this partnership, hence a "symbiotic" relationship.

        d. The most significant experimental test of her hypothesis came when the DNA of the modern organelles was analyzed and compared to the DNA of bacteria.  Indeed, the DNA in mitochondria and chloroplasts is more similar to bacteria DNA than to the DNA in the nucleus of that very same eukaryotic cell! (Your cells have mitochondria, and the DNA in those mito's is more similar to the DNA in bacteria than to the DNA in the nucleus of your cells.).
 Mitochondria also have a bacteria-like type of ribosome, and they can synthesize some of their own proteins - so  they have some very independent properties.  They also reproduce  themselves by division; they are not produced by the host cell.