Chapter Seven: The Classification of Unicellular Organisms

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Chapter outline

1. General classification of organisms
   1. Prokaryotes versus eukaryotes
      1. Prokaryotes, also called bacteria, do not have membrane-bound organelles. Their DNA is arranged in a circle.
      2. Since prokaryotes evolved before eukaryotes, they are much simpler.
      3. Eukaryotes have many specialized membrane-bound organelles. Their DNA is linear and is wrapped around histones.
   2. Heterotrophs versus autotrophs
      1. Heterotrophs are organisms which obtain their food from the environment.
      2. Organisms which produce their own food are autotrophs.
   3. Aerobes versus anaerobes
      1. Aerobes are organisms which require oxygen for respiration.
      2. Anaerobes are organisms which do not need oxygen to survive.
   4. Official classification of organisms
      1. Organisms are classified into five different kingdoms. Two of them, kingdom Monera and kingdom Protista, consist of unicellular organisms. The subcategory under a kingdom is called a phylum.
      2. Kingdom Monera consists only of prokaryotes.
      3. All unicellular eukaryotes are found in kingdom Protista.
2. Spirochetes, Myxobacteria, and Cyanobacteria
   1. Spirochetes
      1. Spirochetes are a class of bacteria which may be aerobic or anaerobic.
      2. All spirochetes have a spiral shape, formed by wrapping around special flagella called axial filaments.
      3. A spirochete can move by rotating its axial filaments.
      4. Since heat denatures proteins, the genetic information had to be some other material.
   2. Myxobacteria
      1. Myxobacteria are aerobic, heterotrophic bacteria which secrete a slimy mucus-like material.
      2. They do not have flagella but move using fibrils on the inside of the cell.
      3. Myxobacteria reproduce by forming large clusters called fruiting bodies which release small clusters of cells called cysts.
   3. Cyanobacteria
      1. Cyanobacteria are autotrophic bacteria, sometimes called blue-green algae. They may be aerobic or anaerobic.
      2. Unlike most prokaryotes, cyanobacteria have specialized membranes which help perform specific functions.
      3. Cyanobacteria do not normally have flagella.
      4. They may form large colonies in which certain cells take on specific roles.
3. Algae
   1. Algae refers informally to autotrophs in kingdom Protista.
   2. Algae usually live near the surface of large bodies of water where light (necessary for photosythesis) is abundant.
   3. They may form large colonies, and some varieties of algae are multicellular.
4. Euglenas
   1. Euglenas are a type of algae, so they have chloroplasts in which photosynthesis occurs.
   2. They do not have a cell wall and can act as a heterotroph if the environment does not foster photosynthesis.
   3. The stigma is a light-sensitive structure which directs the euglena towards light for photosynthesis.
   4. The pyrenoid bodies store food in the form of paramylum, a type of starch.
   5. Euglenas can move using their long flagellum or their pellicle, a structure made of protein lying next to the cell membrane which can wiggle.
5. Chrysophytes and Pyrrophytes
   1. Both chrysophytes and pyrrophytes are types of algae.
   2. Chrysophytes
      1. Chrysophytes contain the green pigment chlorophyll and another pigment, fucoxanthin, which gives them a yellow-brown color.
      2. Chrysophytes store their food in the form of oils.
      3. The cell walls of chrysophytes are strengthened by silicon compounds.
      4. They usually live in marine environments.
   3. Pyrrophytes
      1. Pyrrophytes are also known as dinoflagellates.
      2. Most pyrrophytes have two flagella.
      3. Pyrrophytes are bioluminescent; they naturally give off a small amount of light, enough to glow in the dark.
      4. Pyrrophytes store their food as both starch and oils.
      5. Like chrysophytes, they usually are found in marine environments.
6. Protozoa
   1. Protozoans informally refers to heterotrophs in kingdom Protista.
   2. Most protozoans reproduce asexually, but some types have complex sexual reproduction cycles.
7. Phyla Mastigophora and Sarcodina
   1. Mastigophores
      1. Mastigophores are the most primitive protozoans.
      2. They can have many flagella, and they also move using pseudopodia.
      3. Most mastigophores reproduce asexually.
      4. Mastigophores are usually parasites, living inside of another organism at that organism's expense.
   2. Sarcodines
      1. Sarcodines freely form pseudopodia to move and capture prey.
      2. They are usually free-living, existing in both freshwater and marine environments.
      3. Foraminiferans are sarcodines with calcerous shells (they are made of CaCO₃).
      4. Radiolarians have shells containing silica. They live in marine environments.
      5. Heliozoans have shells made of silica, and they lve in freshwater.
8. Phyla Sporozoa and Ciliophora
   1. Sporozoans
      1. Sporozoans are parasites; they obtain nutrients from their host organism.
      2. They do not have cilia or flagella.
      3. Sporozoans often have complex life cycles. The Plasmodium vivax finds itself in both a mosquito's stomach and a human liver during its lifetime.
   2. Ciliates
      1. Ciliates have great amounts of cilia.
      2. They have trichocysts, organelles which can be discharged from the cell to anchor it or paralyze prey.
      3. Ciliates can live in freshwater or marine environments.
      4. They usually feed on bacteria or other protists.
      5. Paramecia
         1. Paramecia, a type of ciliate, have a gullet to ingest large food particles.
         2. Most paramecia have a contractile vacuole.
         3. Paramecia can exchange DNA with one another through conjugation. This involves lining up their gullets and allowing DNA to pass through.

Terms to know

• aerobe - An organism which uses oxygen during respiration as an acceptor of hydrogen atoms to form water.
• algae - An informal term which refers to photosynthetic eukaryotes.
• anaerobe - An organism which does not use oxygen during respiration.
• autotroph - An organism which produces its own food through a method of autotrophic nutrition (such as photosynthesis).
• axial filaments - Special flagella found in spirochetes around which the cell wraps itself, resulting in a spiral shape. The rotation of the axial filaments can be used for locomotion.
• conjugation - A sharing of genetic information between two paramecia through their gullets.
• contractile vacuole - An organelle which pumps excess water of a cell to prevent it from bursting.
• eukaryote - Cells which contain membrane-bound organelles and are generally more complex than prokaryotes.
• fruiting body - A large cluster of cells of the phylum myxobacteria used in reproduction.
• heterotroph - An organism which must obtain its food from the environment.
• kingdom - One of five large categories into which all organisms can be classified. Prokaryotes are found in kingdom Monera, and kingdom Protista consists of unicellular eukaryotes.
• paramylum - The form of starch which euglenas store as a spare energy source.
• pellicle - A structure in euglenas made of protein lying just next to the cell membrane. The wiggling of the pellicle can be used for locomotion.
• phylum - A subcategory under the general category of kingdom.
• prokaryote - Cells which do not contain membrane-bound organelles and are in general less complex than eukaryotes. They are also referred to as bacteria.
• protozoan - An informal term which refers to a heterotrophic eukaryote.
• pseudopod - Extensions of the cytoplasm toward which the rest of the cytoplasm tends to flow. Pseudopodia can be used for movement and the capture of prey.
• pyrenoid body - A structure found in the euglena which stores starch in the form of paramylum.
• stigma - A light-sensitive structure found in the euglena which directs the euglena's movement toward light.