Classification of Prostist

4 Key excerpts on "Classification of Prostist"

• 

  eBook - ePub

  Fungi, Algae, and Protists

  • Britannica Educational Publishing, Kara Rogers(Authors)
  • 2010(Publication Date)
  • Britannica Educational Publishing

    (Publisher)

  CHAPTER 4Protists

  I n 1866 German zoologist Ernst Haeckel proposed the establishment of the kingdom Protista to embrace “lower” organisms. However, his conception failed to gain widespread support during his lifetime. Much later, in the 1970s and ’80s, the work of American biologists R.H. Whittaker and Lynn Margulis, as well as others, led to widespread support for considering living organisms as constituting five separate kingdoms, one of which was kingdom Protista. This grouping consisted of eukaryotic, predominantly unicellular microscopic organisms known generally as protozoans, algae, and “fungi-like” organisms.

  Protists often share certain morphological and physiological characteristics with animals, plants, and fungi. However, protists are neither animals nor plants, nor are they fungi. In fact, although the protists were given their own kingdom, recent genetic and biochemical studies have demonstrated that many of the traditional members of the kingdom do not share a common evolutionary history. Thus, despite the work of Whittaker and Margulis, most scientists have abandoned the use of kingdom Protista in formal classification schemes.

  Because of their somewhat ambiguous and diverging evolutionary histories, protists are extraordinarily heterogeneous, far more so than fungi and lichens. This diversity can be seen in the various ways in which they acquire nutrients, whether through the breakdown of organic matter or through symbiosis, parasitism, or other types of interactions with plants and animals. Most protists are motile, often having either flagella, cilia, or pseudopodia, which allow them to navigate within aqueous habitats. Many protists are pathogenic (disease-causing) in plants or animals, and some cause serious diseases—such as malaria and sleeping sickness—in humans. Thus, although the protists are no longer recognized as a formal group in current biological classification systems, they still are important members of life on Earth.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  The Handy Science Answer Book

  • James Bobick, Naomi E. Balaban, James Bobick, Naomi E. Balaban(Authors)
  • 2019(Publication Date)
  • Visible Ink Press

    (Publisher)

  In 1969, American plant ecologist Robert Harding Whitaker (1920–1980) proposed a system of classification based on five different kingdoms. The groups suggested by Whitaker were the bacteria group Prokaryote (originally called Monera), Protista, Fungi (for multicellular forms of nonphotosynthetic heterotrophs and single-celled yeasts), Plantae, and Animalia. This classification system is still widely accepted.

  A six-kingdom system of classification, as proposed in 1977 by American microbiologist and biophysicist Carl Woese (1928–2012), included Archaebacteria and Eubacteria (both for bacteria), Protista, Fungi, Plantae, and Animalia. In 1981, Woese further proposed a classification system based on three domains (a level of classification higher than a kingdom): Bacteria, Archaea, and Eukarya. The domain Eukarya is further subdivided into four kingdoms: Protista, Fungi, Plantae, and Animalia. The system involving three domains is the most current classification scheme that is generally accepted by biologists.

  What is cladistics?

  Cladistics is one of the newest approaches to classification. It is often defined as a set of concepts and methods for determining cladograms, which portray branching patterns of evolution. Overall, it is a method of classification of organisms (e.g., a group of Gram-negative bacteria) according to the proportion of measurable characteristics that they have in common. Thus, the organisms are grouped together based on whether or not they have one or more shared characteristics that are unique and that come from the group’s last common ancestor; in addition, the characteristics cannot be present in more distant ancestors.

  CELLS What is a cell?

  A cell is the basic structural and functional unit of all forms of life. It is a membranebound unit that contains hereditary material (DNA) and cytoplasm. The study of microscopic cell structure is cytology and was established as a branch of biology in 1892 by the specialized investigations of Oscar Hertwig (1849–1922).

  What is the cell theory?

  The cell theory is the concept that all living things are made up of essential units called “cells.” Diverse forms of life exist as single-celled organisms. More complex organisms, including plants and animals, are multicellular—cooperatives of many kinds of specialized cells that could not survive for long on their own. All cells come from preexisting cells and are related by division to earlier cells that have been modified in various ways during the long evolutionary history of life on Earth. Everything an organism does occurs fundamentally at the cellular level.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Cell Physiology Sourcebook

  A Molecular Approach

  • Nicholas Sperelakis(Author)
  • 2001(Publication Date)
  • Academic Press

    (Publisher)

  SECTION VII P r o t o z o a a n d B a c t e r i a This page intentionally left blank Michael Levandowsky and Thomas E. Gorreil Physiological Adaptations of Protists I. Introduction This chapter introduces the reader to the great diversity to be found in the physiology of the single-celled eukaryotes, or protists. These include a variety of groups, some auto-trophic or plant-like, some phagotrophic or osmotrophic and thus animal-like, and many with a combination of these traits. First, a word about terminology and classification. Trad-itionally, these organisms comprised the algae and the pro-tozoa. The early classifications of protozoa divided them into three groups based on their locomotion (ciliates, flag-ellates, amoebae) and a parasitic group (sporozoa). The algae were classified largely on the basis of pigments (red algae, brown algae, golden-brown algae, green algae) and obvious structural differences (cryptophytes, di-noflagellates). Many of the flagellated groups appeared in both classifications. These characteristics are clearly im-portant, and the early classifications and terminology tend to persist in informal usage. However, subsequent work with the electron microscope, biochemical advances, and molecular approaches have changed many of our views on phylogenetic relationships, and revised classification schemes have been proposed (Cavalier-Smith, 1993; Patterson 1994). In this chapter, the term Protista, or simply protists, is used to describe all of these groups. Figure 1 gives an infor-mal indication of current thinking about relationships among protistan and other eukaryotic groups, as indicated by molecular evidence, particularly rRNA homology (Sogin, 1989, 1991) as well as ultrastructural information. Molecular and other evidence suggests that many protist lines may have originated over a billion years ago.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Origins of Sex

  • Mark Carlson Williams(Author)
  • 1977(Publication Date)
  • Yale University Press

    (Publisher)

  THE EMERGENCE OF PROTISTS • 75 Knowledge of protoctists has been limited by the traditional ways in which they have been studied. Scattered in the literatures of zoology, botany, and mycology, general information about protists is difficult to obtain. Even today there are arguments about whether the nature of these organisms is primaril. plant, fungal, or animal. It is none of these. Those of us who study this extraordinarily variable kingdom are beginning to recognize over thirty dis-tinctive groups (phyla) of protoctists (fig. 22). We expect fundamentally differ-ent new phyla of smaller protoctists to be revealed by natural history and electron microscopy within the next decade or so. Although protoctists are defined as eukaryotes excluded from the animal, plant, and fungal kingdoms, they have positive identifying features as well. Protoctists are aquatic organisms that vary from the size of bacteria (for exam-ple, Nanochlorum, the smallest photosynthetic protist known, is a little larger than one micrometer in diameter; Zahn, 1984) to giant seaweeds a hundred meters long. Unlike animals, fungi, and plants, protoctists may lack mitosis, microtubules, mitochondria, and other common eukaryotic organelles. The distribution, including the absence, of these features considered essential to the sexual meiotic process provides us with fascinating clues to its origins. All protoctists lack embryos and embryonic development, major features in animals. Except for a few groups, members of a given protoctist phylum tend to have characteristic cell structures, including kinetids. Kinetids (fig. 23) are unit structures that always include at least one kinetosome (fig. 24). They consist of microtubules and fibers arranged in patterns surrounding the one or more kinetosomes at the bases of undulipodia. Nutritionally protoctists are heterotrophs, feeding by absorption (os-motrophically), by ingestion (phagotrophically), or they are photosynthetic.

  Sign up to read

  Learn more about book