The Kingdom Protista

<h1>Text Preview</h1> <p>The first eukaryotes arose from prokaryotic ancestors about two billion years ago. </p> <p>Because they were unicellular, those early eukaryotes would be classified as protists if they existed today. What innovations distinguish eukaryotes from prokaryotes?</p> <p>The most conspicuous difference is the presence of organelles within eukaryotic cells. Organelles are membrane-enclosed structures in which different cellular functions take place. The defining feature of a eukaryote is its nucleus, the compartment containing its DNA. The nuclear envelope and the endoplasmic reticulum may have arisen from an infolding of the plasma membrane in the ancestral eukaryote. Biologists hypothesize that the organelles called mitochondria and chloroplasts are the result of an extreme form of association between different types of organisms.</p> <p>We saw an example of a type of association called symbiosis in our survey of prokaryotic organisms. Bacteria can live in the digestive tracts of animals, where they help break down substances in food, like cellulose. Symbiosis is the living together of two dissimilar organisms in close association. Mitochondria and chloroplasts are the result of an association called endosymbiosis, in which one organism lives within the body of another organism. </p> <p>Biologists believe that primitive eukaryotic cells engulfed ancient symbiotic eubacteria without digesting them. The resulting combination gave the organism advantages that its competitors lacked. The cells that ingested the forerunner of mitochondria could use oxygen to break down organic compounds for usable chemical energy. The cells that ingested the forerunner of chloroplasts could convert sunlight to chemical energy.</p> <p>Some modern protists have symbiotic bacteria in contact with their plasma membrane. Their relationship may be a lot like the symbiosis that led to the formation of mitochondria and chloroplasts. </p> <p>Think of the evolutionary implications of endosymbiosis. Until now, we’ve only considered changes in organisms in a single line of descent, which would sometimes branch into new species. In the rise of eukaryotes containing mitochondria and chloroplasts, the merger of two distant lines of descent led to a new type of organism—the protists.</p> <p>Protists are amazingly diverse. Single-celled protists vary greatly in size. Some are as small as bacteria; others are over a millimeter in length—visible to the naked eye. At the other extreme of size, multicellular protists called kelps can grow tens of meters in length.</p> <p>Many protist cells are extremely complex. In some, certain regions of the cell surface are devoted to specific functions, such as the ingestion of food, or the expulsion of wastes. Some even have a light-sensitive structure that functions as a simple eye. Many protists have specialized surface structures to protect them from adverse conditions in their environment. Many have compartments for the storage and digestion of food, or for collecting excess water within the cell.</p> <p>Many types of protists use flagella to move. These protists are referred to as flagellates—a term that’s descriptive, not taxonomic. A protist's flagellum is structurally very different from a bacterial flagellum. In a protist, the flagellum is covered by the plasma membrane and supported by bundles of microtubules.</p> <p>Many protists, not necessarily related, have the form of amoebas. They have a constantly changing shape, and move and feed using extensions of their cytoplasm called pseudopodia.</p> <p>Protists are nutritionally diverse. Those possessing chloroplasts are photoautotrophs, using sunlight as a source of energy and carbon dioxide as a raw material for growth.</p> <p>Others are chemoheterotrophs, depending on organic compounds for energy and building materials. Of these, some are absorptive, taking in food molecules through their plasma membranes; and some are ingestive, engulfing their food. Some protists can even switch between an autotrophic and heterotrophic state, depending on the availability of sunlight!</p> <p>Protists reproduce both asexually and sexually. Asexual reproduction occurs by mitosis, often with variations in the molecular details. Some protists undergo meiosis and produce haploid gametes, which unite to form a genetically distinct diploid cell. Some protists exchange genetic material in the process of conjugation. Although the process doesn’t increase the number of cells, it does produce genetic variation, because the resulting cells are genetically distinct from either of the original cells. </p> <p>Some Protists have complex life cycles. In some, the form of their cell changes. A protist with a flagellum at one stage of its life cycle may change into an amoeba at a later stage. Some plantlike protists even undergo alternation of generations. Each of the two generations is multicellular. The sporophyte generation is a diploid organism that produces spores, reproductive cells that can develop into an adult without interacting with any other cells. The gametophyte generation is a haploid organism that produces gametes. One gamete must fuse with another in order to produce a new organism.</p> <p>With that general background on the kingdom Protista, let’s consider some important groups of protists, starting with the protozoans.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>