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|Protista are the simplest of the eukaryotes. Protists are an unusual group of organisms that were put together because they don't really seem to belong to any other group. Some protists perform photosynthesis like plants such as the diatoms (see Figure 11-42.) while others move around and act like animals such as the amoeba (see Figure 11-43), but protists are neither plants nor animals.|
Figure 11-42 Diatoms
Figure 11-43 Amoeba
As with all eukaryotic cells, protists contain membrane-bound nuclei and endomembrane systems, as well as numerous organelles. Movement is often provided by one or more flagella, and cilia are often present on the plasma membrane as sensory organelles. Unlike prokaryotes, protistan nuclei contain multiple DNA strands, though the total number of nucleotides is significantly less than that in more complex eukaryotes. Protists can reproduce mitotically, and some are capable of meiosis for sexual reproduction. Cellular respiration in the kingdom is primarily an aerobic process, but some protists, including those that live in mud below ponds or in animal digestive tracts, are strict or facultative anaerobes.
Protists represent an important step in early evolution, evolving from prokaryotes and eventually giving rise to the entire line of eukaryotes. The first protists probably evolved 1.7 billion years ago, 2.3 billion years after the origin of life, from simple communities of prokaryotic cells. Membrane infolding was one of the defining processes in this evolution: in some prokaryotic cells, parts of the plama membrane folded into the cell to create the nuclear envelope and the other organelles of the endomembrane system. The second major step in the evolution of protists from bacteria was the process of endosymbiosis, which introduced the mitochondrion and chloroplast as organelles of eukaryotic cells. Small prokaryotic cells capable of cellular respiration or photosynthesis entered eukaryotic cells, either as parasites or indigestible food, and these prokaryotes evolved into mitochondria and chloroplasts as they developed a symbiotic relationship with the host cell. (Because mitochondria are present in all eukaryoptic cells, this process probably happened to mitochondria first.) As a result of these two processes, protists evolved as sucessful organisms. Eventually, colonial protists evolved into plants, fungi, and animals, of the eukaryotic kingdom, which came to dominate the earth.
Protista is divided into four major groups by lifestyle: the protozoans, the slime molds, the unicellular algae, and the multicellular algae. Protozoans include all protists that ingest their food, and thus they live primarily in aquatic habitats, such as ponds, drops of water in soil, or the digestive tracts of animals. In the latter capacity, a small number of protozoans function as parasites. The slime molds in the second group are unique in having both unicellular and multicellular stages. When sufficient bacteria (food) are present, cellular slime molds are single amoeboid cells; however, when food becomes scarce, they aggregate into slug-like colonies, which become large reproductive structures. Plasmodial slime molds also exist as single cells when nutrients are plentiful, but each cell can grow into a large, branching plasmodium with many nuclei. This differentiates into reproductive structures when food is scarce. The third and fourth groups of protists, the algae, contain chloroplasts and photosynthesize like plants; these can be unicellular, colonial, or multicellular. Multicellular marine algae, the seaweeds, are similar to marine plants, and many biologists support moving seaweed into the plant kingdom.
Diatoms (see Figure 11-42.) have a golden brown accessory pigment in their chloroplasts that can mask the color of chlorophyll. The structure of a diatom often is compared to a box because the cell wall has 2 halves, or valves, with the larger valve acting as a "lid" for the smaller valve. The cell wall of the diatom has an outer layer of silica, a common ingredient of glass. Diatoms are among the most numerous of all unicellular algae in the oceans. As such, they serve as an important source of food for other organisms.
Amoeba proteus (see Figure 11-43.) are a small mass of cytoplasm without any definite shape. They move about and feed by means of cytoplasmic extensions called pseudopodia, or false feet. A pseudopodium forms when the cytoplasm streams forward in the particular direction. The organelles within an amoeba include food or digestive vacuoles and contractile vacuoles (for expelling waste).Further details of micro-organisms evolution are described in the special topic on Evolution of Micro-organisms and Plants.