Infolding Plasma Membrane

Protists Defined

► In this book we define the protists simply as all eukaryotes that are not plants, fungi, or animals. The protists are a para-phyletic group, not a clade.

The Origin of the Eukaryotic Cell

► The modern eukaryotic cell arose from an ancestral prokary-ote in several steps. Probable steps include the loss of the cell wall and infolding of the plasma membrane. Review Figure 28.2

► In subsequent steps, an infolded plasma membrane attached to the chromosome may have led to the formation of a nuclear envelope. A primitive cytoskeleton evolved. Review Figure 28.3

► The first truly eukaryotic cell was larger than its prokaryote ancestor, was probably a phagocyte, and may have possessed one or more flagella of the eukaryotic type.

► The incorporation of prokaryotic cells as endosymbionts gave rise to eukaryotic organelles. Peroxisomes, which protected the host cell from an oxygen-rich atmosphere, may have been the first organelles of endosymbiotic origin. Mitochondria evolved from once free-living proteobacteria, and chloroplasts evolved from once free-living cyanobacteria. Review Figure 28.3

General Biology of the Protists

► Most protists are aquatic; some live within other organisms. The great majority are unicellular and microscopic, but many are multicellular and a few are enormous.

► "Protozoan" is an outdated term sometimes applied to pro-tists, mostly ingestive heterotrophs, that were once classified as animals. "Alga" is an outdated term sometimes applied to pho-tosynthetic protists.

► Protists vary widely in their modes of nutrition and locomotion. Some protist cells contain contractile vacuoles, and some digest their food in food vacuoles. Review Figures 28.5, 28.6. See Web/CD Tutorial 28.1

► Protists have a variety of cell surfaces, some of them protective.

► Many protists contain endosymbionts. Some protists are endosymbionts in other cells, including other protists. Some endosymbiotic protists perform photosynthesis, to the advantage of their hosts.

► Most protists reproduce both asexually and sexually. Protist Diversity

► Molecular and other techniques are enabling biologists to identify many clades of protists. Review Table 28.1 and Figure 28.9

Diplomonads and Parabasalids

► Diplomonads and parabasalids may have the most ancient roots of today's protists. Both lack mitochondria, having apparently lost them during their evolution.

► Diplomonads have two nuclei and multiple flagella. Review Figure 28.10a

► Parabasalids have flagella and undulating membranes. Review Figure 28.10b


► The euglenozoans are a clade of unicellular protists with flagella.

► Euglenoids are euglenozoans that are often photosynthetic and have anterior flagella. Review Figure 28.11

► Kinetoplastids are euglenozoans that have a single, large mitochondrion, in which RNA is edited.


► The alveolates are a clade of unicellular organisms with cavities, called alveoli, beneath their plasma membranes.

► Dinoflagellates are marine alveolates with a golden-brown color that results from their photosynthetic and accessory pigments. They are major contributors to world photosynthesis. Many are endosymbionts; in that role they are important contributors to coral growth. Dinoflagellates are responsible for toxic "red tides."

► Apicomplexans are parasitic alveolates with an amoeboid body form. Their spores, containing a mass of organelles at the apical end, are adapted to the invasion of host tissue. The api-complexan Plasmodium, which causes malaria, uses two alternate hosts (humans and Anopheles mosquitoes). Review Figure 28.14

► Ciliates are alveolates that move rapidly by means of cilia and have two kinds of nuclei. The macronuclei control the cell by means of transcription and translation. The micronuclei are responsible for genetic recombination, accomplished by conjugation, a process that is sexual, but not reproductive. Review Figures 28.16, 28.17. See Web/CD Activity 28.1


► Stramenopiles typically have two flagella of unequal length, the longer bearing rows of tubular hairs. Some stramenopile groups are photosynthetic.

► Diatoms are unicellular stramenopiles, many of which have complex, two-part, glassy cell walls. They contribute extensively to world photosynthesis. Review Figure 28.19

► The brown algae are predominantly multicellular, photosyn-thetic stramenopiles. They include the largest of all protists, and some show considerable tissue differentiation.

► In many multicellular photosynthetic protists and in all plants, both haploid and diploid cells undergo mitosis, leading to an alternation of generations. The diploid sporophyte generation forms spores by meiosis, and the spores develop into hap-loid organisms. This haploid gametophyte generation forms gametes by mitosis, and their fusion yields zygotes that develop into the next generation of sporophytes. Review Figure 28.22

► Oomycetes are a group of nonphotosynthetic stramenopiles including water molds and downy mildews. The oomycetes are coenocytic. They are diploid for most of their life cycle.

Red Algae

► Red algae are multicellular, photosynthetic protists. They differ from the other photosynthetic protist groups in having a characteristic storage product (floridean starch) and lacking flagellated reproductive cells.


► The chlorophytes, a clade of green algae, are often multicellu-lar. Like plants, they contain chlorophylls a and b and use starch as a storage product.

► The chlorophytes are sister to a clade that includes other green algae and the plant kingdom.

► The chlorophytes have diverse life cycles; among these are the isomorphic alternation of generations of Ulva and the hap-lontic life cycle of Ulothrix. Review Figures 28.26, 28.27. See Web/CD Activities 28.2 and 28.3


► The choanoflagellates are colonial protists with flagella and a body type similar to the most characteristic type of cell found in sponges. The choanoflagellates are sister to the animal kingdom.

A History of Endosymbiosis

► Primary endosymbiosis of a cyanobacterium and a eukaryote gave rise to the chloroplasts of green algae, plants, and red algae. Review Figure 28.29. See Web/CD Tutorial 28.2

► Secondary endosymbioses of eukaryotes with unicellular green or red algae gave rise to the chloroplasts of euglenoids, stramenopiles, and other groups. One of those groups has given rise to another type of chloroplast by tertiary endosymbiosis.

Some Recurrent Body Forms

► Some similar body forms are found in several different, unrelated protist groups.

► Amoebas, which appear in many protist groups, move by means of pseudopods.

► Actinopods have thin, stiff pseudopods that serve various functions, including food capture.

► Foraminiferans also use pseudopods for feeding, and they secrete shells of calcium carbonate.

► Acellular slime molds and cellular slime molds are superficially similar, moving as slimy masses and producing stalked fruiting structures. However, they differ at the cellular level. Acellular slime molds are coenocytes with diploid nuclei. Cellular slime molds consist of individual haploid cells that aggregate into masses consisting of distinct cells.

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  • Lidya
    What is the function of infoldings in the plasma membrane?
    8 years ago
    How are infoldings in a heterotroph an advantage?
    8 years ago
  • eric
    What is infolding of plasma membrane?
    7 years ago

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