Bellringer Look at the pictures of different kinds of protists. Organize the protists into subgroups based on the characteristics you see.

What types of organisms are classified as protists? Key Ideas What types of organisms are classified as protists? What methods of reproduction do protists use? Why is the classification of protists likely to change in the future?

What Are Protists? From tiny glass stars that float in the ocean to slimy green fuzz that carpets rocks on the shore, a wide variety of organisms make up the group we call protists. The kingdom Protista is made up of organisms that do not belong in any of the other kingdoms. As a result, the members of this kingdom are quite diverse, Protists are eukaryotic organisms that cannot be classified as fungi, plants, or animals.

What Are Protists?, continued Several important characteristics evolved in protists. Those characteristics are: membrane-bound organelles complex cilia and flagella sexual reproduction with gametes multicellularity

What Are Protists?, continued Organelles, including mitochondria and chloroplasts, allow single cells to perform a wide variety of functions. Complex cilia and flagella like those found in protists are also found in many other types of cells.

What Are Protists?, continued Sexual reproduction allows for greater genetic diversity than reproduction by binary fission does. Multicellularity allows cells to specialize, which in turn allows for the development of tissues, organs, and organ systems.

Visual Concept: Characteristics of Protists (this answers a question on the notes!)

Reproduction Asexual reproduction results in offspring that are genetically identical to the parent. Sexual reproduction results in offspring that are genetically different from either parent. Sexual reproduction involves the union of reproductive cells, usually called gametes. Gametes are haploid cells that join to form a diploid zygote.

Reproduction, continued Protists can reproduce asexually by binary fission, budding, and fragmentation. Protists can also reproduce sexually by fusion of gametes.

Reproduction, continued Asexual Reproduction Binary fission occurs when a unicellular organism reproduces by splitting in half after replicating its DNA. The cells of a multicellular organism reproduce by mitosis with cytokinesis, often simply called mitosis. Multicellular organisms do not undergo binary fission.

Reproduction, continued Asexual Reproduction Budding is a form of asexual reproduction in which a part of the parent organism pinches off and forms a new organism. Budding can occur in unicellular and multicellular organisms. Budding differs from binary fission in that the offspring is smaller than the parent.

Reproduction, continued Asexual Reproduction In fragmentation, part of a multicellular organism breaks off and starts a new organism. Fragmentation differs from budding in that budding is an action that is performed by the organism itself. Fragmentation is the result of an action that is done to an organism. For example, an accident can result in fragmentation, but not budding.

Reproduction, continued Sexual Reproduction In many protists, sexual reproduction occurs as a response to environmental stress. In some protists, the zygote secretes a tough outer coating and becomes a zygospore. Zygospores can survive freezing, drying, and UV radiation.

Reproduction, continued Visual Concept Sexual Reproduction In most unicellular protists a mature organism is haploid. A haploid cell divides by binary fission to produce haploid gametes. Two gametes fuse to form a diploid zygote and a zygospore. When environmental conditions improve, meiosis occurs within the zygospore. Haploid cells break out of the zygospore and grow into mature cells.

Reproduction, continued Sexual Reproduction Many multicellular protists can reproduce both sexually and asexually. This process, called alternation of generations, consists of multicellular haploid and multicellular diploid phases. The diploid, spore-producing phase is called the sporophyte generation.

Reproduction, continued Sexual Reproduction The adult sporophyte has sporangia, reproductive cells that produce haploid spores by meiosis. The spores grow into multicellular haploid organisms. The haploid, gamete-producing phase is called the gametophyte generation. The mature gametophyte produces haploid gametes by mitosis.

Reproduction, continued Visual Concept Sexual Reproduction Two gametes fuse to form a diploid zygote. The zygote divides to form a multicellular diploid organism. This step begins the first stage of a new sporophyte generation.

Classifying Protists When living things are classified, relationships between groups are inferred from specific shared characteristics. Some protists share characteristics of plants or animals, while others are more like fungi. Understanding the relationships between protists and developing a classification system that reflects these relationships are ongoing challenges.

Classifying Protists, continued Scientists use Information from DNA, RNA, and protein to infer relationships among protists and other kingdoms. The classification of protists is likely to change as scientists learn more about protists are related to each other and to members of other kingdoms.

Classifying Protists Molecular studies suggest that protists could be classified into up to 20 kingdoms! The characteristics that protists share with plants, animals, and fungi provide information about the evolution of these organisms. A new classification system for protists is currently being developed.