Section 1: Characteristics of 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? Protists are eukaryotic organisms that cannot be classified as fungi, plants, or animals. Several important characteristics evolved in protists. Those characteristics are: membrane-bound organelles complex cilia and flagella sexual reproduction with gametes multicellularity

Visual Concept: Characteristics of Protists

Visual Concept: Origin of Eukaryotic Cells

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.

Reproduction, continued Asexual Reproduction Binary fission unicellular organism reproduces by splitting in half after replicating its DNA. Multicellular organisms do not undergo binary fission. Budding part of the parent organism pinches off and forms a new organism. can occur in unicellular and multicellular organisms. offspring is smaller than the parent.

Reproduction, continued Asexual Reproduction Fragmentation part of a multicellular organism breaks off and starts a new organism. budding is an action that is performed by the organism itself. Fragmentation is the result of an action that is done to an organism.

Reproduction, continued Unicellular Sexual Reproduction response to environmental stress. Zygospore Protective structure that contains a zygote can survive freezing, drying, and UV radiation.

Reproduction, continued Unicellular Sexual Reproduction Most mature unicellular protists are haploid 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 Haploid cells break out Grow to mature cells

Visual Concept: Sexual Reproduction in Unicellular Protists

Reproduction, continued Multicellular Sexual Reproduction Many multicellular protists can reproduce both sexually and asexually. This process is called alternation of generations Two or more different methods of reproduction

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

Reproduction, continued Multicellular 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.

Visual Concept: Reproduction in Ulva

Classifying Protists Some protists share characteristics of plants or animals, while others are more like fungi. Classified by DNA, RNA, protein Likely to change as scientists learn more could be classified into up to 20 kingdoms! characteristics that protists share with plants, animals, and fungi provide information about the evolution of these organisms.

Summary Protists are eukaryotic organisms that cannot be classified as fungi, plants, or animals. Protists can reproduce asexually by binary fission, budding, and fragmentation. Protists can also reproduce sexually by fusion of gametes. The classification of organisms currently grouped in the kingdom Protista is likely to change as scientists learn more about how these organisms are related to each other and to members of other kingdoms.