Protist Adaptations – Key Concepts

• Definition of Protists

  • Protists encompass all eukaryotes excluding plants, fungi, and animals.

  • The term is useful but not cladistic, as it represents a paraphyletic group.

• Adaptations

  • Protists exhibit various adaptations:

    • Nuclei:

      • Possibilities include single nuclei, two identical nuclei, two different-sized nuclei, or a single "supercell" with millions of nuclei.

      • These adaptations indicate synapomorphies for different protist lineages.

    • Support/Protection:

      • Cell Wall:

        • Diatoms: Silica

        • Dinoflagellates: Cellulose

      • External Shell:

        • Foraminiferans: Calcium Carbonate

      • Interior Support:

        • Excavatans: Microtubules

        • Euglenoids: Pellicle Proteins

        • Alveolates: Alveoli Sacs

    • Cell Number:

      • Primarily unicellular, but multicellularity has evolved in some lineages.

      • Colonial: Cells perform all life functions and live together (e.g., Volvox).

      • Multicellular: Specialized, interdependent cells (e.g., Kelp).

• Feeding Adaptations

  • Protists obtain energy through various mechanisms:

    • Ingestive: Phagocytosis (e.g., Amoeba, Stentor).

    • Absorptive: Decomposers (e.g., Slime Molds); some are parasites absorbing organic molecules.

    • Photosynthetic: Responsible for ~50% of Earth's photosynthesis, contributing significantly to ecosystems (e.g., Diatoms in Lake Washington).

• Motility Adaptations

  • Amoeboid Motion: Pseudopodia (e.g., Amoeba).

  • Ciliated: (e.g., Paramecium).

  • Flagellated: (e.g., Dinoflagellates).

Adaptations in Reproduction

• Asexual Reproduction:

  • Through mitotic cell division, producing genetically identical daughter cells.

• Sexual Reproduction:

  • Through meiotic cell division, resulting in genetically diverse gametes that are haploid.

  • Sexual reproduction is exclusive to eukaryotes.

• Comparison:

  • Binary Fission: Similar to mitosis in asexual reproduction.

  • Meiosis: Followed by fusion of gametes.

Why Sexual Reproduction?

• Promotes genetic variability, particularly advantageous during environmental changes or stressful conditions.

• Mixed dominance in life cycles:

  • Can be haploid or diploid dominated.

Alternation of Generations

• Many multicellular protists alternate between multicellular haploid and diploid forms.

• Life Cycle Stages:

  • Gametophyte:

    • Multicellular haploid form producing gametes via mitosis.

  • Sporophyte:

    • Multicellular diploid form producing spores via meiosis.

• Definitions:

  • A spore is a single haploid cell; gametes are haploid cells that fuse to form a diploid zygote, developing into a sporophyte.

Examples of Alternation of Generations

• Similar Appearance: Ectocarpus siliculosus exhibits haploid and diploid forms that look the same.

• Differing Appearance: Laminaria species have distinct haploid and diploid forms; both have specialized structures for meiosis and mitosis.