Protists

Protists are eukaryotic organisms classified into various groups based on characteristics.
Traditional classification divided protists into plant-like and animal-like forms, but modern views recognize the diversity of protists as a polyphyletic group.
The term "Kingdom Protista" refers to an artificial grouping of over 64,000 single-celled life forms.
Protists mostly lack the tissue organization present in higher eukaryotes.
They thrive in a range of moist habitats, often being free-living or parasitic, with some species causing diseases in humans and animals.

Nutritional strategies include:
- Chemoheterotrophy: Protozoa obtain nutrients from dead organic matter via enzymatic degradation (osmotrophy) and phagocytosis (holozoic nutrition).
- Photoautotrophy: Some protists utilize photosystems I and II for oxygenic photosynthesis, classified as strict aerobes.
- Mixotrophy: Organisms that can use both organic and inorganic carbon simultaneously.

Plasma membranes of protists resemble those of multicellular plants and animals.
Cytoplasm is often divided into:
- Ectoplasm: A gelatinous outer layer just beneath the plasma membrane.
- Endoplasm: A fluid inner region.
Structure provides support and aids in nutrient uptake.

Encystment: The process where protists enter a dormant state (cyst) to withstand unfavorable environmental changes and possibly assist in transmission between hosts in parasitic forms.
Excystment: The transition from a cyst back to active form upon returning to favorable conditions, critical in parasitic life cycles after transmission between hosts.

Protists can reproduce via:
- Asexual reproduction: Commonly through binary fission.
- Sexual reproduction: Involving gamete fusion through syngamy, which can occur within one individual (autogamy) or between individuals (conjugation).

Includes subgroups: Fornicata, Parabasilia, Euglenozoa.
Organisms characterized by a suspension feeding groove (cytostome) aiding in particle capture.

Microaerophilic protists, e.g., Giardia, known for causing diarrheal diseases.
Flagellated and lack mitochondria, possess mitosomes (mitochondria-like organelles).

Flagellated, often endosymbionts in animals.
Lack distinct cytosomes and feed via phagocytosis, with two major groups: Trichonymphida (associated with termites) and Trichomonadida (human pathogens).

Found predominantly in freshwater; approximately one-third are photoautotrophic; Euglena is the most renowned.
Exhibits unique features like a pellicle for structural support and a stigma for light orientation.
Pathogenic trypanosomes can cause severe diseases, including Chagas' disease and certain forms of sleeping sickness, demonstrating antigenic variation to evade host defenses.

Identified by amoeboid motility using pseudopodia.
Two types: Naked amoebae (only plasma membrane) and testate amoebae (encased in protective structures).
Reproduce via binary or multiple fission, with various pseudopod shapes classified accordingly.

Classified into myxogastria (acellular slime molds) and dictyostelia (cellular slime molds).
Myxogastria features a plasmodium state with multiple nuclei, capable of forming fruiting bodies.

Marine protists characterized by reticulopodia and complex tests; contribute to carbon cycling in ecosystems.

Crucial for global carbon cycling with unique frustule structures and high primary productivity levels in oceans.

Contains organisms with photosynthetic plastids derived from ancient cyanobacteria endosymbiosis, including all higher plants and various algal species.

Includes diverse green algae with variations in morphology, and reproductive strategies encompass both sexual and asexual reproduction.