121.3 Biology chapter 27

Domain Eukarya: Introduction and Unique Features

Overview of Eukaryotes

Eukaryotes are organisms whose DNA is supercoiled and packaged within a membrane-bound nucleus. They are typically larger than prokaryotic cells and can form cooperative aggregations, leading to multicellular organisms, tissues, and organs. The domain Eukarya contains the most familiar organisms, including fungi, plants, animals, and protists.

• Eukaryotic cells are distinguished by their nucleus and complex internal structures.

• Four main kingdoms of multicellular eukaryotes: Plants (cell walls of cellulose, large vacuoles, chloroplasts), Fungi (cell walls of chitin, septa), Animals (no cell walls, no chloroplasts, small vacuoles), and Protists.

Eukaryotic Origins

Chemical traces suggest the first eukaryotes appeared about 2.7 billion years ago, with microfossils dating back 1.5 billion years. Modern eukaryotic cells are likely much different from the originals due to horizontal gene transfers, endosymbiosis, and membrane infolding.

• Key evolutionary processes: Endosymbiosis (engulfing other cells), membrane infolding.

• Eight modern clades: Alveolates, Excavates, Stramenopiles, Plants, Rhizarians, Amoebozoans, Fungi, Animals.

Origins of Eukaryotic Organelles

Nucleus and Endoplasmic Reticulum

The nucleus and endoplasmic reticulum (ER) are integral to eukaryotic cells. Both are believed to have formed by infolding of the cell membrane.

• Nucleus: Contains genetic material, regulates gene expression.

• Endoplasmic reticulum: Protein translation, transport, storage.

• Membrane invaginations serve as passageways and are common in eukaryotes.

• The nuclear envelope is an extension of the ER.

Mitochondria and Chloroplasts: Endosymbiont Hypotheses

• Mitochondria: Believed to have originated from bacteria living inside larger prokaryotes. They detoxified oxygen and later coupled reduction to water with ATP production. Mitochondria have their own genome and replicate by binary fission.

• Chloroplasts: Originated from photosynthetic cyanobacteria engulfed by red algae (primary endosymbiosis), then by other protists (secondary and tertiary endosymbiosis).

• Endosymbiosis is not rare; other structures (flagella, microtubules, centrioles) may have resulted from similar processes.

Kingdom Protista: Diversity and Characteristics

General Features of Protists

Protists are the simplest eukaryotes, comprising about 200,000 species. Most are unicellular and very diverse in shape, size, and life history.

• Defined more by what they are not: not plants, fungi, or animals; do not develop from embryos; minimal cell differentiation.

• Cell surfaces vary: some have only a membrane, others have silica shells or cysts.

• Locomotion: flagella, pseudopodia, or cilia.

• Nutrition: mostly heterotrophic, some photosynthetic, some both.

• Reproduction: both asexual (budding, schizogony) and sexual (meiosis, mitosis).

Protist Diversity and Classification

Protists are a loose grouping defined by unicellularity, lack of embryonic cleavage, heterotrophy, and possession of a membrane-bound nucleus. They are found in all habitats, with some symbiotic and pathogenic forms.

Significant Protist Phyla

Overview of Major Phyla

The following table summarizes the main protist phyla, their characteristics, and classification:

Phylum	Main Features	Classification
Diplomonadida	Paired nuclei, lack mitochondria (mitosomes), flagella	Excavates
Parabasilida	Anaerobic, parabasal fiber, hydrogenosomes, flagella	Excavates
Euglenida	Flexible pellicle, flagella, autotrophic/heterotrophic	Excavates
Kinetoplastida	Kinetoplast DNA, undulating membrane, parasitic	Excavates
Ciliophora	Cilia, alveoli, macro/micronucleus, conjugation	Alveolata
Apicomplexa	Apical complex, parasitic, complex life cycles	Alveolata
Dinoflagellata	Flagella, bioluminescent, red tides, alveoli	Alveolata
Stramenopila	3-part flagellar hairs, diatoms, brown algae	Stramenopila
Actinopoda	Axopodia, silica skeleton, radial symmetry	Rhizaria
Foraminifera	Tests (shells), complex life cycles, benthic	Rhizaria
Rhodophyta	Red algae, phycobilisomes, alternation of generations	Archaeplastida
Loboseans	Amoeboid, pseudopodia, free-living/parasitic	Amoebozoa
Slime Molds	Plasmodial/cellular, fruiting bodies, spores	Amoebozoa
Choanoflagellata	Collar of villi, single flagellum, animal ancestor	Opisthokonta
Microspora	Intracellular parasites, mitosomes, spores	Fungi (Microsporidia)
Chlorophyta	Green algae, chlorophylls, cell walls	Archaeplastida

Excavates: Diplomonadida, Parabasilida, Euglenida, Kinetoplastida

• Diplomonadida: Paired nuclei, lack mitochondria, flagella. Example: Giardia lamblia.

• Parabasilida: Anaerobic, parabasal fiber, hydrogenosomes, flagella. Example: Trichomonas vaginalis.

• Euglenida: Flexible pellicle, flagella, autotrophic/heterotrophic. Example: Euglena.

• Kinetoplastida: Kinetoplast DNA, undulating membrane, parasitic. Example: Trypanosomes (sleeping sickness).

Alveolata: Ciliophora, Apicomplexa, Dinoflagellata

• Ciliophora: Cilia, alveoli, macro/micronucleus, conjugation. Example: Paramecium.

• Apicomplexa: Apical complex, parasitic, complex life cycles. Example: Plasmodium (malaria).

• Dinoflagellata: Flagella, bioluminescent, red tides, alveoli. Example: Red tide dinoflagellates.

Stramenopila: Diatoms, Brown Algae

• 3-part flagellar hairs, diatoms, brown algae, heterotrophic/photosynthetic.

• Important producers in marine food webs.

Rhizaria: Actinopoda, Foraminifera

• Actinopoda: Axopodia, silica skeleton, radial symmetry.

• Foraminifera: Tests (shells), complex life cycles, benthic.

Amoebozoa: Loboseans, Slime Molds

• Loboseans: Amoeboid, pseudopodia, free-living/parasitic.

• Slime Molds: Plasmodial/cellular, fruiting bodies, spores.

Opisthokonta: Choanoflagellata

• Collar of villi, single flagellum, animal ancestor.

Fungi: Microspora

• Intracellular parasites, mitosomes, spores.

Archaeplastida: Chlorophyta, Rhodophyta

• Chlorophyta: Green algae, chlorophylls, cell walls.

• Rhodophyta: Red algae, phycobilisomes, alternation of generations.

Representative Images of Protist Diversity

• Ciliophora: Paramecium (example of ciliate structure and movement)

• Chlorophyta: Micrasterias (green algae, cell wall structure)

• Stramenopila: Synura (example of colonial stramenopile)

Summary Table: Protist Phyla and Their Features

Phylum	Key Features	Example
Diplomonadida	Lack mitochondria, paired nuclei	Giardia lamblia
Parabasilida	Anaerobic, hydrogenosomes	Trichomonas vaginalis
Euglenida	Flexible pellicle, flagella	Euglena
Kinetoplastida	Kinetoplast DNA, parasitic	Trypanosomes
Ciliophora	Cilia, alveoli, conjugation	Paramecium
Apicomplexa	Apical complex, parasitic	Plasmodium
Dinoflagellata	Flagella, red tides	Dinoflagellates
Stramenopila	3-part flagellar hairs	Diatoms, brown algae
Actinopoda	Axopodia, silica skeleton	Radiolarians
Foraminifera	Tests, benthic	Foraminiferans
Rhodophyta	Red algae, phycobilisomes	Nori, agar
Loboseans	Amoeboid, pseudopodia	Entamoeba
Slime Molds	Plasmodial/cellular, spores	Myxogastria
Choanoflagellata	Collar, flagellum	Choanoflagellates
Microspora	Intracellular parasites	Microsporidia
Chlorophyta	Green algae, chlorophylls	Volvox, Micrasterias

Key Terms and Concepts

• Endosymbiosis: The process by which one organism lives inside another, leading to organelle formation.

• Pellicle: Flexible protein strips under the cell membrane, providing shape and flexibility.

• Axopodia: Thin pseudopodia radiating outward, used for feeding and locomotion.

• Alternation of Generations: Life cycle alternating between haploid and diploid stages.

• Binary Fission: Asexual reproduction by splitting into two cells.

• Conjugation: Sexual process in ciliates involving exchange of genetic material.

Summary

The domain Eukarya encompasses a vast diversity of organisms, with protists representing the most varied and foundational group. Understanding their structure, origins, and classification is essential for grasping the evolution and complexity of eukaryotic life. Protist phyla are classified based on unique structural, reproductive, and ecological features, and their study provides insight into the origins of multicellular life and the evolutionary history of eukaryotes. Additional info: Some details about evolutionary relationships and classification were inferred for completeness and clarity.