Domain Eukarya and Protist Diversity: Structure, Evolution, and Photosynthesis

Domain Eukarya

Characteristics of Eukaryotic Cells

The domain Eukarya includes all organisms with eukaryotic cells, which are distinguished by their complex structure and organization compared to prokaryotes.

• Large cell size: Eukaryotic cells are generally larger than prokaryotic cells.

• Membrane-bound organelles: Includes nucleus, mitochondria, chloroplasts, endoplasmic reticulum, and others.

• Well-developed cytoskeleton: Provides structural support and facilitates cell movement.

• Reproduction: Both asexual and sexual reproduction are common; mitosis and meiosis are key processes.

• Origins: Eukaryotes originated through endosymbiosis, where ancestral cells incorporated other cells as organelles.

Eukaryotic Kingdoms

Major Groups and Protists

Eukaryotic kingdoms include Animals, Plants, Fungi, and a diverse group known as protists. Protists are defined by what they are not: they are not animals, plants, or fungi.

• Protists: A paraphyletic group, meaning they do not form a single evolutionary lineage.

• Evolution: Protists evolved approximately 1.5 billion years ago.

• Classification: Modern classifications reflect monophyletic lineages, organized into four supergroups.

Protist Diversity

Metabolism and Habitats

Protists exhibit a wide range of metabolic strategies and are most abundant in aquatic environments.

• Photoautotrophs: Use light energy to synthesize organic compounds (e.g., algae).

• Heterotrophs: Obtain energy by consuming other organisms (e.g., protozoa).

• Mixotrophs: Can switch between autotrophic and heterotrophic modes.

• Unicellularity: Most protists are unicellular, though some form colonies or multicellular structures.

• Sexual reproduction: Most protists are capable of sexual reproduction.

Evolution of Eukaryotic Characteristics

Origins and Endosymbiosis

Eukaryotic cells evolved from prokaryotic ancestors through a series of key innovations, including the development of internal membranes and the acquisition of mitochondria and chloroplasts via endosymbiosis.

• Three Domains of Life: Bacteria, Archaea, and Eukarya share a common ancestor.

• Endosymbiosis: Mitochondria and chloroplasts originated as free-living bacteria that were engulfed by ancestral eukaryotic cells.

• Membranous organelles: Evolved through infolding of the plasma membrane.

Evolution of Eukaryotic Cells

Membrane Infolding and Organelle Acquisition

The evolution of eukaryotic cells involved the development of internal compartments and the integration of symbiotic bacteria as organelles.

• Infolding: The plasma membrane folded inward to form the nuclear envelope and endomembrane system.

• Mitochondria and Chloroplasts: These organelles originated from endosymbiotic events with proteobacteria and cyanobacteria, respectively.

Cyanobacteria and Chloroplasts

Structural and Functional Similarities

Cyanobacteria and chloroplasts share many structural and functional features, supporting the endosymbiotic theory of chloroplast origin.

• Thylakoid membranes: Both contain internal membrane structures for photosynthesis.

• Genetic material: Both have circular DNA.

• Photosynthetic pigments: Both use chlorophyll a.

Photosynthesis in Eukaryotes and Prokaryotes

Comparative Table of Photosynthetic Characteristics

Photosynthesis varies among eukaryotes and prokaryotes in terms of electron donors, pigments, and environments.

Characteristic	Eukaryotes (Algae, Plants)	Cyanobacteria	Prokaryotes (Green Bacteria)	Prokaryotes (Purple Bacteria)
Substance That Reduces CO2	H2O	H2O	Sulfur compounds	Sulfur compounds, organic compounds
Oxygen Production	Oxygenic	Oxygenic	Anoxygenic	Anoxygenic
Photosynthetic Pigment	Chlorophyll a	Chlorophyll a	Bacteriochlorophyll a or b	Bacteriochlorophyll a or b
Site of Photosynthesis	Thylakoids (chloroplasts)	Thylakoids	Chromatophores	Chromatophores
Environment	Aerobic	Aerobic (and anaerobic)	Anaerobic	Anaerobic

Evolution of Multicellularity in Green Algae

Transition from Unicellular to Multicellular Forms

Green algae demonstrate a range of organizational complexity, from unicellular species to multicellular colonies and true multicellular organisms.

• Unicellular forms: Chlamydomonas

• Colonial forms: Tetrabaena, Gonium, Eudorina

• Multicellular forms: Volvox

• Evolutionary trend: Increasing cell specialization and cooperation leads to true multicellularity.

Additional info: The transition to multicellularity is a key evolutionary innovation that enabled the development of complex life forms in the domain Eukarya.