🔬General Biology I Unit 23 – Protists

What Are Protists?

• Eukaryotic organisms that are not plants, animals, or fungi
• Mostly unicellular, but some are multicellular or colonial
• Highly diverse group with a wide range of morphologies and lifestyles
• Inhabit various environments, including aquatic and terrestrial habitats
• Play crucial roles in ecosystems as primary producers, consumers, and decomposers
• Include both free-living and parasitic species
• Historically grouped together based on their lack of specialized tissues and organs
• Paraphyletic group, meaning they do not share a single common ancestor

Diversity of Protists

• Protists exhibit an incredible diversity of forms, sizes, and lifestyles
• Range in size from microscopic (a few micrometers) to macroscopic (several meters long, such as kelp)
• Display a variety of cell organizations, including single cells, colonies, and multicellular structures
• Possess diverse organelles and cellular features adapted to their specific lifestyles
• Exhibit various modes of locomotion, such as flagella, cilia, and pseudopodia
• Occupy a wide range of ecological niches, from free-living to parasitic
• Have evolved multiple times independently from different ancestral lineages
• Demonstrate a remarkable ability to adapt to different environments and conditions

Protist Cell Structure

• Protists are eukaryotic cells with membrane-bound organelles
• Possess a nucleus containing genetic material (DNA) enclosed by a nuclear envelope
• Have a variety of organelles, such as mitochondria, endoplasmic reticulum, and Golgi apparatus
• Some protists have chloroplasts for photosynthesis (autotrophic), while others lack them (heterotrophic)
• Cell surface may be covered by a cell membrane, a pellicle, or a cell wall made of various materials (cellulose, silica, or chitin)
• Locomotion is achieved through flagella, cilia, or pseudopodia, depending on the species
  • Flagella are long, whip-like structures used for swimming
  • Cilia are shorter, hair-like structures that beat in coordination for movement or feeding
  • Pseudopodia are temporary, foot-like projections used for crawling or engulfing food
• Some protists have specialized organelles, such as contractile vacuoles for osmoregulation or extrusomes for defense or prey capture

Protist Metabolism and Nutrition

• Protists exhibit a wide range of metabolic and nutritional strategies
• Autotrophic protists (such as algae) carry out photosynthesis to produce organic compounds from sunlight and carbon dioxide
  • Possess chloroplasts containing chlorophyll and other photosynthetic pigments
  • Contribute significantly to global primary production and oxygen generation
• Heterotrophic protists obtain nutrients by consuming other organisms or organic matter
  • Some are predators that actively hunt and capture prey using specialized structures (such as pseudopodia or extrusomes)
  • Others are filter feeders that strain suspended particles from the surrounding water using cilia or flagella
  • Parasitic protists absorb nutrients from their hosts, often causing disease (such as malaria or toxoplasmosis)
• Mixotrophic protists combine autotrophic and heterotrophic nutrition, depending on environmental conditions
• Some protists have symbiotic relationships with other organisms, exchanging nutrients or providing shelter

Reproduction in Protists

• Protists reproduce through various means, including asexual and sexual reproduction
• Asexual reproduction is common and allows for rapid population growth
  • Binary fission: a parent cell divides into two genetically identical daughter cells
  • Multiple fission: a parent cell undergoes multiple divisions to produce many daughter cells
  • Budding: a small outgrowth (bud) develops on the parent cell and separates to form a new individual
• Sexual reproduction involves the fusion of gametes (haploid reproductive cells) to form a zygote
  • Promotes genetic diversity and allows for adaptation to changing environments
  • May involve the alternation of generations between haploid and diploid stages
• Some protists have complex life cycles that alternate between asexual and sexual reproduction
• Environmental factors (such as nutrient availability or population density) can trigger switches between reproductive modes

Ecological Roles of Protists

• Protists play crucial roles in various ecosystems as primary producers, consumers, and decomposers
• Photosynthetic protists (such as phytoplankton) form the base of aquatic food webs
  • Provide energy and nutrients for higher trophic levels
  • Contribute to the global carbon cycle by fixing atmospheric carbon dioxide
• Heterotrophic protists act as consumers at different trophic levels
  • Microzooplankton (such as ciliates and flagellates) graze on bacteria and other microorganisms
  • Larger protists (such as amoebae and dinoflagellates) consume algae, other protists, and small animals
• Parasitic protists can significantly impact the health and population dynamics of their hosts
  • May cause diseases in humans (malaria, giardiasis), animals (avian malaria, cryptosporidiosis), and plants (downy mildew, red tides)
• Protists contribute to nutrient cycling and decomposition by breaking down dead organic matter
• Some protists form symbiotic relationships with other organisms
  • Coral reefs depend on the symbiosis between coral animals and dinoflagellate algae (zooxanthellae)
  • Termite guts contain flagellated protists that help digest cellulose

Evolutionary Significance

• Protists provide insights into the early evolution of eukaryotic life
• Represent a diverse assemblage of lineages that diverged early in eukaryotic evolution
• Possess a wide range of adaptations and innovations that have contributed to their success
  • Development of chloroplasts through endosymbiosis of photosynthetic bacteria
  • Evolution of mitochondria from ancient bacterial symbionts
  • Emergence of sexual reproduction and complex life cycles
• Serve as models for understanding the transition from unicellular to multicellular life
  • Some protist lineages (such as green algae) gave rise to land plants
  • Others (such as choanoflagellates) are closely related to animals
• Highlight the importance of endosymbiosis in shaping the evolution of eukaryotes
• Provide a framework for understanding the diversification of eukaryotic life and the origins of major lineages

Key Protist Groups to Know

• Algae: photosynthetic protists that include green algae, red algae, and brown algae
  • Green algae (Chlorophyta) are ancestral to land plants and include unicellular, colonial, and multicellular forms
  • Red algae (Rhodophyta) are mostly multicellular and important in marine ecosystems
  • Brown algae (Phaeophyta) are multicellular and include kelps and other seaweeds
• Protozoa: heterotrophic protists that include amoebae, ciliates, and flagellates
  • Amoebae (Amoebozoa) move and feed using pseudopodia and include free-living and parasitic species (Entamoeba)
  • Ciliates (Ciliophora) move using cilia and are important grazers in aquatic environments (Paramecium, Tetrahymena)
  • Flagellates (various phyla) move using flagella and include both free-living and parasitic forms (Euglena, Giardia, Trypanosoma)
• Dinoflagellates (Dinoflagellata): mostly marine protists with two flagella and often armored with cellulose plates
  • Some are photosynthetic and form symbiotic relationships with coral animals
  • Others are heterotrophic and can form harmful algal blooms (red tides)
• Apicomplexans (Apicomplexa): parasitic protists with complex life cycles and specialized structures for host cell invasion
  • Include important human pathogens such as Plasmodium (malaria), Toxoplasma (toxoplasmosis), and Cryptosporidium (cryptosporidiosis)
• Oomycetes (Oomycota): fungus-like protists that include important plant pathogens
  • Cause diseases such as potato late blight (Phytophthora infestans) and sudden oak death (Phytophthora ramorum)
  • Have cell walls made of cellulose rather than chitin (as in true fungi)