Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 28.1 Most Eukaryotes Are Single-Celled Organisms
Protists are eukaryotes that are not classified as plants, animals, or fungi. Though often unicellular, they exhibit remarkable structural and functional complexity. These diverse organisms play key roles in understanding the origins of eukaryotic life and the process of endosymbiosis.
Unicellular but Complex
- Most protists are unicellular, though some form colonies or multicellular structures.
- Despite being single-celled, many protists have complex internal organization, with specialized organelles like:
- Nucleus, ER, Golgi, lysosomes (common in eukaryotes)
- Unique organelles (e.g., contractile vacuoles, ocelloids—eye-like structures in dinoflagellates)
- Protists perform all essential life functions using subcellular organelles rather than organs.
Nutritional Diversity
- Protists show a wide range of nutritional strategies:
- Photoautotrophs: Use light and have chloroplasts.
- Heterotrophs: Absorb or ingest organic material.
- Mixotrophs: Combine photosynthesis and heterotrophy.
- These nutritional modes have evolved independently in different lineages.
Reproduction and Life Cycles
- Protists exhibit diverse reproductive strategies:
- Asexual reproduction is common.
- Many undergo sexual reproduction or use meiosis and fertilization.
- All three basic sexual life cycle types are found among protists, often with unique variations.
Endosymbiosis and Eukaryotic Evolution
- Much of protist diversity stems from endosymbiosis, where one cell lives inside another.
- Mitochondria originated from engulfed alpha proteobacteria.
- Plastids (e.g., chloroplasts) evolved from cyanobacteria via primary endosymbiosis.
- This primary endosymbiosis gave rise to red and green algae, which were later engulfed in secondary endosymbiosis, producing groups like:
- Chlorarachniophytes
- Euglenids
- Stramenopiles
- Alveolates
- Some retained vestiges of engulfed eukaryotic nuclei, like nucleomorphs in chlorarachniophytes.
The Abandoned Protist Kingdom and Supergroup Classification
- Traditional “kingdom Protista” is obsolete; protists do not form a monophyletic group.
- Many protists are more closely related to animals, fungi, or plants than to each other.
- Scientists now classify eukaryotes into four supergroups (e.g., Excavata, SAR, Archaeplastida, Unikonta), though relationships among them continue to evolve with new data.
- Some groups (e.g., haptophytes, cryptophytes, hemimastigotes) still have unresolved phylogenetic positions.
In a Nutshell
Protists are highly diverse eukaryotes with complex structures, varied nutrition, and dynamic reproductive strategies. Much of their diversity traces back to endosymbiosis, a key event in the origin of mitochondria and plastids. Although once grouped as “Protista,” modern classification recognizes multiple distinct lineages based on molecular and structural evidence.