3, Protists and Eukaryotic Evolution

General Characteristics
- Protists are a diverse group of eukaryotic organisms not classified as fungi, plants, or animals.
- They exhibit vast differences in cellular structure and habitat.
- Can be unicellular, colonial, or multicellular.
- Sizes range from microscopic to quite large.
- Varied forms and symmetries with all types of nutritional strategies.

Eukaryotic Cell Features
- More complex than prokaryotes (bacteria and archaea).
- Presence of a complex cytoskeleton and compartmentalization (nucleus and organelles).
- Evidence of eukaryotic life dates back approximately 1.5 billion years.
Nucleus and ER Formation
- Nucleus and endoplasmic reticulum arose from the infoldings of a prokaryotic cell membrane.
- Some archaea display similar membrane infoldings.
Endosymbiosis Theory
- Organelles such as mitochondria and chloroplasts evolved through endosymbiosis involving ancestral eukaryotes and bacterial cells.
- Mitochondria originated from aerobic bacteria engulfed by a prokaryotic host (possibly archaea).
- Chloroplasts originated from engulfed photosynthetic bacteria, specifically cyanobacteria.
- Brown algae exhibit a secondary endosymbiosis where they engulf red algae containing chloroplasts.

Mitochondria and chloroplasts contain their own circular DNA, similar in size and structure to bacterial DNA.
Ribosomes in these organelles are similar to bacterial ribosomes and susceptible to antibiotics.
Replication of mitochondria and chloroplasts occurs by binary fission, unlike eukaryotic mitosis.

Mitosis evolved in eukaryotes as a mechanism to ensure the proper segregation of multiple chromosomes during cell division.
Prokaryotes typically possess a single DNA molecule, whereas eukaryotes have multiple linear chromosomes.

Classification of Protists
- Protists do not form a single monophyletic group; they are paraphyletic.
- Present in all five recognized eukaryotic supergroups:
- Excavata
- SAR (Stramenopiles, Alveolates, Rhizaria)
- Archaeplastida (includes land plants)
- Amoebozoa
- Opisthokonta (includes fungi and animals).
Cell Surface Characteristics
- Varied cell surface structures including plasma membranes and extracellular matrices (ECM).
- Certain groups like diatoms have silica shells; cysts serve as dormant phases with protective coverings.
Locomotion Methods
- Flagella: one or more whip-like structures.
- Cilia: shorter, numerous hair-like external structures for movement.
- Pseudopodia: extendable parts of the cell used primarily by amoebas.

Types of Nutrition
- Autotrophs: photosynthetic or chemoautotrophic.
- Heterotrophs: including phagotrophs that ingest particulate food.
- Mixotrophs: both autotrophic and heterotrophic capabilities.

Asexual Reproduction
- Commonly employs various methods, including:
- Mitosis (producing equal-sized daughter cells).
- Budding (producing one smaller daughter cell).
- Schizogony (multiple nuclear divisions followed by cellular division).
Sexual Reproduction
- Some protists reproduce sexually, especially under stress.
- Involves meiosis to produce haploid gametes allowing genetic recombination.

Protists demonstrate evolutionary trends from single-celled forms to complex multicellular structures.
Key innovations fostering cellular specialization and adaptation.

Consists of diplomonads, parabasalids, and euglenozoans characterized by unique cytoskeletal features and specific DNA profiles.

Diplomonads: unicellular, multiple flagella, known for Giardiasis (Giardia duodenalis) affecting intestinal health and lack functional mitochondria.

Comprises three major branches:
- Stramenopiles: includes brown algae and diatoms.
- Alveolates: includes dinoflagellates and ciliates.
- Rhizaria: includes foraminifera and cercozoa.

Composed of Rhodophyta (red algae), Chlorophyta (green algae), Charophytes, and land plants, all of which obtained chloroplasts through primary endosymbiosis.

Split into Chlorophyta and Charophytes, with Charophytes being the ancestors of land plants. Certain green algal species, such as Chlamydomonas reinhardtii, represent significant models in studies of eukaryotic photosynthesis and multicellularity.

Found in some chlorophytes showcasing both gametophyte and sporophyte generations, as seen in species like Ulva lactuca that go through distinct stages in their reproductive life cycles.