BISC 120 Midterm 3 | 16 Protists and Endosymbiosis

How do organelles and the cytoskeleton contribute to eukaryotic complexity?

Membrane-bound organelles separate cellular functions. The cytoskeleton provides shape and anchors organelles.

What does the mix of archaeal-like and bacterial-like genes suggest about eukaryotic origins?

Information-processing genes often resemble archaeal genes, while many metabolic genes resemble bacterial genes. This supports a merger involving an archaeal-related host and bacterial ancestry. Horizontal gene transfer also contributed some bacterial genes.

What two lineages were involved in the origin of mitochondria?

An archaeal-related host acquired an alphaproteobacterium. The precise route of internalization remains uncertain.

Where would you draw mitochondrial and plastid acquisition on the eukaryote tree?

Place alphaproteobacterial acquisition on the early eukaryote lineage before its main branches split. Place cyanobacterial acquisition later on the branch leading to red algae, green algae, and plants.

What happened during primary plastid endosymbiosis?

A heterotrophic eukaryote acquired a photosynthetic cyanobacterium, producing the ancestral plastid inherited by red and green algae.

What distinguishes primary from secondary plastid endosymbiosis?

In primary endosymbiosis, a host acquires a bacterium. In secondary plastid endosymbiosis, a eukaryote acquires another eukaryote that already has a plastid.

What features support a bacterial origin for mitochondria and plastids?

Circular DNA, bacteria-like ribosomes, and similarities in inner membrane structure and function.

What clues suggest that a plastid came through a secondary endosymbiotic event?

Extra plastid membranes and evidence of an algal intermediate support secondary endosymbiosis. Cyanobacterial ancestry is expected in both models.

Why is photosynthesis patchily distributed on the eukaryote tree?

Two explanations are early acquisition followed by losses and repeated acquisition through endosymbiosis.

Why is the informal grouping called protists paraphyletic?

It includes many eukaryotic descendants of a shared ancestor while excluding plants, animals, and fungi.

What four eukaryotic supergroups does the course tree use?

Excavata, SAR, Archaeplastida, and Unikonta.

What distinguishes Excavata, and which three groups are shown?

A characteristic cytoskeleton and, in some members, an excavated feeding groove. The three groups are diplomonads, parabasalids, and euglenozoans.

What identifies a euglenozoan? How diverse is its nutrition?

A spiral or crystalline rod inside each flagellum. Members include heterotrophs, autotrophs, parasites, and mixotrophs. A mixotroph combines photosynthetic and heterotrophic nutrition.

What does SAR stand for?

Stramenopiles, alveolates, and rhizarians.

A protist has a hairy flagellum paired with a smooth one. Where does it fit?

Stramenopiles. Diatoms and brown algae are two examples of stramenopiles.

A unicellular photosynthetic plankter has a two-part silica wall. What is it?

A diatom. The glass-like silica wall is called a frustule.

What are kelps, and what gives them their color?

Kelps are large, mostly marine brown algae in Stramenopiles. Carotenoids in the plastids give the brown color.

Trace alternation of generations in the brown-algal example.

The multicellular 2n sporophyte makes n spores by meiosis. Spores grow into multicellular n gametophytes, which make gametes. Fertilization yields a 2n zygote that grows into the sporophyte.

Why is kelp foundational, and what happens after heavy urchin grazing?

Kelp provides food and nursery habitat, captures energy as a primary producer, and exports detritus. Heavy grazing reduces kelp, habitat complexity, and associated biodiversity.

How do you recognize an alveolate or a dinoflagellate?

Alveolates have membrane sacs under the plasma membrane. Dinoflagellates are alveolates with two flagella in grooves of cellulose plates.

What causes a harmful red tide in the slide example?

A dinoflagellate bloom. Carotenoids can color the water, and some blooms produce toxins that harm marine animals.

What groups belong to Archaeplastida, and what is their shared plastid history?

Red algae, green algae, and land plants share plastid ancestry from primary cyanobacterial endosymbiosis.

How does phycoerythrin help red algae live at depth?

It masks chlorophyll and absorbs blue-green light that reaches deeper water.

What are the two green-algal groups shown, and why is green algae excluding plants paraphyletic?

Chlorophytes and charophytes. Land plants descend from the green lineage and are closest to charophytes, so excluding plants leaves out descendants.

What groups make up Unikonta in the course diagram?

Amoebozoans and opisthokonts. The root of the eukaryotic tree is uncertain, so Unikonta-first is a hypothesis.

What is the opisthokont flagellar clue, and which protists are closest to animals and fungi?

A single posterior flagellum in flagellated stages. Choanoflagellates are closest to animals and nucleariids are closest to fungi in the course tree.

What identifies a choanoflagellate, and why are its growth forms relevant to animal origins?

A flagellum surrounded by a collar of microvilli. Both unicellular and colonial forms provide a comparison for the evolution of animal multicellularity.