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Question 1

endosymbiosis

Answer 1

There is abundant evidence that much of protist diversity
has its origins in endosymbiosis, the process in
which certain unicellular organisms engulf other cells, which
become endosymbionts and ultimately organelles in the host
cell.

Question 2

algae

Answer 2

• Much evidence also indicates that later in eukaryotic history, a lineage of heterotrophic eukaryotes acquired an additional endosymbiont—a photosynthetic cyanobacterium—that then evolved into plastids. this plastid-bearing lineage gave rise to two lineages of photosynthetic protists, or algae: red algae and green algae.
• This hypothesis is supported by the observation that the DNA of plastid genes in red algae and green algae closely resembles the DNA of cyanobacteria.
• In addition, plastids in red algae and green algae are surrounded by two membranes. Transport proteins in these membranes are homologous to proteins in the inner and outer membranes of cyanobacterial endosymbionts, providing further support for the hypothesis

Question 3

secondary endosymbiosis

Answer 3

On several occasions during eukaryotic evolution, red algae
and green algae underwent secondary endosymbiosis: They
were ingested in the food vacuoles of heterotrophic eukaryotes
and became endosymbionts themselves. For example, protists
known as chlorarachniophytes likely evolved when a heterotrophic
eukaryote engulfed a green alga. Evidence for this
process can be found within the engulfed cell, which contains a
tiny vestigial nucleus, called a nucleomorph. Genes from the nucleomorph
are still transcribed, and their DNA sequences indicate
the engulfed cell was a green alga. Also consistent with the
hypothesis that chlorarachniophytes evolved from a eukaryote
that engulfed another eukaryote, their plastids are surrounded
by four membranes. The two inner membranes originated as
the inner and outer membranes of the ancient cyanobacterium.
The third membrane is derived from the engulfed alga’s plasma
membrane, and the outermost membrane is derived from the
heterotrophic eukaryote’s food vacuole.

Question 4

oldest lineage of living

eukaryotes

Answer 4

many biologists thought that the oldest lineage of living
eukaryotes consisted of the amitochondriate protists, organisms
without conventional mitochondria and with fewer membranebounded
organelles than other protist groups. But recent structural
and DNA data have undermined this hypothesis. Many of
the so-called amitochondriate protists have been shown to have
mitochondria—though reduced ones—and some of these organisms
are now classified in entirely different groups.