BSC 116 Chapter Notes - Chapter 28: Brown Algae, Symbiogenesis, Heterokont

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Section 28.1: Most eukaryotes are single-celled organisms
Protists- group of very small, mostly unicellular eukaryotes
o Exist in the domain Eukarya
Eukaryotic cells have a nucleus and other membrane enclosed organelles
o Mitochondria and Golgi apparatus
Provide specific locations where particular cellular functions are accomplished,
making the structure and organization of eukaryotic cells more complex than those of
prokaryotic cells
o Have a well-developed cytoskeleton that extends throughout the cell
Provides the structural support that enables eukaryotic cells to have asymmetric
forms, as well as to change in shape as they feed, move, or grow
Protists exhibit more structural and functional diversity than the eukaryotes with which we are
most familiar-- plants, animals, and fungi
Many protists are unicellular, although there are some colonial and multicellular species
o Single-celled protists are justifiably considered the simplest eukaryotes
o At the cellular level, many protists are very complex
Unicellular protists carry out the same biological functions as multicellular organisms, but do so
using subcellular organelles
o Certain protists also rely on organelles not found in most other eukaryotic cells
Contractile vacuoles- pump excess water from the protistan cell
Protists are also very diverse in their nutrition
o Some are photoautotrophs
Contain chloroplasts
o Some are heterotrophs
Absorb organic molecules or ingest larger food particles
o Some are mixotrophs
Combine photosynthesis and heterotrophic nutrition
o Phototrophy, heterotrophy, and mixotrophy have all arisen independently in many different
protist lineages
Reproduction and life cycles also are highly varied among protists
o Some are known to only reproduce asexually
o Others can reproduce sexually or at least employ the sexual process of meiosis and
fertilization
The ongoing changes in our understanding of the phylogeny of protists pose challenges to
students and instructors alike
o Hypotheses about these relationships are a focus of scientific activity, changing rapidly as
new data cause previous ideas to be modified or discarded
Endosymbiosis- a relationship between two species in which one organism lives inside the cell or
cells of another organism
Structural, biochemical, and DNA sequence data indicate that mitochondria and plastids are
derived from prokaryotes that were engulfed by the ancestor of early eukaryotic cells
o The evidence also suggests that mitochondria evolved before plastids
Thus, a defining moment in the origin of eukaryotes occurred when a host cell
engulfed a bacterium that would later become an organelle found in all eukaryotes--
the mitochondrion
To determine which prokaryotic lineage gave rise to mitochondria, researchers have compared
the DNA sequences of mitochondrial genes (mtDNA) to those found in major clades of bacteria
and archaea
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o Collectively, such studies indicate that mitochondria arose from an alpha proteobacterium
o Also indicate that the mitochondria of protists, animals, fungi, and plants descended from a
single common ancestor, thus suggesting that mitochondria arose only once over the course
of evolution
Questions remain about the identity of the host cell that engulfed an alpha proteobacterium
o According to recent genomic studies, the host came from an archaeal lineage, but which
lineage remains undetermined
While the host may have been an archaean, it is also possible that the host was a
member of the lineage that was related to, but had diverged from its archaeal
ancestors
Algae- photosynthetic protists/cyanobacterium
o Red algae
o Green algae
o Cyanobacteria are gram-negative
Have two cell membranes, an inner plasma membrane and an outer membrane that is
part of the cell wall
Plastids in red algae and green algae are also surrounded by two membranes
Transport proteins in these membranes are homologous to proteins in the inner
and outer membranes of cyanobacteria, providing further support for the
hypothesis that plastids originated from a cyanobacterial endosymbiont
Excavata:
o Some members of this supergroup have an "excavated" groove on one side of the cell body
o Two major clades- parabasalids and diplomonads- have mitochondria
Others have flagella that differ in structure from those of other organisms
o Include parasites, such as Giardia, as well as many predatory and photosynthetic species
"SAR" Clade:
o Contains three large and very diverse clades
Stramenopila
Include some of the most important photosynthetic organisms on Earth
diatoms
Alveolata
Include many photosynthetic species as well as important pathogens
Plasmodium- causes malaria
Rhizaria
Includes many species of amoebas, most of which have pseudopodia that are
threadlike in shape
Pseudopodia- extensions that can bulge from any portion of the cell
Used in movement and in capture of prey
o Many of the key groups of photosynthetic stramenopiles and alveolates are thought to have
arisen by secondary endosymbiosis
Archaeplastida:
o Includes red algae and green algae, along with land plants
Red algae and green algae include unicellular species, colonial species, and
multicellular species
Many of the large algae known informally as "seaweeds" are multicellular red or
green algae
o Protists in Archaeplastida include key photosynthetic species that form the base of the food
web in some aquatic communities
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Unikonta:
o Includes amoebas that have lobe- or tube- shaped pseudopodia, as well as animals, fungi,
and non-amoeba protists that are closely related to animals or fungi
o According to one current hypothesis, the unikonts may have been the first group of
eukaryotes to diverge from other eukaryotes
However, this hypothesis has yet to be widely accepted
Section 28.2: Excavates include protists with modified mitochondria and protists with unique flagella
Excavata- a clade that was originally proposed on morphological studies of the cytoskeleton
o Some members of this diverse group also have an "excavated" feeding groove on one side
of the cell body
The excavates include the diplomonads, parabasalids, and euglenozoans
Diplomonads- have reduced mitochondria called mitosomes
o These organelles lack functional electron transport chains and hence cannot use oxygen to
help extract energy from carbohydrates and other organic molecules
Instead, diplomonads get the energy they need from anaerobic biochemical pathways
o Many diplomonads are parasites
ex. Giardia intestinalis- inhabits the intestines of mammals
o Have two equal-sized nuclei and multiple flagella
Extensions of the cytoplasm, consisting of bundles of microtubules covered by the
cell's plasma membrane
Parabasalids- also have reduced mitochondria called hydrogenosomes
o These organelles generate some energy anaerobically, releasing hydrogen gas as a by-
product
Euglenozoans- belong to a diverse clade that includes predatory heterotrophs, photosynthetic
autotrophs, mixotrophs, and parasites
o The main morphological feature that distinguishes protists in this clade is the presence of a
rod with either a spiral or a crystalline structure inside each of their flagella
o The two best studied groups of euglenozoans are the kinetoplastics and the euglenids
Kinetoplastids- have a single, large mitochondrion that contains an organized mass of DNA called a
kinetoplast
o These protists include species that feed on prokaryotes in freshwater, marine, and moist
terrestrial ecosystems, as well as species that parasitize animals, plants, and other protists
ex. kinetoplastids in the genus Trypanosoma infect humans and cause sleeping
sickness, a neurological disease
Infection occurs via the bite of a vector organism (the African tsetse fly)
Also can cause Chagas' disease, which is transmitted by bloodsucking insects
and can lead to congestive heart failure
Euglenid- has a pocket at one end of the cell from which one or two flagella emerge
o Some are mixotrophs
o Many engulf prey by phagocytosis
Section 28.3: The "SAR" clade is a highly diverse group of protists defined by DNA similarities
"SAR" clade- proposed recently based on whole-genome DNA sequence analyses
o Studies have found that three major clades of protists-- the stramenopiles, alveolates, and
rhizarians-- form a monophyletic supergroup
This supergroup contains a large, extremely diverse collection of protists
Because red algae are thought to have originated by primary endosymbiosis, such an origin for the
stramenopiles and alveolates is referred to as secondary endosymbiosis
Stramenopiles- one major supergroup of the SAR clade
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