BIOEE 1780 Lecture Notes - Lecture 6: Cyanobacteria, Red Algae, Nuclear Membrane
28 Apr 2018
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Lecture 6 – Endosymbiosis and the Rise of Eukaryotes
Eukaryotes are monophyletic.
• A single eukaryotic ancestor diversified into the protists, plants, fungi, and animals.
Eukaryotes are more closely related to archaea than bacteria.
The mitochondria and chloroplasts of eukaryotes are derived from bacteria.
Eukaryote Characteristics
1. Membrane-enclosed nucleus/organelles
2. Relatively large
3. Relatively complex
Events that led to the modern eukaryotic cell:
1. The origin of a flexible cell surface
a. Ancient prokaryotes had firm cell walls.
b. Ancestral prokaryotes lost this cell wall.
i. Allowed for larger cell size, which is limited by surface area to volume ratio
1. Infolding created more surface area for gas and nutrient exchange.
ii. Cell can exchange materials with its environment rapidly enough to sustain a larger volume
and more rapid metabolism.
iii. Allows for endocytosis (membrane pinches off around pieces of the environment, bringing
them into the cell)
2. The origin of a cytoskeleton
a. Allowed for the formation of flagellum
b. Simple cytoskeletons evolved in prokaryotes before the origin of eukaryotes
3. The origin of a nuclear envelope, which enclosed a genome organized into chromosomes
a. Ribosome-studded internal membranes were also created
4. The appearance of digestive vacuoles
5. The acquisition of certain organelles via endosymbiosis
a. Phagocytosis: one cell engulfs another
b. Mutualistic relationship developed between bacterium and host eukaryote:
i. Host cell provided a safe environment and nutrients for the bacterium.
ii. The bacterium performed oxidative metabolism that was necessary for the host's survival.
c. Mitochondria evolved from a proteobacterium.
i. When the mitochondria began forming ATP during cellular respiration, the modern
eukaryotic cell was complete.
d. Chloroplasts arose from cyanobacteria, which generate O2 as a product of photosynthesis.
i. All chloroplasts trace their ancestry to the engulfment of one cyanobacterium by a larger
eukaryotic cell.
1. This gave rise to the photosynthetic eukaryotes.
2. Known as primary endosymbiosis
a. Gave rise to red algae, green algae, and land plants
3. The cyanobacterium had an inner and outer membrane, giving the original chloroplasts
two membranes.
4. Some peptidoglycan remnants
ii. Represents a synapomorphy of plants
iii. Secondary Endosymbiosis: uptake of a unicellular green alga; an organism containing a
primary endosymbiont gets engulfed
1. The chloroplast was retained, but the rest of the alga contents were lost
2. These chloroplasts have 3 membranes.
iv. Tertiary Endosymbiosis: a dinoflagellate lost its chloroplast and took up another protist that
had acquired its chloroplast through secondary endosymbiosis
Evidence of Endosymbiosis of Mitochondria
1. Double membrane
2. Have mtDNA (mitochondria DNA)
3. Mitochondria and proteobacteria have a single circular chromosome
Evidence of Endosymbiosis of Chloroplasts
1. Double membrane
2. Have cpDNA (chloroplast DNA)
3. cpDNA and cyanobacteria DNA are circular
Primary Endosymbiosis Organisms
1. Red Algae
2. Glaucophyte
3. Green Algae
Primary endosymbiosis organisms were engulfed, creating secondary endosymbiosis organisms:
1. Engulf red algae:
a. Stramenopiles
b. Ciliates
c. Alveolates
d. Dinoflagellates
2. Engulf green algae
a. Euglenids
Some dinoflagellates have tertiary endosymbiosis.
8 Major Clades of Eukaryotes
1. Alveolates
a. Possess sacs, called alveoli, beneath their cell membranes
i. Alveoli support the cell surface
b. Unicellular
c. Most are photosynthetic
i. Acquired chloroplasts through secondary endosymbiosis of red algae
d. Examples:
i. Dinoflagellates
1. Some are freshwater, but mostly marine plankton
a. Primary producers of organic matter in oceans
2. Most are photosynthetic
3. Chloroplast was acquired through tertiary endosymbiosis
4. Have 2 flagella, one in an equatorial groove (middle groove) and one in a longitudinal
groove
