1. Chapter 28
Protists
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2. Kingdom Protista
Until recently, unicellular eukaryotes were grouped into the Kingdom Protista.
Scientific advances in molecular genetics/biology and systematics has caused the entire kingdom to be reshaped.
The Kingdom was once thought to be monophyletic.
It is now known to be paraphyletic.
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3. Protists
Protists are the most functionally diverse group of organisms known
They share a few common features:
1. Most are unicellular.
2. They are the simplest eukaryotes.
3. They are the most functionally diverse.
Phototrophs--contain chloroplasts
Heterotrophs--absorb organic molecules, ingest food particles.
Mixotrophs--combine photosynthesis and heterotrophic nutrition.
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4. Protists
Protists can be divided into 3 groups based on their nutritional modes. These groups, however, do not refer to their monophyletic grouping.
1. Photosynthetic protists or algae.
2. Ingestive protists or protozoans.
3. Absorptive protists.
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5. Protist Diversity
Much of the protist diversity comes from endosymbiosis.
Biologists have hypothesized and a lot of evidence supports the fact that certain unicellular organisms engulfed other cells which ultimately became organelles within the cell.
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6. Protist Diversity
An example: One postulate is that a heterotrophic eukaryote acquired a photosynthetic cyanobacterium which evolved into a plastid.
Support for this comes from the fact that the DNA of the plastid genes in red algae and green algae closely resembles that of the DNA found in cyanobacteria.
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7. Protist Diversity-Red and Green Algae
Several times throughout evolutionary history, red and green algae underwent secondary endosymbiosis.
Occurred when red and green algae were ingested by another heterotrophic eukaryote and became endosymbiotic themselves.
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8. Secondary Symbiosis-Chlorarachinophytes
For example: Chlorarachinophytes evolved from a eukaryote that engulfed another eukaryote. Their plastids are surrounded by 4 membranes.
The two inner membranes originated as the inner and outer membranes of ancient cyanobacterium
The third membrane is derived from the engulfed algae’s plasma membrane.
The outermost membrane is derived from the heterotrophic eukaryote’s food vacuole.
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