Lecture #25—Protista

Protista are generally single-celled eukaryotic organisms. They are larger and much more complex than prokaryotes, having a nucleus with multiple chromosomes and numerous membranous organelles. They originated more than 1.5 billion years after the first prokaryotes and well after the beginning of the oxygen revolution. In this lecture we consider the origin of the eukaryotic cell and the different kinds of eukaryotes. The increase in oxygen in the atmosphere undoubtedly played a major role in the evolution of the eukaryotes.

Eukaryotes have two organelles that play key roles related to the Oxygen Revolution: Chloroplasts which produce O2 by photosynthesis and mitochondria which use the O2 to generated energy.

In the last lecture we considered the evolution of photosynthesis and left off just as the oxygen levels were raising. Recall that O2 is toxic and caused mass extinction of anaerobic prokaryotes. But some prokaryotes developed metabolic reactions to use the oxygen to accept 2 electrons from other reactions. The oxygen then picked up 2 hydrogen ions (H+) to form water.

Oxygen atom + 2 electrons + 2 H+ H20

They did this by using the pyruvic acid that was leased at the end of glycolysis and passing it into a series of chemical reactions called the Krebs Cycle and some electron carrier molecules the last ones passing their electrons to oxygen. Along with these reactions there was a huge amount of energy (ATP) produced. (In eukaryotes these events occur in mitochondria, which are aerobic mutualist bacteria with the Krebs cycle and electron transport chain.)

Where did the Eukaryotes come from?

Answer:They evolved from the prokaryotes by two mechanisms: 1) Infolding of the plasma membrane to form the nuclear membrane, Golgi, vesicles, endoplasmic reticulum, etc.

2) Endosymbiosis—joining together of prokaryotic cells to form mitochondria and chloroplasts inside a host cell.

Horizontal gene transfer was involved because eukaryotic cells have both eubacterial and archaeal genes: Archaeal genes run processes involving DNA & RNA (informational functions): replication, transcription, & translation. Bacterial genes run metabolic & housekeeping chores (operational functions).

Kingdom Protista is considered to polyphyletic. That is, taxonomists think the group is so diverse that they wish to split it into many phyla. For convenience we will categorize the phyla into 3 large groups and these are themselves polyphyletic:

1) Animal-like Protista = Protozoa

(Lack chloroplasts; classified by locomotion; heterotrophic)

2) Fungal-like Protista

(Lack chloroplasts; produce spores; heterotrophic)

3) Plant-like Protista = Unicellular algae

(Have chloroplasts; photosynthetic; classified by pigments)

Protozoa:

1) Flagellates (move with flagella)

2) Ciliates (move with cilia)

3) Amoebae (move with pseudopods); some exist in silicon shells (radiolarians), or calcium carbonate shells (Foraminiferans).

4) Sporozoa (lack movement, produce spores)

Slime Molds: Fungal characteristics (have amoebae as part of their life cycle)

Algae: Protista with chloroplasts. Classified by the different types of chlorophyll.

1)Algae with chlorophylls a & c: dinoflagellates, diatoms, multicellular brown algae (kelp)

2)Algae with chlorophylls a & b: euglenas, multicellular green algae, multicellular red algae

Terms/Concepts to Define:

Krebs cycle

Electron transport chain

Infolding Hypothesis

Endosymbiotic Hypothesis

Protista

Protozoa

Polyphyletic

Choanoflagellates

Eumetazoa

Parazoa

Slime mold

Diatoms

Dinoflagellates

Phaeophyta

Chlorophyta

Rhodophyta

Can you answer these questions?

  1. Ultraviolet light kills organisms and cells. The ozone layer of the atmosphere blocks this radiation. How does the oxygen revolution set the stage for the eventual invasion of life on land?
  2. What major selective advantages did the advent of the Krebs cycle and its electron transport chain bring to some prokaryotes?
  3. Describe where chloroplasts came from?
  4. Describe where mitochondria came from?
  5. Give three lines of evidence indicating that chloroplasts and mitochondria are mutualistic prokaryotes.
  6. Why do we believe that Choanoflagellates are animals’ protozoan ancestors?
  7. Which Protist is the most likely plant ancestor and why?