biol10005

Summary of Protists (because I hate them)

Eukaryotes not belonging to plant, animal or fungal kingdoms and have a variety of ways of gaining nutrition

  • Can be uni/multicellular or colonial and have many cellular forms including flagellates and plasmodia
  • Majority are aquatic and have flagella or cilia
  • Gain nutrition by photosynthesis, parasitism, predation and absorption

First eukaryotic organisms were probably similar to modern day protists

  • The nuclear membrane and endomembrane system probably evolved from infolding of the bacterial cell membrance that enveloped the nucleoid. Plastids and mitochondria are derived from endosymbiotic bacteria that have become organelles in eukaryotic cells.

Major eukaryotic lineages, such as animals and fungi, arose from different protist lineages, but many more independent protist lineages exist

  • choanoflagellates - animals
  • slime moulds - fungi

Photosynthetic protists, commonly called algae, are diverse and not all related.  Primary plastids arose in the ‘green lineage’ which includes glaucophytes, red algae and green algae

  • Glaucophytes - photosynthetic flagellates with primitive chloroplast that have a peptidoglycan wall like bacteria
  • Red Algae - multicullular and lack flagella, contain chlorophyll a ad phycobilin pigments
  • green algae - unicellular, colonial and multicellular forms, contain chlorophyll a and b

Secondary plastid acquisitions created an enormously diverse new array of protists, some which abandoned photosynthesis to become heterotrophic or parasitic

  • Chromists (cryptomonads, coccolithophorids, diatoms, brown algae, oomycetes) 
  • characterized by flagellar architecture and secondary plastids, typically have one smooth posterior flagellum and one hairy anterior flagellum
  • most photosynthetic with chlorophyll a and c
  • oomycetes absorb food through hypae
  • cryptomonads clearly ingested eukaryotic cells, nucleomorph is relict nucleus of engulfed cell

Alveolates are unicells with distinctive vesicles, cortical alveoli, beneath the cell membrane

  • Dinoflagellates - 2 flagella, many photosynthetic containing chlorophyll a and c, some predatory
  • Apicomplexans - intracellular parasites of animals, cause diseases such as malaria, apical complex used to penetrate host cells
  • Ciliates - predatory unicells characterized by 2 types of nuclei and a covering of cilia

Euglenoids are closely related to flagellates that all have an anterior depression from which the flagella emerge

  • euglenoids are freeliving, some have cloroplasts and some engulf prey through anterior depression (gullet)
Evolution of the Eukaryotic Cell

What do we know about the origin of the Eukaryotic cell?

  • 1.3 - 2 billion years before the present
  • mechanism of evolution is not known
  • There is good evidence for the origin of mitochondria and chloroplasts from primary endosymbiosis

The first Eukaryotic cells to evolve were protists and they are an extremely diverse group of organisms.

When do Eukaryotic cells first appear in the fossil record?

  • Precambrian acritarch fossils are the first known of eukaryotic cells – about 1.3 – 2 billion years old.
  • Multicellular (filamentous), eukaryotic organisms appear about 1.4 billion years ago
  • Early seaweed fossils – from 635 million years ago

Where did Eukaryotic cells come from?

It is generally thought that eukaryotes eveolved from prokaryotic organisms.

The nuclear membrane and endomembrane system of eukaryotes probably evolved from a prokaryote where invaginations of the bacterial cell membrane enveloped the nucleoid.

Mitochondria and plastids (or choloroplasts, but choloroplast usually refers to ‘green plastids’ reseverved for plants or green algae) of eukaryotes arose by endosymbiosis, which refers to an organism living inside another.

Plastids derive from cyanobacteria and mitochondria are descended from purple bacteria. Plastids and mitochondria are derived from endosymbiotic bacteria that have become organelles in eukaryotic cells.

Mitochondria –> animals & fungi

Chloroplasts (Plastids) –> green algae and plants 

What is the Key Evidence for the Endosymbiotic Origin of Mitochondria and Chloroplasts?

  1. Mitochondria and chloroplasts are semiautonomous, retaining their own genome (DNA, RNA). Their genomes resemble those of prokaryotes (i.e., purple bacteria, cyanobacteria)
  2. They also retain their own machinery for synthesizing proteins, including ribosomes.

  3. Their metabolism is like existing prokaryotic organisms (e.g., a cyanobacteria for chloroplast).

  4. ftsZ protein involved in division of prokaryotes and organelles

  5. Some chloroplasts still have the bacterial peptidoglycan wall between the inner and outer membranes.

Protists with primary chloroplasts have 3 genomes, while all protists with secondary chloroplasts have either 3 or 4 genomes.

Cyanobacteria

Green and purple bacteria lack PSII, do not use H20 as a source of electrons and, therefore do not produce oxygen. With the evolution of cyanobacteria came PSII, which provided the mechanism for using H20 as a source of electrons and provided the earth with an atmosphere that includes oxygen.

Cyanobacteria contain chlorophyll a and water soluble pigments called phycobilins. Blue phycobilins, present in most species give them a blue-green appearance.

Cyanobacteria have specialised cells such as akinetes and heterocysts. An akinete is a spore that develops from a cell which becomes enlarged and filled with food reserves. The spore can remain dormant and then germinate to produce a new filament. A heterocyst is relatively colourless, has a thick, transparent cell wall, may be involved in asexual reproduction and is a sige of nitrogen fixation.

What characteristics do Cyanobacteria share with chloroplasts?

  1. Their photosynthetic thylakoids contain chlorophyll A, as do all chloroplasts.
  2. Their accessory pigments, phycocyanin and phycoerythrin, are found in the chloroplasts of several protists.
  3. Chloroplast genomes show they are related to Cyanobacteria