Protists Notes

Protistas

• Informal name for a group of eukaryotic organisms that are difficult to classify.
• Belong to the Domain Eucaria.
• Mostly unicellular and microscopic.
• Genetic evidence shows that some are more related to plants, animals, or fungi than to other protists.
• The protist group is like the drawer where we put everything we don't know where it goes.

Diversity in Protists

• The group with the greatest structural and functional diversity of eukaryotes.
• Unicellular and multicellular organisms.
• Some organisms form colonies.
• Sexual, asexual, or both types of reproduction.
• Can live in terrestrial or aquatic environments, inside other organisms, or in dead organic matter.
• Nutritional diversity:
  • Photoautotrophs: Have chloroplasts and perform photosynthesis.
  • Heterotrophs: Need to feed on organic matter.
  • Mixotrophs: Combine photosynthesis with heterotrophic nutrition.

General characteristics of Eukaryotes

• Nucleus: Contains the genetic material, separated from the cytoplasm by a membrane.
• Organelles: Each performs specific functions.
• Mitochondria: Organelle specialized in aerobic respiration.
• Cytoskeleton: Allows the cell to alter its shape to move, eat, and grow.
• Carry out Mitosis.
• Chromosomes organized by histones.

Evolution of eukaryotic organisms

• Endosymbiosis: Relationship between two species in which one organism lives inside the cells of the other organism (the host).
• Mitochondria and chloroplasts are derived from prokaryotic organisms that were swallowed by the ancestor of eukaryotic cells.
• Mitochondria: Endosymbiosis of an alpha proteobacteria (aerobic).
• Chloroplasts: Endosymbiosis of a cyanobacteria (photosynthetic).
• "Protoeukaryotic" host cell: Related to archaea.

Endosymbiosis

• Folds in the plasma membrane give way to the first organelles in a proto-eukaryotic cell.
• First endosymbiosis: the cell swallows an aerobic bacterium that will become the mitochondrion.
• Second endosymbiosis: the eukaryotic cell swallows a cyanobacterium that will become the chloroplast.

Secondary endosymbiosis

• Plastid
• Green alga
• Euglenids
• Inner plastic membrane
• Chlorarachniophytes
• Nucleomorph
• Outer plastid membrane
• Nuclear pore-like gap
• Dinoflagellates
• Red alga
• Plastid
• Stramenopiles

Evidence for the theory of endosymbiosis

• Mitochondria:
  • Have their own DNA, a circular chromosome.
  • Ribosomes and tRNA very similar to those of prokaryotes.
  • Their shape is the same as that of alpha-proteobacteria.
  • Double membrane.
  • Divide independently of the eukaryotic cell by binary fission.
  • Has lost genes, which explains why it cannot live independently.
• Chloroplasts:
  • Have their own DNA, a circular chromosome.
  • Ribosomes similar to those of cyanobacteria.
  • Double membrane and some a layer of peptidoglycan between both membranes.
  • Divide independently of the eukaryotic cell by binary fission.
  • Has lost genes, which explains why it cannot live independently.

Four supergroups of Eukaryotes

• Archaeplastida
• Unikonta
• SAR (Stramenopila, Alveolata, Rhizaria)
• Excavata (Formerly called Discova)

Archaeplastida (Green and red algae)

• Autotrophs.
• Unicellular, unicellular that live in colonies, and multicellular.
• Sexual and asexual reproduction.
• Cell walls with cellulose.
• Life cycle alternation of generations.
• Group that includes terrestrial plants.

Amoebozoa

• Heterotrophs.
• Many show lobed pseudopods that they use to move and eat (phagocytosis).
• Some are parasites.
• Unicellular and multicellular.
• Sexual and asexual reproduction.
• Includes Slime molds.
• Amoeba movement by pseudopods
  • This type of movement is possible thanks to the cytoskeleton that these organisms possess.

Opisthokonta

• Heterotrophs.
• Name comes from its single flagellum in the posterior part.
• The protists in this group are unicellular and colonial.
• Includes choanoflagellates.
• Filters.
• Sexual and asexual reproduction.
• Aquatic.

Rhizaria

• Heterotrophs and mixotrophs.
• Some have gained organelles by secondary endosymbiosis.
• Includes amoebas with fine pseudopods like needles or threads.
• Some create calcium carbonate or silica armor.
• All unicellular.
• Sexual and asexual reproduction.
• Aquatic and some parasitic.

Chromalveolates

• Includes Alveolates and Stramenopiles.
• Arise from secondary endosymbiosis with a red algae.
• Some have lost this plastid.
• Autotrophs, heterotrophs, and mixotrophs.
• Unicellular and multicellular.
• Sexual and asexual reproduction.
• Complex life cycles.
• Aquatic and some parasitic.

Alveolates

• They are characterized by having membrane-bound sacs (alveoli) just below the plasma membrane.
• This group includes:
  • Dinoflagellates
  • Apicomplexes
  • Ciliates

Alveolates: Dinoflagellates

• red tide
• Bioluminescent bays

Alveolates: Apicomplexes

• All are parasites of animals and some cause serious diseases.
• One end of the cell contains a complex of organelles specialized to penetrate host cells.
• They have complex life cycles.
• Require two or more hosts to complete it.

Life cycle of Plasmodium

• Apicomplex that causes malaria.
• It has two hosts: Mosquito of the genus Anopheles and Humans

Alveolates: Ciliates

Stramenopiles

• Includes some of the most important photosynthetic organisms on the planet.
  • Diatoms
  • Golden algae
  • Brown algae
• Important producers of $O_2$ on the planet.
• Sequester carbon at the bottom of the sea for a long time.
• Unicellular and multicellular.

Stramenopiles: Brown algae

• Pneumatocysts: air-filled vesicles that allow these algae to float in the water

Excavata

• General characteristics
  • Feeding grooves (excavated).
  • Multiple flagella.
  • Autotrophs, heterotrophs, mixotrophs and some parasitic.

Excavata: Euglenozoas

• Protists commonly found in pond waters.
• They have two flagella that emerge from one side of the cell.
• They are mixotrophs.
• They have no cell wall.