Chapter 29 – Protists
-paraphyletic group- they represent some, but not all, of the descendents of a single common ancestor.
- Protists are all eukaryotes that are not green plants, animals or fungi.
29.1 Why Do Biologists Study Protists?
Impacts on Human Health and Welfare
Malaria: Caused byApicomplexa. World’s most chronic health problem. Four species of the protist
Plasmodium are capable of parasitizing humans and causing malaria.
Plasmodium spends part of its life cycle inside mosquitoes; most antimalaria campaigns have focused on
controlling these insects.
HarmfulAlgal Blooms: Due to photosynthetic protists called dinoflagellates. Harmful at high population
density because synthesize toxins to protect themselves from predation. Blooms known as red tides
Ecological Importance of Protists
-10 percent of euks
Protists Play a Key Role inAquatic Food Chains
Plankton: small organisms that live near the surface of oceans or lakes and that drift along or swim only
-Sugars and organic compounds produced by phytoplankton are the basis of food chains in freshwater and
Could Protists Help Reduce Global Warming?
Global carbon cycle: Movement of carbon atoms from CO2 molecules in the atmosphere to organisms in
the soil or ocean, and then back to atmosphere.
-Phytoplankton die and sink to bottom of ocean:
1. Limestone and other carbon-containing rocks that form on the ocean floor can lock up carbon atoms.
2. Dead bacteria, archaea, protists accumulate at bottom of the ocean. C2 then passes from the atmosphere
to the body of a photosynthetic protest and then to petroleum is removed from the atmosphere.
29.2 How Do Biologists Study Protists?
Microscopy: Studying Cell Structure
- Detailed studies of cell structure revealed that protists could be grouped according to
characteristic overall form, according to organelles with distinctive features or both.
- Synapomorphies: shared, derived traits that distinguish major monophyletic groups. Evaluating Molecular Phylogenies:
Discovering New Lineages via Direct Sequencing
29.3 What Themes Occur in the Diversification of Protists?
What Morphological Innovations Evolved in Protists?
The Nuclear Envelope: Derived from infoldings of plasma membrane
Evolution of envelope advantageous because it separated transcription and translation. Early
morphological innovation gave early euks a new way to manage and process genetic info.
Endosymbiosis theory: Mitochondria originated when a bacterial cell took up residence inside a
-Euk cells started to use their cytoskeletal elements to surround and engulf smaller prey. Instead of being
digested, an engulfed bacterium began to live inside its euk host. Engulfed cell survived by absorbing
carbon with high potential energy from host and oxidizing it using oxygen as final electron acceptor.
Host supplied bacterium with protection from its other prey and carbon compounds, while bacterium
produced much more ATP than host cell could.
Following points support endosymbiosis theory:
1. Mitochondria are size of avg bacterium and replicate by fission like them. Duplication takes place
independently of division by host cell.
2. Mitochondria have their own ribosomes and manufacture their own proteins. Ribosomes closely
resemble bacterial ones in size and composition and are poisoned by antibiotics.
3. Double membranes, consistent with the engulfing mechanism of origin
4. Have their own genomes which are organized as circular molecules – much like a bacterial
5. Mitochondrial gene sequences turned out to be much more closely related to the sequences from
the a-proteobacteria than nuclear DNAof euks.
Structures for Support and Protection
- Diatoms surrounded by silicon-oxide shell
- Dinoflagellates have a cell wall make of cellulose plates
- Some lineages of Foraminifera secrete an intricate chambered test (hard external structure) of
- The parabasalids have a distinctive internal support rod, consisting of cross-linked microtubules - The euglenids have a collection of protein strips located under plasma membrane. Strips
supported by microtubules and stiffen the cell
- The alveolates have distinctive sac-like structures alveoli, under membrane that stiffen it.
- Is a synapomorphy shared by all of brown algae and all of plasmodial and cellular slime moulds.
Also arose in some lineages of red algae.
How Do Protists Find Food?
- The engulfing process is possible in protists that lack a cell wall
- Flexible membrane and dynamic cytoskeleton gives these species the ability to swallow prey
- When nutrients are taken up directly from environment across plasma membrane