23.1: Eukaryotic Origins
Living things fall into three large groups: Archaea, Bacteria, and Eukarya. The first two have prokaryotic cells, and the third contains all eukaryotes. A relatively sparse fossil record is available to help discern what the first members of each of these lineages looked like, so it is possible that all the events that led to the last common ancestor of extant eukaryotes will remain unknown. However, comparative biology of extant organisms and the limited fossil record provide some insight into
What event is thought to have contributed to the evolution of eukaryotes?
- global warming
- volcanic activity
- oxygenation of the atmosphere
Which characteristic is shared by prokaryotes and eukaryotes?
- nuclear envelope
- DNA-based genome
Mitochondria most likely evolved by _____________.
- a photosynthetic cyanobacterium
- cytoskeletal elements
- membrane proliferation
Which of these protists is believed to have evolved following a secondary endosymbiosis?
- green algae
- red algae
Describe the hypothesized steps in the origin of eukaryotic cells.
Eukaryotic cells arose through endosymbiotic events that gave rise to the energy-producing organelles within the eukaryotic cells such as mitochondria and chloroplasts. The nuclear genome of eukaryotes is related most closely to the Archaea, so it may have been an early archaean that engulfed a bacterial cell that evolved into a mitochondrion. Mitochondria appear to have originated from an alpha-proteobacterium, whereas chloroplasts originated as a cyanobacterium. There is also evidence of secondary endosymbiotic events. Other cell components may also have resulted from endosymbiotic events.
There are over 100,000 described living species of protists, and it is unclear how many undescribed species may exist. Since many protists live as commensals or parasites in other organisms and these relationships are often species-specific, there is a huge potential for protist diversity that matches the diversity of hosts. As the catchall term for eukaryotic organisms that are not animal, plant, or fungi, it is not surprising that very few characteristics are common to all protists.
Protists that have a pellicle are surrounded by ______________.
- silica dioxide
- calcium carbonate
Protists with the capabilities to perform photosynthesis and to absorb nutrients from dead organisms are called ______________.
Which of these locomotor organs would likely be the shortest?
- a flagellum
- a cilium
- an extended pseudopod
- a pellicle
Alternation of generations describes which of the following?
- The haploid form can be multicellular; the diploid form is unicellular.
- The haploid form is unicellular; the diploid form can be multicellular.
- Both the haploid and diploid forms can be multicellular.
- Neither the haploid nor the diploid forms can be multicellular.
Explain in your own words why sexual reproduction can be useful if a protist’s environment changes.
The ability to perform sexual reproduction allows protists to recombine their genes and produce new variations of progeny that may be better suited to the new environment. In contrast, asexual reproduction generates progeny that are clones of the parent.
Giardia lamblia is a cyst-forming protist parasite that causes diarrhea if ingested. Given this information, against what type(s) of environments might G. lamblia cysts be particularly resistant?
As an intestinal parasite, Giardia cysts would be exposed to low pH in the stomach acids of its host. To survive this environment and reach the intestine, the cysts would have to be resistant to acidic conditions.
23.3: Groups of Protists
In the span of several decades, the Kingdom Protista has been disassembled because sequence analyses have revealed new genetic (and therefore evolutionary) relationships among these eukaryotes. Moreover, protists that exhibit similar morphological features may have evolved analogous structures because of similar selective pressures—rather than because of recent common ancestry. This phenomenon, called convergent evolution, is one reason why protist classification is so challenging.
Which protist group exhibits mitochondrial remnants with reduced functionality?
- slime molds
Conjugation between two Paramecia produces ________ total daughter cells.
What is the function of the raphe in diatoms?
- capturing food
What genus of protists appears to contradict the statement that unicellularity restricts cell size?
The chlorophyte (green algae) genera Ulva and Caulerpa both have macroscopic leaf-like and stem-like structures, but only Ulva species are considered truly multicellular. Explain why.
Unlike Ulva, protists in the genus Caulerpa actually are large, multinucleate, single cells. Because these organisms undergo mitosis without cytokinesis and lack cytoplasmic divisions, they cannot be considered truly multicellular.
Why might a light-sensing eyespot be ineffective for an obligate saprobe? Suggest an alternative organ for a saprobic protist.
By definition, an obligate saprobe lacks the ability to perform photosynthesis, so it cannot directly obtain nutrition by searching for light. Instead, a chemotactic mechanism that senses the odors released during decay might be a more effective sensing organ for a saprobe.
Protists function in various ecological niches. Whereas some protist species are essential components of the food chain and generators of biomass, others function in the decomposition of organic materials. Still other protists are dangerous human pathogens or causative agents of devastating plant diseases.
An example of carbon fixation is _____________.
Which parasitic protist evades the host immune system by altering its surface proteins with each generation?
- Paramecium caudatum
- Trypanosoma brucei
- Plasmodium falciparum
- Phytophthora infestans
How does killing Anopheles mosquitoes affect the Plasmodium protists?
Plasmodium parasites infect humans and cause malaria. However, they must complete part of their life cycle within Anopheles mosquitoes, and they can only infect humans via the bite wound of a mosquito. If the mosquito population is decreased, then fewer Plasmodium would be able to develop and infect humans, thereby reducing the incidence of human infections with this parasite.
Without treatment, why does African sleeping sickness invariably lead to death?
The trypanosomes that cause this disease are capable of expressing a glycoprotein coat with a different molecular structure with each generation. Because the immune system must respond to specific antigens to raise a meaningful defense, the changing nature of trypanosome antigens prevents the immune system from ever clearing this infection. Massive trypanosome infection eventually leads to host organ failure and death.