Question 1

According to the endosymbiotic theory, why was it adaptive for the larger (host) cell to keep the engulfed cell alive, rather than digesting it as food?

Accepted Answer

The engulfed cell eventually evolved into mitochondria, which are responsible for producing ATP through cellular respiration. It was therefore adaptive for the host cell to keep the engulfed cell alive to continue to benefit from its ATP production.

Question 2

A particular species of protist has obtained a chloroplast via secondary endosymbiosis. You know this because the chloroplasts ________.

Accepted Answer

The presence of three or four membranes around the chloroplast indicates that it was obtained via secondary endosymbiosis. In this process, a eukaryotic cell engulfs a photosynthetic eukaryotic cell, which then becomes the chloroplast. The engulfed cell's plasma membrane becomes the first membrane, and the host cell's plasma membrane becomes the second membrane. Additional membranes can be added if the engulfed cell had already evolved strategies to protect itself from predation.

A is incorrect because while the chloroplast may contain genes from both the engulfed cell and the host cell, this is not a defining characteristic of secondary endosymbiosis. 
B is incorrect as the size of the chloroplast is not a defining characteristic of secondary endosymbiosis. 
D is incorrect as the number of pigments in the chloroplast is not a defining characteristic of secondary endosymbiosis.

Question 3

Which of the following pairs of protists and their ecological roles are correctly matched?

Accepted Answer

Apicomplexans, such as Plasmodium (which causes malaria) and Toxoplasma (which can infect humans and other animals), are known for being parasitic to animals. Euglenozoans, such as Euglena, are primarily photosynthetic but can also ingest food and are considered mixotrophic. Dinoflagellates can be both photosynthetic and heterotrophic, but they are known for causing harmful algal blooms and are not parasites of plants. Entamoebas are primarily known for being parasitic to animals, including humans.

Question 4

Use the following information to answer the question.
Giardia intestinalis is an intestinal parasite of humans and other mammals that causes intestinal ailments in most people who ingest the cysts. Upon ingestion, each cyst releases two motile cells, called trophozoites. These attach to the small intestine's lining via a ventral adhesive disk. The trophozoites anaerobically metabolize glucose from the host's intestinal contents to produce ATP. Reproduction is completely asexual, occurring by longitudinal binary fission of trophozoites, with each daughter cell receiving two haploid nuclei (n = 5). A trophozoite will often encyst as it passes into the large intestine by secreting around itself a case that is resistant to cold, heat, and dehydration. Infection usually occurs by drinking untreated water that contains cysts.
The cysts of Giardia function most like the ________.

Accepted Answer

The cysts of Giardia are resistant structures that allow the parasite to survive harsh conditions such as cold, heat, and dehydration. Endospores of bacteria are also resistant structures with similar functions. The mitochondria of ancestral diplomonads, nuclei of archaeans, and capsids of viruses are not analogous structures to the cysts of Giardia.

Question 5

The chloroplasts of land plants are thought to have been derived according to which evolutionary sequence?

Accepted Answer

The most widely accepted evolutionary sequence is that cyanobacteria were engulfed by ancient eukaryotic cells, which eventually evolved into green algae. From green algae, land plants evolved, and the chloroplasts in land plants are thought to have originated from the chloroplasts in green algae.

Question 6

Trypanosome infections evade attacks by host immune systems through which of the following mechanisms?

Accepted Answer

Trypanosomes are able to evade attacks by host immune systems by producing new cell-surface proteins with a different molecular structure by each new generation. This allows the trypanosome to continuously change its surface antigens, making it difficult for the host immune system to recognize and mount a response against the parasite. Option B is not correct as trypanosomes are not known to produce toxins that kill lymphocytes. Option C is not correct as trypanosomes do not insert their DNA into the nuclear DNA of host cells. Option D is partially correct as trypanosomes can infect lymphocytes, but this does not necessarily lead to a decline in the host's ability to fight infection.

Question 7

Use the following information to answer the question.
Paulinella chromatophora is one of the few cercozoans that is autotrophic, carrying out aerobic photosynthesis with its two elongated "chromatophores." The chromatophores are contained within vesicles of the host cell, and each is derived from a cyanobacterium, though not the same type of cyanobacterium that gave rise to the chloroplasts of algae and plants.
Which process could have allowed the nucleomorphs of chlorarachniophytes to be reduced, without the net loss of any genetic information?

Accepted Answer

Horizontal gene transfer could allow genetic information to be transferred from the nucleomorphs to the host genome, reducing the nucleomorphs while retaining the genetic information.

Question 8

Use the following information to answer the question.
Giardia intestinalis is an intestinal parasite of humans and other mammals that causes intestinal ailments in most people who ingest the cysts. Upon ingestion, each cyst releases two motile cells, called trophozoites. These attach to the small intestine's lining via a ventral adhesive disk. The trophozoites anaerobically metabolize glucose from the host's intestinal contents to produce ATP. Reproduction is completely asexual, occurring by longitudinal binary fission of trophozoites, with each daughter cell receiving two haploid nuclei (n = 5). A trophozoite will often encyst as it passes into the large intestine by secreting around itself a case that is resistant to cold, heat, and dehydration. Infection usually occurs by drinking untreated water that contains cysts.
The mitosome of Giardia has no DNA within it. If it did contain DNA, then what predictions should we be able to make about its DNA?

Accepted Answer

The mitosome of Giardia does not contain DNA, so if it did contain DNA, it is more likely to be circular with few introns, like the mitochondrial DNA of many other organisms. It is also not likely to be associated with histone proteins, as those are typically found in the nucleus of eukaryotic cells.

Question 9

An individual mixotroph loses its plastids, yet continues to survive. Which of the following most likely accounts for its continued survival?

Accepted Answer

Mixotrophs are organisms that are capable of both autotrophic and heterotrophic nutrition. If a mixotroph loses its plastids, it can still survive by engulfing organic material by phagocytosis or by absorption. This allows it to obtain energy and nutrients from external sources, rather than relying solely on photosynthesis. Options A and B are incorrect because a mixotroph cannot rely on photosystems floating freely in the cytosol, and it would not necessarily gain extra mitochondria when it lost its plastids. Option D is incorrect because endospores are usually dormant structures that are formed to help the organism survive in adverse environmental conditions, and are not likely to play a role in the survival of a mixotroph that has lost its plastids.

Question 10

Use the following information to answer the question.
Paulinella chromatophora is one of the few cercozoans that is autotrophic, carrying out aerobic photosynthesis with its two elongated "chromatophores." The chromatophores are contained within vesicles of the host cell, and each is derived from a cyanobacterium, though not the same type of cyanobacterium that gave rise to the chloroplasts of algae and plants.
The closest living relative of P. chromatophora is the heterotroph P. ovalis. P. ovalis uses threadlike pseudopods to capture its prey, which it digests internally. Which of the following, if observed, would be the best reason for relabeling P. chromatophora as a mixotroph instead of an autotroph?

Accepted Answer

If P. chromatophora was observed with a vacuole containing food, it would indicate that it is also capable of heterotrophic nutrition in addition to its autotrophic capabilities. This would make it a mixotroph rather than purely autotrophic. None of the other options listed would indicate a shift towards heterotrophy.

Question 11

Which two genera have members that can evade the human immune system by frequently changing their surface proteins?

Accepted Answer

Plasmodium and Trypanosoma are known for their ability to frequently change their surface proteins, allowing them to evade the human immune system. Plasmodium is the causative agent of malaria, and Trypanosoma causes diseases such as sleeping sickness and Chagas disease. This antigenic variation is a key factor in their survival and pathogenicity within human hosts.

Question 12

Use the following information to answer the question.
Giardia intestinalis is an intestinal parasite of humans and other mammals that causes intestinal ailments in most people who ingest the cysts. Upon ingestion, each cyst releases two motile cells, called trophozoites. These attach to the small intestine's lining via a ventral adhesive disk. The trophozoites anaerobically metabolize glucose from the host's intestinal contents to produce ATP. Reproduction is completely asexual, occurring by longitudinal binary fission of trophozoites, with each daughter cell receiving two haploid nuclei (n = 5). A trophozoite will often encyst as it passes into the large intestine by secreting around itself a case that is resistant to cold, heat, and dehydration. Infection usually occurs by drinking untreated water that contains cysts.
Giardia's mitosome can be said to be "doubly degenerate," because it is a degenerate type of ________, an organelle that is itself a degenerate form of ________.

Accepted Answer

Giardia's mitosome is a degenerate form of a mitochondrion, which is itself a degenerate form of a proteobacterium. The mitosome of Giardia lacks the DNA and respiratory capacity found in typical mitochondria, making it doubly degenerate.

Question 13

Many parasitic members of the excavates lack plastids and have highly reduced mitochondria. Which of the following statements explains these observations?

Accepted Answer

The highly reduced mitochondria and lack of plastids in parasitic members of the excavates suggest that these traits did not confer a fitness advantage in their dark, low-oxygen environments. As a result, selection pressure favored loss of genes for these organelles, which were replaced by alternative metabolic pathways.

Question 14

All protists are ________.

Accepted Answer

All protists are eukaryotic, meaning their cells contain a nucleus and other membrane-bound organelles. While most protists are unicellular, there are some multicellular forms as well. Not all protists are symbionts or mixotrophic, so choices C and D are incorrect.

Question 15

Consider the following data:
(a) Most ancient eukaryotes are unicellular.
(b) All eukaryotes alive today have a nucleus and cytoskeleton.
(c) Most ancient eukaryotes lack a cell wall. Which of the following conclusions could reasonably follow the data presented? The first eukaryote may have been ________.

Accepted Answer

Since most ancient eukaryotes lacked a cell wall, it suggests they were capable of phagocytosis, which requires the cell to be able to change its shape and form temporary pseudopodia to engulf prey. This is a characteristic present in many unicellular eukaryotes today. While the first eukaryote may have had similar features to plant cells, it cannot be concluded with certainty based on the given data. There is no evidence to suggest that the first eukaryote was anaerobic or photosynthetic.

Question 16

Use the following information to answer the question.
Giardia intestinalis is an intestinal parasite of humans and other mammals that causes intestinal ailments in most people who ingest the cysts. Upon ingestion, each cyst releases two motile cells, called trophozoites. These attach to the small intestine's lining via a ventral adhesive disk. The trophozoites anaerobically metabolize glucose from the host's intestinal contents to produce ATP. Reproduction is completely asexual, occurring by longitudinal binary fission of trophozoites, with each daughter cell receiving two haploid nuclei (n = 5). A trophozoite will often encyst as it passes into the large intestine by secreting around itself a case that is resistant to cold, heat, and dehydration. Infection usually occurs by drinking untreated water that contains cysts.
If the mitosomes of Giardia contain no DNA, yet are descendants of what were once free-living organisms, then where are we likely to find the genes that encode their structures, and what accounts for their current location there?

Accepted Answer

The genes that encode the structures of the mitosomes in Giardia are likely found in the nucleus, and their current location there is likely due to horizontal gene transfer. This is because Giardia is thought to have acquired these organelles through the endosymbiosis of a bacterium, and the transfer of genes from the bacterium's genome to the nuclear genome of the host cell is a common mechanism for the retention of organelles. Plasmids and S phase are not likely to be involved in the structure and location of the mitosomes. Conjugation and transformation may involve the transfer of genetic material, but they are not relevant to the question.

Question 17

When a mosquito infected with Plasmodium first bites a human, the Plasmodium ________.

Accepted Answer

When a mosquito infected with Plasmodium bites a human, the Plasmodium cells enter the bloodstream and travel to the liver. In the liver, they infect liver cells and multiply rapidly, causing damage to the liver. It is only after this stage that the Plasmodium cells can infect red blood cells and cause symptoms of malaria.

Question 18

Which of the following have chloroplasts (or structures since evolved from chloroplasts) thought to be derived from ancestral green algae?

Accepted Answer

Chlorarachniophytes are believed to have acquired their chloroplasts via endosymbiosis with green algae. The other options (stramenopiles, apicomplexans, and dinoflagellates) have chloroplasts that were acquired through secondary endosymbiosis, where an ancestor of these groups engulfed an algal cell that already had a chloroplast. Therefore, their chloroplasts are not thought to be derived from ancestral green algae directly.

Question 19

Prokaryotic and eukaryotic flagella ________.

Accepted Answer

Prokaryotic and eukaryotic flagella have different structures. Prokaryotic flagella are powered by a rotary motor in the cell membrane and have a simple filamentous structure, while eukaryotic flagella are powered by the movement of microtubules and have a more complex structure.

Question 20

Use the following information to answer the question.
Giardia intestinalis is an intestinal parasite of humans and other mammals that causes intestinal ailments in most people who ingest the cysts. Upon ingestion, each cyst releases two motile cells, called trophozoites. These attach to the small intestine's lining via a ventral adhesive disk. The trophozoites anaerobically metabolize glucose from the host's intestinal contents to produce ATP. Reproduction is completely asexual, occurring by longitudinal binary fission of trophozoites, with each daughter cell receiving two haploid nuclei (n = 5). A trophozoite will often encyst as it passes into the large intestine by secreting around itself a case that is resistant to cold, heat, and dehydration. Infection usually occurs by drinking untreated water that contains cysts.
Given its mode of reproduction and internal structures, which of the following should be expected to occur in Giardia at some stage of its life cycle?

Accepted Answer

Giardia reproduces asexually through binary fission, which involves the separation of daughter chromosomes. Crossing over, meiosis, and synapsis are associated with sexual reproduction and do not occur in Giardia's life cycle.

Quiz 28: Protists