23: Protists Flashcards
Eukaryotic Origins, Characteristics of Protists, Groups of Protists, Ecology of Protists
What are some examples of protists?
Protists range from the microscopic, single-celled Acanthocystis turfacea and the ciliate Tetrahymena thermophila, which can be seen using light microscopy, to the enormous, multicellular kelps (Chromalveolata) that extend for hundreds of feet in underwater “forests”.
What is the origin of the term “protist”?
This name was first suggested by Ernst Haeckel in the late 19th century.
What are some examples of protists with large, macroscopic cells?
Plasmodia (giant amoebae) of myxomycete slime molds, and marine green alga Caulerpa.
What are some examples of multicellular protists?
Red, green, and brown seaweeds.
What is endosymbiosis?
Engulfment of one cell within another such that the engulfed cell survives, and both cells benefit; the process responsible for the evolution of mitochondria and chloroplasts in eukaryotes.
What is the endosymbiotic theory?
Theory that states that eukaryotes may have been a product of one cell engulfing another, one living within another, and evolving over time until the separate cells were no longer recognizable as such.
What is a plastid?
One of a group of related organelles in plant cells that are involved in the storage of starches, fats, proteins, and pigments.
In what kinds of environments are microscopic protists commonly found?
Soil, freshwater, brackish and marine environments, as well as the digestive tracts of animals and in the vascular tissues of plants. Others invade the cells of other protists, animals, and plants.
What is the oldest fossils found?
The earliest fossils found appear to be Bacteria, most likely cyanobacteria. They are about 3.5 billion years old and are recognizable because of their relatively complex structure and, for prokaryotes, relatively large cells.
When do the first eukaryotes appear in the fossil record?
Structures of size 10 micrometers and greater, which might be fossils, appear in the geological record about 2.1 billion years ago.
Which characteristics must have been common in the most recent common ancestor of eukaryotic organisms?
- Cells with nuclei surrounded by a nuclear envelope with nuclear pores.
- Mitochondria.
- A cytoskeleton containing the structural and motility components called actin microfilaments and microtubules.
- Flagella and cilia.
- Chromosomes, each consisting of a linear DNA molecule coiled around basic (alkaline) proteins called histones.
- Mitosis.
- Sex.
- Cell walls.
What is the prevalence of mitochondria in eukaryotes?
Some extant eukaryotes have very reduced remnants of mitochondria in their cells, whereas other members of their lineages have “typical” mitochondria.
What is the prevalence of the cytoskeleton in eukaryotes?
All extant eukaryotes have actin microfilaments and microtubules.
What is the prevalence of flagella and cilia in eukaryotes?
Some extant eukaryotes lack flagella and/or cilia, but they are descended from ancestors that possessed them.
What is the prevalence of histones in eukaryotic chromosomes?
The few eukaryotes with chromosomes lacking histones clearly evolved from ancestors that had them.
What is the prevalence of mitosis in eukaryotes?
Mitosis is universally present in eukaryotes.
What is karyogamy?
A stage where two haploid nuclei fuse together to create a diploid zygote nucleus.
What is the prevalence of cell walls in eukaryotes?
Members of all major lineages have cell walls, and it might be reasonable to conclude that the last common ancestor could make cell walls during some stage of its life cycle. However, not enough is known about eukaryotes’ cell walls and their development to know how much homology exists among them. If the last common ancestor could make cell walls, it is clear that this ability must have been lost in many groups.
What was likely the origin of eukaryotes?
All extant eukaryotes are descendants of a chimeric organism that was a composite of a host cell and the cell(s) of an alpha-proteobacterium that “took up residence” inside it.
Which metabolic processes are found in prokaryotes, and in eukaryotes?
Many important metabolic processes arose in prokaryotes, and some of these, such as nitrogen fixation, are never found in eukaryotes. The process of aerobic respiration is found in all major lineages of eukaryotes, and it is localized in the mitochondria. Aerobic respiration is also found in many lineages of prokaryotes, but it is not present in all of them, and many forms of evidence suggest that such anaerobic prokaryotes never carried out aerobic respiration nor did their ancestors.
What is the history of atmospheric oxygenation?
Various metabolic processes evolved that protected organisms from oxygen, one of which, aerobic respiration, also generated high levels of ATP. It became widely present among prokaryotes, including in a group we now call alpha-proteobacteria. Organisms that did not acquire aerobic respiration had to remain in oxygen-free environments. Originally, oxygen-rich environments were likely localized around places where cyanobacteria were active, but by about 2 billion years ago, geological evidence shows that oxygen was building up to higher concentrations in the atmosphere. Oxygen levels similar to today’s levels only arose within the last 700 million years.
When were the first organelles observed?
Mitochondria were first observed by light microscopists in the late 1800s, where they appeared to be somewhat worm-shaped structures that seemed to be moving around in the cell.
What is the history of the endosymbiotic theory?
In the 1960s, American biologist Lynn Margulis developed endosymbiotic theory. In 1967, Margulis introduced new work on the theory and substantiated her findings through microbiological evidence. Although Margulis’ work initially was met with resistance, the once-revolutionary hypothesis is now widely (but not completely) accepted, with work progressing on uncovering the steps involved in this evolutionary process and the key players involved.
What are some features of mitochondria?
Mitochondria arise from the division of existing mitochondria; they may fuse together; and they may be moved around inside the cell by interactions with the cytoskeleton. However, mitochondria cannot survive outside the cell.
How are mitochondria created?
Mitochondria divide independently by a process that resembles binary fission in prokaryotes. Specifically, mitochondria are not formed from scratch (de novo) by the eukaryotic cell; they reproduce within it and are distributed with the cytoplasm when a cell divides or two cells fuse. Their reproduction is synchronized with the activity and division of the cell.
Where are genes for respiratory proteins found?
Many of the genes for respiratory proteins are located in the nucleus, but in some eukaryotic groups, they are found in mitochondria.
What is some evidence from respiratory protein genes for the endosymbiont theory?
The genes of respiratory proteins in eukaryotes, when compared to those of other organisms, appear to be of alpha-proteobacterial origin. The fact that in many eukaryotes they are found in the nucleus and not mitochondria has been interpreted as evidence that the genes have been transferred from the endosymbiont chromosome to the host genome. This loss of genes by the endosymbiont is probably one explanation why mitochondria cannot live without a host.
Are mitochondria found in anaerobic eukaryotes?
Some anaerobic eukaryotes appear to lack organelles that could be recognized as mitochondria. Findings suggest that reduced organelles are found in most, if not all, anaerobic eukaryotes, and that all eukaryotes appear to carry some genes in their nuclei that are of mitochondrial origin.
What are some functions associated with the reduced mitochondrion-derived organelles of anaerobic eukaryotes?
The generation of clusters of iron and sulfur for use as cofactors.
Which type of molecule is found in plastids that is involved in the light-harvesting chemical reactions of photosynthesis?
Plastids are rich in the pigment chlorophyll a and a range of other pigments, called accessory pigments, which are involved in harvesting energy from light.
What is the primary endosymbiotic origin of plastids?
Plastids are derived from cyanobacteria that lived inside the cells of an ancestral, aerobic, heterotrophic eukaryote. Plastids of primary origin are surrounded by two membranes.
Why is it suggested that primary endosymbiosis has occurred twice in the history of photosynthetic eukaryotes?
In one case, the common ancestor of the major lineage/supergroup Archaeplastida took on a cyanobacterial endosymbiont; in the other, the ancestor of the small amoeboid rhizarian taxon, Paulinella, took on a different cyanobacterial endosymbiont. Almost all photosynthetic eukaryotes are descended from the first event, and only a couple of species are derived from the other.
What is one of the differences between cyanobacteria and most other prokaryotes?
Cyanobacteria have extensive, internal membrane-bound sacs called thylakoids.
What are some similarities between chloroplasts and cyanobacteria?
Chloroplasts of primary origin have thylakoids, a circular DNA chromosome, and ribosomes similar to those of cyanobacteria.
Which plastids have a relictual cyanobacterial wall?
Each chloroplast is surrounded by two membranes. In the group of Archaeplastida called the glaucophytes and in Paulinella, a thin peptidoglycan layer is present between the outer and inner plastid membranes. All other plastids lack this relictual cyanobacterial wall.
What are the origins of the two plasma membranes of plastids?
The outer membrane surrounding the plastid is thought to be derived from the vacuole in the host, and the inner membrane is thought to be derived from the plasma membrane of the symbiont.
What are some endosymbiotic characteristics of plastids?
There is, as with the case of mitochondria, strong evidence that many of the genes of the endosymbiont were transferred to the nucleus. Plastids, like mitochondria, cannot live independently outside the host. In addition, like mitochondria, plastids are derived from the division of other plastids and never built from scratch.
When did the endosymbiotic event that led to Archaeplastida occur?
Researchers have suggested that the endosymbiotic event that led to Archaeplastida occurred 1 to 1.5 billion years ago, at least five hundred million years after the fossil record suggests that eukaryotes were present.
What is secondary endosymbiosis?
A cell containing an endosymbiont to itself became engulfed.
What are some characteristics of secondary plastids?
Secondary plastids are surrounded by three or more membranes, and some secondary plastids even have clear remnants of the nucleus of the endosymbiotic alga. Others have not “kept” any remnants.
Are all plastids in eukaryotes derived from primary endosymbiosis?
Not all plastids in eukaryotes are derived directly from primary endosymbiosis. There are cases where tertiary or higher-order endosymbiotic events are the best explanations for plastids in some eukaryotes.
What is a chlorarachniophyte?
Chlorarachniophytes are rare algae indigenous to tropical seas and sand that can be classified into the rhizarian supergroup. Chlorarachniophytes extend thin cytoplasmic strands, interconnecting themselves with other chlorarachniophytes, in a cytoplasmic network. Molecular and morphological evidence suggest that the chlorarachniophyte protists are derived from a secondary endosymbiotic event, where a eukaryote engulfed a green alga, the latter of which had already established an endosymbiotic relationship with a photosynthetic cyanobacterium.
What is the evidence that chlorarachniophytes evolved from secondary endosymbiosis?
The chloroplasts contained within the green algal endosymbionts are still capable of photosynthesis, making chlorarachniophytes photosynthetic. The green algal endosymbiont also exhibits a stunted vestigial nucleus. Additionally, the plastids of chlorarachniophytes are surrounded by four membranes.
Why is it suggested that chlorarachniophytes are the products of an evolutionary recent secondary endosymbiotic event?
Their plastids are surrounded by four membranes, whereas in other lineages that involved secondary endosymbiosis, only three membranes can be identified around plastids, which is explained as a sequential loss of a membrane during the course of evolution.
How are the four membranes of chlorarachniophytes explained?
The first two correspond to the inner and outer membranes of the photosynthetic cyanobacterium, the third corresponds to the green alga, and the fourth corresponds to the vacuole that surrounded the green alga when it was engulfed by the chlorarachniophyte ancestor.
What are some examples of secondary endosymbiosis, excluding chlorarachniophytes?
Secondary endosymbiosis of green algae led to euglenid protists, whereas secondary endosymbiosis of red algae led to the evolution of dinoflagellates, apicomplexans, and stramenopiles.
What evidence is there that mitochondria were incorporated into the ancestral eukaryotic cell before chloroplasts?
All eukaryotic cells have mitochondria, but not all eukaryotic cells have chloroplasts.
What is a mixotroph?
Organism that can obtain nutrition by autotrophic or heterotrophic means, usually facultatively.
What is a pellicle?
Outer cell covering composed of interlocking protein strips that function like a flexible coat of armor, preventing cells from being torn or pierced without compromising their range of motion.
What is a phagolysosome?
Cellular body formed by the union of a phagosome containing the ingested particle with a lysosome that contains hydrolytic enzymes.
How diverse are protists?
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 the 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.
What are some characteristics of protist cell structure?
The cells of protists are among the most elaborate of all cells. Most protists are microscopic and unicellular, but some true multicellular forms exist. A few protists live as colonies that behave in some ways as a group of free-living cells, and in other ways as a multicellular organism. Still other protists are composed of enormous, multinucleate, single cells that look like amorphous blobs of slime, or in other cases, like ferns. In fact, many protist cells are multinucleated; in some species, the nuclei are different sizes and have distinct roles in protist cell function.
What is the size and protective features of protists?
Single protist cells range in size from less than a micrometer to three meters in length to hectares. Protist cells may be enveloped by animal-like cell membranes or plant-like cell walls. Others are encased in glassy silica-based shells or wound with pellicles of interlocking protein strips.
What are the stages of phagocytosis?
The engulfment of a food particle, the digestion of the particle using hydrolytic enzymes contained within a lysosome, and the expulsion of undigested materials from the cell.
What is a saprobe?
A subtype of heterotrophs, which absorb nutrients from dead organisms or their organic wastes.
What modes of movement have protists evolved for motility?
The majority of protists are motile, but different types of protists have evolved varied modes of movement. Some protists have one or more flagella, which they rotate or whip. Others are covered in rows or tufts of tiny cilia that they coordinately beat to swim. Others form cytoplasmic extensions called pseudopodia anywhere on the cell, anchor the pseudopodia to a substrate, and pull themselves forward.
What is taxis?
The movement toward or away from a stimulus.
What is phototaxis?
Movement toward light, which is accomplished by coupling a locomotion strategy with a light-sensing organ.
What are examples of protists which use different transportation methods?
Paramecia waves hair-like appendages called cilia to propel itself; amoebas use lobe-like pseudopodia to anchor itself to a solid surface and pull itself forward; Euglena use a whip-like tail called a flagellum to propel itself.
What are the two modes of binary fission in protists?
Transverse or longitudinal, depending on the axis of orientation.
What is an example of a protist which exhibits binary fission?
Paramecium.
What is an example of a protist which exhibits multiple fission?
True slime molds.
