Chapter 5 - Structure & Function in Protists Flashcards
Most eukaryotes are ___________________.
Microbes
What is another name for eukaryotic microbes?
Protists
How long did eukaryotic microbes have before macroscopic lineages (i.e., animals) arose?
1 billion years
Eukaryotic cells are characterized by what two things. What are they?
- An endomembrane system
- A cytokeletal system
Why is it believed that the endomembrane and cytoskeletal system led to the evolutionary success of the eukaryotes?
Because these two systems are the basis for the ability to phagocytose particular food
What were the crucial first events in the evolution of the eukaryotic cell?
The development of a rudimentary cytoskeletal system and the ability to manage the flow of material from one membrane to the next
What 7 organelles make up the endomembrane system?
- The cell membrane
- The endoplasmic reticulum
- The Golgi body
- The endosome
- The nuclear membrane
- The lysosome
- The vacuole
What is not included as part of the endomembrane system?
The mitochondrial and cholorplast membrane
What does the cytoskeletal system consist of?
Microtubules and microfilaments
What targets proteins to particular places in the eukaryotic cell?
Special signal sequences
There are two types of targeting mechanisms in the eukaryotic cell. What are they?
Cotranslation and posttranslation
Why do eukaryotic cells require proteins to have signal sequences?
Because the endomembrane system partitions the cell into multiple compartments each of which has its own distinctive proteins
What type of protein undergoes cotranslational targeting?
Those targeted for the ER and/or secretion
Proteins destined for the mitochondria and/or chloroplast undergo the __________________________ mechanism for protein sorting.
Post-translation
What is required for post-translational signaling?
A chaperone protein
What are the steps to co-translational targeting?
- mRNA exists the nucleus
- Ribosome binds and begins translation in the cytosol
- The amino-terminal 20 or so amino acids constitute a signal sequence that targets the protein to the ER
- The signal recognition particle (SRP) binds to the ribosome and the signal peptide, halting translation
- The ribosome binds to the translocation complex on the ER membrane, displacing the SRP and recommencing translation
- When translation is completed, the signal sequence is cleaved and degraded
- The translocation complex dissociates
What binds to the ribosome and signal peptide, halting translation?
The signal recognition particle
To what does the ribosome bind on the ER membrane in co-translational targeting?
The translocation complex
What are the steps for post-translation protein targeting?
- As mitochondrial or chloroplast proteins are translated, chaperone proteins bind to them
- The chaperone conducts the protein to the organelle where it is translocated to the interior
- The signal sequence is cleaved off and the protein folds into its mature configuration
How many signal sequences do proteins that cross two membranes have?
Two signal sequences: one directs them to the mitochondrion or chloroplast and is cleaved off as the polypeptide enters the organelle; as the polypeptide enters, it is bound by organelle chaperone proteins to keep it unfolded; these chaperones conduct the protein to a second translocation complex where the second signal sequence directs their transfer across the second membrane
What type of destinations require two signal sequences?
Intermembrane spaces (mitochondria or chloroplast)
Thylakoids
How does the endomembrane system exchanges materials by ________________ and ___________________ of membrane vesicles
Budding
Fusion
How are vesicles transported in the cell?
Organized and moved by the cytoskeleton
Is budding and fusion energy-dependent or energy-independent?
Energy-dependent; it requires ATP
What encloses the chromosomes?
The nuclear envelope
What is a specialized region of the ER that consists of two concentric membranes, the outer surface of which is studded with ribosomes?
The nuclear envelope
Is the outer membrane of the nuclear envelope continuous with the ER?
Yes, it is
The nuclear envelope surrounds the ______________________, the material within the nuclear envelope
Nucleoplasm
Why is the nucleoplasm chemically very different from the cytoplasm?
Because of the selectivity of the pores that perforate the nuclear envelope
The nucleoplasm is especially high in what type of molecule?
RNA
What is the site of rRNA transcription and ribosome assembly?
Nucleolus
Form what part of the ER do vesicles bud? The rough or smooth ER?
The smooth ER
Is rRNA translated?
No, it is never translated; it is simply a structural component of ribosomes
The nuclear envelope is composed of two ________________ ______________
Phospholipid bilayers
Does the area closest to or farther from the nucleus have more ribosomes?
Closest to the nucleus
What do chromosomes consist of?
Chromatin
What is chromatin?
A complex of DNA and histone proteins
Free DNA is wrapped around ______________________ cores in chromatin, most of which is coiled into a ________ ____ ____________ in the interphase nucleus. In mitosis and meiosis, this fiber is condensed by linked to a protein scaffold of ______________, which is further coiled into the fully condensed chromosome - 10,000 times shorter than the molecule of DNA it contains
Nucleosomes
30 nm fiber
Condensins
Before chromatin can be transcribed, what must be done to it?
It must be remodeled, a process in which the 30 nm fiber uncoils and the DNA unwraps from the nucleosomes
What type of chemical modification increases transcription?
Acetylation
What regulates the passage of materials between the cytoplasm and nucleoplasm?
Nuclear pore complexes
What partially blocks the nuclear pores?
A group of proteins called the pore complex
What is the maximum sized molecule that can pass between the nucleoplasm and cytoplasm without disrupting the pore complex?
50 kDa
What are the principle macromolecules that pass through the core cmplex?
mRNA
tRNA
Partially assembled ribosomes
How does the nuclear pore complex distinguish between which mRNAs to export and retain?
5’ methyguanosine cap
3’ polyadenyl tail
Where are the enzymes required for mRNA capping located?
Nucleus
What effect does the placement of capping enzymes have on polypeptide transcription?
mRNA transcription and capping are coupled
What recognizes the special sequence for polyadenlyation?
The tailing enzyme
Why does the cell cap and tail mRNA molecules?
To protect mRNA molecules from exonucleases - methylguanosine and poly-adenylated tails do not look like “normal” nucleotides that would be the targets of endonucleases
What unit of the ribosome recognizes the 5’ cap and stabilizes the mRNA molecule?
The small ribosomal subunit
The 5’ cap and 3’ tail are involved in signaling, mRNA protection, and _____________ _________________
Alternative splicing
When is the poly-A tail added?
Post-transcriptionally
What are the steps to poly-A tailing?
- Cleavage factors bind to the cleavage and adenylation specificity factor (CPSF)
- Poly-A polymerase binds
- The mRNA is cleaved
- Poly-A polymerase adds the poly-A tail one nucleotide at a time
To what do cleavage factor proteins bind for polyadenylation?
Cleavage and polyadenylation specificity factor (CPSF)
What helps poly-A polymerase to continue adding adenine nucleotides?
Nuclear poly-A binding protein (PABPN1)
What is the nuclear poly-A binding protein?
A protein that helps poly-A polymerase to keep adding nucleotides to the mRNA molecule
Approximately how many adenine nucleotides are added to a mRNA molecule in mammals?
250
How does the function of the poly-A tail differ in eukaryotes and prokaryotes?
In eukaryotes the poly-A tail extends the life of the mRNA while in prokaryotes it shortens the mRNA life as it targets the mRNA for degradation
Which organelle is responsible for membrane synthesis?
The ER
What is the principal site of lipid synthesis and membrane protein synthesis in the eukaryotic cell?
The ER
Lipid sythesis is accomplished by ______________ embedded in the _____ membrane.
Enzymes
ER membrane
Membrane protein synthesis is done by ___________________ attached to the _______________ surfaces of the __________.
Ribosomes
Cytoplasmic surface
ER
True or false?
The ER is also the site of synthesis of secretory proteins, digestive enzymes, cell walls, and protein glycosylation.
True
What does the Golgi do?
Sorts the mixed contents of ER membrane and lumen into different vesicles (“protein sorting”)
Where are peptide bonds always catalyzed?
By ribosomes in the cytoplasm
Why are proteins destined for secretion or other organelles not embedded into the ER membrane as they leave the ER?
Because they lack the hydrophobic amino acid residues typical of membrane insertion
How does the Golgi sort proteins?
By destination
Materials are transported from the ER to the Golig in membrane ________________. Once at the Golgi, the vesicles fuse with the cisterna on one face of the Golgi, the ___-Golgi. Materials are transferred from one cisterna to the next through the stack by _____________ ________________ and _________________. Vesicles are produced at the other face of the Golgi, the _____-Golgi. These vesicles are ___________________ with contents and membrane proteins appropriate to their destination.
Vesicles
cis-Golgi
Vesicle budding and fusion
trans-Golgi
Homogenous
Although still not fully understood, what is believed to be the underlying mechanism by which the Golgi sorts proteins together?
Glycosylation
What oligosaccharide modification is associated with the lysosome?
Mannose-6-phosphate
What are endosomes?
Vesicles involved in sorting proteins taken in during endocytosis and sends lysomal enzymes to the lysosome
What are secretory vesicles?
Vesicles that carry membrane proteins and materials to be exported from the cell
What is the general term for the process in which vesicles fuse with the cell membrane to release their contents to the outside?
Exocytosis
What is one of the main functions of the endosome?
To separate materials from their receptors
How does the endosome separate materials from their receptors?
Lysosomal enzymes are transported to the endosome in a vesicle; in the vesicle, lysomal enzymes are bound to a membrane receptor that recognizes the M6P signal; the vesicle binds to and fuses with the endosome; once in the endosome, the enzymes dissociate from their receptors because the pH of the endosome is low; the endosome then sorts the membrane receptor and lysosomal enzymes into different transport vesicles
What are the three steps discussed regarding sorting of lysosomal enzymes and receptors in the endosome?
- Lysosomal enzymes dissociate from their receptors in the low pH of the endosome
- The endosome sorts lysosomal proteins into one kind of vesicle for the lysosome
- The free receptors are sorted into a different vesicle for the Golgi
What is the first step in intracellular digestion and in surface receptor recycling?
Endocytosis
What do surface receptors do?
Sense the chemical conditions of the extracellular environment by binding specifically to particular small molecules called ligands
Do all eukaryotic cells obtain their food via endocytosis?
No, but many do
What happens when a ligand binds its receptor?
A conformational change occurs on the inside of the cell reveals an active site that creates a response or triggers a response
What recycles membrane proteins?
Endosomes
What are the five steps regarding endocytosis discussed in class?
- Receptor protein and bound ligand are phagocytosed
- The phgaocytosed materials are sent to the endosome
- In the acidic endosome, the ligand detaches
- The ligand is sent to the lysosome for digestion
- The receptor protein is recycled back to the cell surface
_________________________ eliminates indigestible residues from the cell
Exocytosis
What is a phagolysosome?
A hybrid vesicle composed of a lysosome and endosome fused together for materials endocytosed that are to be digested
Some protists, notably the _________________________, have a defined pathway that phagolysosomes take
Ciliates
In the ciliates, there is a morphologically defined spot on the cell surface where phagocytosis takes place, often at the base of a groove or a pit. What is the spot called?
Cytostome
Where do undigested residues exit in ciliates?
The cytoproct
What five steps occur in the endocytosis and exocytosis in ciliates?
- Cilia sweep food particles into the cytostome
- Phagocytosis occurs at the base of the cystosome
- The phagosome fuses with the lysosomes
- Intracelluar digestion begins as the phagolysosome is slowly moved around the periphery of the cell
- Undigested residue is expelled at the cytoproct
How is the phagolysosome transported to the cytoproct in ciliates?
Via microtubules
How is the cytoproct regenerated after exocytosis occurs?
Excess membrane is recovered as small vesicles and regenerates the cytoproct
In ciliates, the phagolysosome moves along microtubules until it reaches the cytoproct. What happens once it arrives?
The phagolysosome membrane fuses with the cell membrane at the cytoproct
Many protists are armed with ______________________.
Extrusomes
What are extrusomes?
Sacs that lie beneath the cell surface and contain a coiled or compressed filament ending in a sharp dart-like structure or other mechanism for attacking prey
An extrusome can serve two functions. What are they?
Defense
Predation
What category of extrusome contains a proteinaceous rod that is though to length many times when hydrated, thereby stabbing prey or predator when water rushes in?
Trichocysts
Are mitochondria and chloroplast part of the endomembrane system?
No; they are not linked by vesicle fusion or budding to any other membrane systems of the cell
Some anaerobic protists have __________________________ instead of mitochondria that partially oxidize pyruvate produced by fermenting sugar and produce hydrogen gas.
Hydrogenosomes
Mitochondria and chloroplasts have their own ______________________ and their own protein-synthesizing systems
Chromosomes
True or false?
Many mitochondrial and chloroplast proteins, and some lipids, are imported from the cytoplasm.
True; although they have their own chromosome and protein synthetic system, they don’t encode all their own proteins, and in most cases, their proteins are made on cytoplasmic ribosomes and then transported to their respective destinations
What proteins extract specific phospholipids from the ER and transfer them to the mitochondria and chloroplasts?
Phospholipid transfer proteins
What allows mitochondria and chloroplasts to exchange small molecules with the cytoplasm in their inner membranes?
Permeases
How do material transverse the outer membranes of mitochondrion and chloroplasts?
Via porins that allow small molecules to cross; these molecules freely diffuse between the cytoplasm and the intermembrane space, leading to similar chemical environments
What types of small molecules can diffuse between the cytoplasm and intermembrane spaces of mitochondrion and chloroplasts?
Water, carbon dioxide, oxygen
How do solutes transverse the inner membrane?
Small molecules (like water, carbon dioxide, oxyge) can diffuse; other molecules can only cross if there are specific permeases
What is the endosymbiotic theory?
The endosymbiotic theory states that some of the organelles in today’s eukaryotic cells were once prokaryotic microbes. In this theory, the first eukaryotic cell was probably an amoeba-like cell that got nutrients by phagocytosis and contained a nucleus that formed when a piece of the cytoplasmic membrane pinched off around the chromosomes. Some of these amoeba-like organisms ingested prokaryotic cells that then survived within the organism and developed a symbiotic relationship. Mitochondria formed when bacteria capable of aerobic respiration were ingested; chloroplasts formed when photosynthetic bacteria were ingested. They eventually lost their cell wall and much of their DNA because they were not of benefit within the host cell. Mitochondria and chloroplasts cannot grow outside their
What evidence is there in support of the theory of endosymbiosis?
- Chloroplasts are the same size as prokaryotic cells, divide by binary fission, and, like bacteria, have Fts proteins at their division plane. The mitochondria are the same size as prokaryotic cells, divide by binary fission, and the mitochondria of some protists have Fts homologs at their division plane.
- Mitochondria and chloroplasts have their own DNA that is circular, not linear.
- Mitochondria and chloroplasts have their own ribosomes that have 30S and 50S subunits, not 40S and 60S.
- Several more primitive eukaryotic microbes, such as Giardia and Trichomonas have a nuclear membrane but no mitochondria.
Outline the endosymbiotic theory as done in the book.
- A fermentative eukaryote phagocytoses a gram-negative respiratory bacterium
- Rather than being digested, the bacterium breaks out of the phagosome and multiplies in the host cell cytoplasm
- If the rate of bacterial multiplication was the same as that of the host, a stable coexistance could be established; probably however the usualy outcome was that the bacterium multiplied rapidly and killed its host
- But over many generations of host and parasite, the parasite loses its ability to make murein and most biosynthetic enzyes
- The former bacterial cell begins to export much of the ATP it makes by respiration to it shost; both partners now benefit from the arrangement
- Many bacterial genes are lost, and others are transferred to the host nucleus; the bacterium is now a mitochondrion
It is believed that one event led to the development of mitochondrion in fermentative eukaryotes. What about chloroplasts?
It is believed that there have been several separate endosymbiotic events with different cyanobacteria inhabiting the cytoplasm of different respiratory host cells, leading to different groups of photosynthetic eukaryotes
Which evolved first: hydrogenosomes, mitochondria, or chloroplasts?
Hydrogenosomes then mitochondrion and finally chloroplasts (multiple different times)
True or false?
Hydrogenosomes are thought to be another relic of symbiosis.
True; they appear to be relics of ancient endosymbioses between a fermentative protist and a bacterium
How are hydrogenosomes and mitrochondrion similar? How are they different?
They are similar in size, but the hydrogenosome lacks the convulutions of the inner membrane
They both appear to be relics of symbiosis between a fermentative protist and bacterium; however, unlike mitochondrion, hydrogenosome are descended from fermentative ancestors and thus lack the TCA cycle and electron transport chains
Insteading of producing NADH from oxidized pyruvate, hydrogenosomes produce hydrogen gas, butyrate and acetate
Provide an overview of the hydrogenosome’s metabolic pathway
Glucose is converted into two molecules of pyruvate, producing 2 ATP and 2 NADH as part of glycolysis (cytoplasm)
One molecule of pyruvate is reduced to butyrate, reoxidizing both molecules of NADH to NAD+, thereby allowing for the continuation of glycolysis (cytoplasm)
The other molecule of pyruvate enters the hydrogenosome, where it is oxidized to carbon dioxide and acetate, producing an additional ATP; hydrogen gas represents the reduced product (hydrogenosome)
Hydrogenosomes produce molecular hydrogen throug _________________.
Fermentation
Hydrogenosomes lack ____________________ and usually lack DNA
Cytochromes
The most current theory suggests that hydrogenosomes evolved similarly to chloroplasts. What does this mean?
That they underwent several repeated evolutions
What three diseases are caused by fermentative protists and hydrogenosomes?
- Giardia lamblia (Giardiasis)
- Entamobea (amoebic dysentery)
- Trichomonas vaginalis (vaginitis/urethritis) (STI)
How are diseases caused by fermentative protists treated?
Metronidazole
What type of compound is metronidazole?
A nitroimidazole
How is metronidazole activated within the body?
Upon intracellular reduction by intracellular transport proteins
How does metronidazole work to combat infections?
Metronidazole is reduced by the pyruvate:ferredoxin oxidoreductase system in obligate anaerobes, which alters its chemical structure. Pyruvate:ferredoxin oxidoreductase normally generates adenosine triphosphate (ATP) via oxidative decarboxylation of pyruvate. With metronidazole in the cellular environment, its nitro group acts as an electron sink, capturing electrons that would usually be transferred to hydrogen ions in this cycle. Reduction of metronidazole creates a concentration gradient that drives uptake of more drug and promotes formation of intermediate compounds and free radicals that are toxic to the cell
What system reduces metronidazole?
Pyruvate-ferredoxin oxidoreductase system
What does pyruvate-ferredoxin oxidoreductase normally do in a fermentative protist?
Generates ATP via oxidative decarboxylation of pyruvate
How does metronidazole disrupt the metabolic processes of fermentative protists?
Its nitro group acts as an electron sink, capturing the electrons that would have been transfered to hydrogen in the hydrogenosome; thus, reduction of metronidazole creates a concentration gradient that drives more uptake of the drug and promotes formation of intermediate metabolites and free radicals toxic to the protist but not the human
What is the typical structure of a chloroplast?
An outer membrane, an inner membrane, and thylakoid membranes
There are more complex chloroplasts. What makes these chloroplasts “complex”?
Some have a third bounding membrane
Some have a functional nucleus or nucleomorph between the two outermost membranes
What two principal proteinaceous structures make up the eukaryotic cytoskeleton?
Microtubules and microilaments
The presence of a cytoskeletal system composed of microtubules and microfilaments is _____________ among eukaryotes.
Universal
Microtubules and microfilaments are involved in _____________ ____________.
Intracellular movements
What binds to microtubules or microfilaments and move along these structures?
Motor proteins
What are microtubules?
Hollow tubes composed of thousands of molecules of tubulin
What monomer constitutes microtubules?
Tubulin
What end of a microtubule is the preferential site of addition or removal of tubulin subunits during growth or shrinkage?
The (+) end
Which end of the microtubule generally gains and loses tubulin subunits slowly?
The (-) end
How are tubulin molecules organized in a microtubule?
Microtubules are composed of tubulin molecules in a dimer that are arranged in a helical pattern
Tubulin is a ___________, consisting of one alpha-tubulin and one beta-tubulin, which associate into a cylinder that is ___ tubulin molecules around.
Dimer
13
A microtubule, as a whole, is ________________.
Asymmetric
Are microtubules static or dynamic structures?
Dynamice; there is a continual process of elongation and shrinkage
What organizes the cell’s microtubule network?
Centrosomes
Most of the microtubules in the interphase eukaryotic cell radiate from a central point, termed a _____________________ __________________ ________________ or centrosome.
Microtubule organizing center
What are centrioles?
Short cylinders composed of nine triplets of fused microtubules
Most microtubules radiate form the _______________
Centriole
Centrioles are located at right angles to each other and are often found where in the cell?
In the middle of the centrosome
What end of the microtubule is located at the centrosome?
The (-) end
What end of the microtubule is oriented toward the periphery?
The (+) end
What is a major function of microtubules?
To provide a set of tracks throughout the cell along which vesicles of the endomembrane system can be transported
What mediates motility of vesicles in the endomembrane system along microtubules?
Motor proteins
In the presence of _______, motor proteins can move along a microtubule, not simply bind to it.
ATP
Motor proteins has two binding sites. What are they?
One binding site is for microtubules; the other is for a membrane vesicle
There are two principal types of microtubular proteins. What are they?
Kinesins
Dyneins
Kinesins move toward the ___ end of microtubules.
(+)
Dyneins move toward the ___ end of microtubules.
(-)
What microtubule motor protein moves toward (+) ends?
Kinesins
What microtubule motor protein moves toward (-) ends?
Dyneins
Microfilaments are chains of _______________ monomers
Actin
What are microfilaments?
They are long, thin, thread-like structures that are around 9 nm thick; they are composed of thousands of monomers of the protein actin; each microfilament has a helical substructure with polarity
What is the the most abudant protein in the eukaryotic cell?
Actin
There is no correlation between the (+) and (-) end of _________________________ and the cell center or periphery
Microfilaments
What are two functions of microfilaments?
Maintain cell shape
Stabilize the membrane
A microfilament consists of how many strands of actin molecules twined around each other?
Two
What motor protein mediates vesicle movement along microfilaments?
Myosins
Where is the role of microfilaments in vesicle transport more important in the cell?
The cell periphery
Why is the role of microfilaments at the cell periphery important?
Because the density of microtubules at the periphery is low
Why is the density of microtubules low at the periphery of the cell?
Because microtubules are radially arranged with their origins at the centrosome near the cell center
What is cytoplasmic streaming?
In algae and fungi, cytoplasm can be seen to be in continuous steady movement around the central vacuole; this is mediated by a band of microfilaments around the periphery of the cell
Many protists have two distinct zones of cytoplasm. What are they?
Ectoplasm
Endoplasm
What is the ectoplasm?
A zone of rigid, gel-like cytoplasm immediately beneath the cell membrane; the mesh of actin filaments is highly cross-linked to form a rigid gel
What is the endoplasm?
A viscous liquid beneath the ectoplasm in which microfilaments are less extensively cross-linked
Cells that lack a cell wall most often move across a solid surface by what type of movement?
Amoeboid movement
Cells may have one or more _____________ or _____________ to aid in motility
Flagella
Cilia
What mediates amoeboid movement?
Microfilaments
Myosin
It is thought that two separate mechanisms are involved in amoeboid motion. What are they?
- Polymerization of actin to form microfilaments at the leading edge of the cell, leading to a protusion called a pseudopod
- Pseudopod extension is driven by hydraulic forces that are created by the contraction of actin and myosin filaments in the ectoplasm at the rear of the cell as a localized thinning of the ectoplasm at the front allows for the pseudopod to move forward
What creates the hydrostatic pressure required to push the pseudopod forward in amoeboid motion?
Contraction of the actin/myosin filaments in the ectoplasm toward the rear of the cell
Flagella and cilia contain a bundle of _________________ that slide against each other
Microtubules
Are eukaryotic flagella and cilia the same as prokaryotic flagella and cilia?
No; they are completely different in structure and mechanism of movement
Flagella and cilia have _________________ structure; they differ in the way they _____________ and in their ______________.
Identical structure
Move
Length
What is the axoneme?
A highly structured bundle of microtubules that runs down the center of flagella and cilia
How is the axoneme structured?
A set of nine peripheral pairs of fused microtubules and a pair of unfused ones in the center; this arrangement is called the 9 + 2 arrangement
Each pair of microtubules in an axoneme is linked to the adjacement one by a set of what proteins?
Dynein motor proteins
What type of motion do flagella and cilia utilize?
Sliding
What provides the power to bend the flagellum?
A pair of dynein arms attached to each pair of microtubules
What pulls the flagellum back to its resting position once dynein arms detach?
Elastic nexin molecules
When is a flagella or cilia limp and flexible?
When the dynein molecules are not attached to the adjacent molecule
When is the flagellum/cilium stiff and rigid?
When the dynein is bound to microtubules but not actively moving
When does the flagellum/cilium actually bend?
When the dyneins on one side move while the dyneins on the other side remain unattached
What ion concentration appears to be critical for flagellum/cilium regulation?
Ca2+
The calcium concentration as well as the calcium binding protein ____________________ and cAMP all appear to be involved in the regulation of flagellum/cilium movement
Calmodulin
What permanently links adjacent pairs of microtubules?
Flexible proteins called nexin
Provide an overview of the flagellar bending mechanism.
- Dynein attaches to an adjacent microtubule pair; the flagellum is stiff
- Binding of ATP causes the dynein to detach and retract; the flagellum is limp
- Hydrolysis of ATP causes the dynein to extend at an angle and attach
- Release of ADP causes the dynein to resume its original (stiff) conformation, sliding the two microtubule pairs relative to each other
- Repetition of the cycle moves the microtubules one more step
Where do flagella and cilia originate?
A centriole-like basal body
What are basal bodies?
The anchoring structures for flagellum and cilium that are very similar or even identical to centrioles; they are found at the base of all flagella and cilia even in cells whose centrosome does not contain a centriole
How are centrioles and basal bodies different?
Centrioles consist of small cylinders composed of nine sets of three fused microtubules
In basal bodies, two of each set of three fused mcirotubules continue past the basal body to become the nine fused pairs that make up the flagellum or cilium core
What stabilizes and interconnects rows of basal bodies in cilia?
Microtubules
How do flagella and cilia differ in their beat pattern and length?
Flagella are generally much longer than cilia
Flagella generally propogate a bend down the length of the filament, beginning at the base, and as the bed moves down, it presses against the medium and moves the cell forward; cilia typically have a pattern of movement that has two distinct phases, a power stroke and a recovery stroke
How does a flagellum move?
Through the propogation of waves via the bending of the flagellum, which usually moves the flagellum forward
How do cilia move?
Via two phases
During the power stroke, most of the filament is rigid, and bending occurs only near the base, which sweeps the cilium through the medium like an oar
During the recovery stroke, the cilium is mostly limp and is pulled back to its original position by a bed opposite that of the power stroke that propagates from the base to the tip
Cilia and flagella are for movement but not always for the organism to move forward. In rotifers, what are cilia utilized for?
Moving water towards the rotifer so that the rotifer can then sift for food
Many protists have a rigid polysaccharide cell wall that resists the cell’s internal _________________ ______________, but none has the chemical or structural complexity of the prokaryotic cell wall
Turgor pressure
Some protists have a _______________ instead of a cell wall, which lies immediately beneat the membrane and is attached to the membrane
Pellicle
Some protists, notably the amoebas, make extracellular shells, or ___________, that protect against predation rather than osmotic stress.
Tests
What usually makes up the eukaryotic cell wall?
Cellulose, pectin, or other polymers of glucose
The wall can have a layered appearance with layers composed of differing polysaccharides, including N-acetylglucosamine, also known as ____________.
Chitin
Ocassionally the polysaccharide wall material is organized into a series of discrete plates that interlock or overlap. In what species does this commonly occur?
Dinoflagellates
Some protists make walls that are heavily impregnanted with inorganic salts like ________________ or ______________.
Silica oxides
Calcium carbonate
What algal group makes elaborately sculptured walls of silica oxides?
Diatoms
What algal group makes cell walls with overlapping plates of calcium carbonate?
Coccolithophorids
What characterizes the pellicle?
A layer of protein or polysaccharide that strengthens the cell membrane in the cytoplasm rather than the exterior
What collects water from the cytoplasm and expels it to the outside via exocytosis?
The contractile vacuole
In protists that lack a rigid wall and inhabit fresh water or other hypotonic environments, water excretion via an elaborate systems of membranes termed the ________________ ____________________ ___________________ is required
Contactile vacuole complex
When the contractile vacuole is full, what happens?
It fuses with the cell membrane and expels its contained water to the outside
Through what structure does exocytosis of water occur in most ciliates and flagellates?
Contractile vacuole pore
What is the contractile vacuole pore?
A localized indentation of the cell membrane into a short cylinder that penetrates through the pellicle
What stabilizes the contractile vacuole pore?
A helical ring of microtubules
How does the contractile vacuole collect water?
Through a system of tubules or vesicles
The process of bringing water into the cell and pushing it out of the cell requires _______.
ATP
The contractile vacuole membrane system has two principal components: the contractile vacuole itself and the _____________, an adjacent system of membranous vesicles or tubules that conveys water to the contracile vesicle
Spongiome
Two broad categories of contractile vacuole complex may be recognized: that characteristic of ________________ in which the contractile vacuole is formed by the coalescence of small vescilers and fragments against into vesicles during discharge; and that characteristic of the ___________ and ___________________ in which a permanent contractile vacuole alternately inflates and collapses
Amoebas
Flagellates
Ciliates
What is closed mitosis?
A process of mitosis in which the nuclear envelope remains intact or nearly so throughout the process
When the nuclear envelope breaks down to allow for microtubule penetration of the nucleoplasm, what type of mitosis occurs?
Open mitosis
Which form of mitosis is most common in protists?
Closed mitosis
There are at least three types of closed mitosis. What are they?
- Mitotic spindle forms entirely within the nucleus; microtubule organizing centers are embedded into the nuclear envelope; nuclear envelope is pinched in middle and nucleus divides into two
- Partial fragmentation of nuclear envelope; gaps develop through which spindle fibers from cytoplasm penetrate nucleoplasm and attach to chromosomes; gaps are repaired and nucleus divides
- Nuclear envelope remains intact; spindle forms in cytoplasm; nucleus wraps around spindle or develops channels through the nucleus; chromosomes attach to nuclear envelope proteins, which attach to the spindle (common in dinoflagellates)
Reproduction is normally ________________ in protists but ________________ reproduction does occur.
Asexual
Sexual
