Chapter 4 - Prokaryotic Cell Structure & Function II Flashcards
Prokaryotes have a _____ __________, which separates the cytoplasm from the exterior environment and allows the cytoplasm to have a chemical composition different from the environment
Cell membrane
Would life be possible without the cell membrane?
No, it would not be; compartmentalization is required for life
What are sterols?
Sterols are molecules in the membranes of eukaryotes that help fill the space caused by unsaturated fatty acid chains and help to maintain the correct fluidity of the membrane over a wide temperature range
Do prokaryotes have sterols in their membranes?
No; very few can make sterols, although a few in the animal body will incorprate host sterols into their membranes
What do prokaryotes have in place of sterols?
Hopanoids
What is the exception to the rule that prokaryotes lack sterols in their membranes?
Mycoplasmas
What are hopanoids?
They are pentacyclic compounds that fold into a shape similar to that of sterols; they are made from a sterol precursor, squalene; the pathway of squalene synthesis is nearly universal among cells, but prokaryotes and eukaryotes differ in the subsequent fate of squalene
What are hopanoids made from?
A sterol precursor called squalene
In prokaryotes, squalene is converted into _____________ while in eukaryotes it is converted into _____________.
Hopanoids
Sterols
The cell membrane is a two-dimensional __________.
Fluid
True or false? The forces that hold individual phospholipids together are weak.
True; it is because of these weak interactions that plasma membranes are fluid with components able to move laterally across the membrane
Although most components of the plasma membrane are fluid, some are not. What is one exception discussed in class?
FtsZ
What is the FtsZ ring?
A protein that starts cell division and is needed in the middle of the cell
What is required for molecules in the plasma membrane to be spatially oriented, meaning that they do not move?
They must be “anchored” in some way to limit their diffusion
The prokaryotic cell membrane has more ________ and different __________ than most membranes.
Protein
Proteins
Why does the plasma membrane of prokaryotes have so many proteins?
Because they lack the specialized membrane-bound organelles present in eukaryotes
Prokaryotic cells have to bring in nutrients and excrete wastes, but their membranes are impermeable to most polar compounds. How then do they get what they need into the cell and get rid of what they do not need?
Permeases
Most prokaryotes have membrane-embedded electron transport proteins to get _____________ from respiration or photophosphorylation.
Energy
What are three primary functions of prokaryotic cell membrane proteins?
- Membrane-embedded electron transport proteins to get energy from respiration or photophosphorylation
- Site of synthesis of cell membrane proteins, outer membrane proteins, periplasmic proteins, secreted proteins, lipids, and cell walls
- Contains sensory proteins that allow cell to determine certain aspects of the chemical or physical conditions in which it resides
Prokaryotic cell membranes are typically about ___% lipid by weight and ___% protein by weight; however, lipids weigh far less, so lipids outnumber proteins by __ to __.
25%
75%
30 to 1
Why do some prokaryotes invaginate their cell membranes?
To provide additional surface area, especially for those that have high respiratory and photophosphorylation rates
Most prokaryotes lack intracellular membrane-bound organelles; however, some prokaryotes do have membrane-bound organelles. The acidocalcisomes store __________ and the anammoxisomes oxidize ammonia and nitrate into _____________ _______.
Phosphate
Nitrogen gas
What is the ExPortal?
An organelle dedicated to the biogenesis of secreted proteins in Streptococcus pyogenes
Although bacterial and eukaryotic cell membranes are always phospholipid bilayers, _____________ cell membranes may be a bilayer or monolayer of ether lipids.
Archael
What phospholipid generally makes up archael cell membranes?
Glycerol-diphytane-diethers
How do glycerol-diphytane-ethers differ from traditional lipids?
- The lipids are branched, usually saturated hydrocarbon chains
- The hydrocarbon side chains are linked to the glycerol by ether bonds rather than ester bonds
In some thermophilic extremophiles, two diethers can be linked covalently tail to tail to form a __________-____________-__________ that span the membrane, leading to a monolayer.
Diglycerol-dibphytane-tetraether
All cells have special structure that contain chromosomes. In eukaryotic cells, the structure is bounded by a double-layered membrane, and the entire structure is called a nucleus. In prokaryotic cells, there is no nuclear envelope, and the structure is called a _____________________.
Nucleoid
In general, obligate symbionts and parasites have genomes much _________ than free-living, host-associated microbes.
Smaller
The nucleoid is _________________ and usually contains a single circular chromosome.
Haploid
Most bacteria have a single, circular chromosome with _____________________ origin of replication.
A single origin of replication
What are plasmids?
Plasmids are small circular DNA molecules that are distinguished from chromosomes by the lack of genes essential for growth under all conditions and for core cellular functions
Is it possible for multiple nucleoids to be present in a prokaryotic cell?
Yes, in prokaryotic cells undergoing rapid multiplication, two to four nucleoids containing identical copies of the chromosome may be seen
The prokaryotic chromosome is ____________________.
Supercoiled
What does it mean to say that a chromosome is supercoiled?
The double helix itself is twisted such that the double helix is under stress to unwind but the unwinding tension is opposed by the hydrogen bonds between bases
What enzyme introduces supercoiling?
DNA gyrase
What category of proteins assist in maintaining the proper amount of supercoiling?
HIstones or histone-like proteins
_________________________ is essential to proper transcription and DNA repair; cells that are unable to engage and maintain such structure will die.
Supercoiling
Is the nucleoid a highly ordered structure?
Yes, it is not simply a concentration of DNA
How is the nucleoid arranged?
The nucleoid is arranged with the origina of replication at one end and the terminus at the other; in between the chromosome is condensed into approximately 500 loops by special condensing proteins; the central portions of the nucleoid are very densely packed
Where does transcription occur?
On the surface of the nucleoid
Why does transcription occur on the surface of the nucleoid?
Because the densely packed core transitions to a more loosely packed region where the longer loops extend into the cytoplasm at the nucleoid surface
How does DNA gyrase work?
It takes a loop of DNA, makes a double stranded cut where the loop crosses over, passes the top part of the loop through the gap, and reseals the broken DNA; this introduces one new twist to the DNA
Transcription and translation in prokaryotes occur simultaneously and are said to be _______________.
Coupled
What does the compaction of DNA into the nucleoid require?
Neutralization of DNA charges by polyamines
Why does neutralization of DNA need to occur for DNA compaction?
Because at neutral pH, the phosphate groups on DNA carry a negative charge, thus making it difficult to pack DNA tightly
In eukaryotic cells, how is the negative charge of DNA neutralized?
Basic histone proteins
What neutralizes charge on DNA in prokaryotes?
Polyamines
What are polyamines?
Small organic compounds with two, three, or four amino groups spaced approximately as far apart as the phosphates in DNA
How are polyamines able to neutralize DNA?
Because their amino groups are protonated and thus positively charged at neutral pH, they can effectively neutralize the charges on DNA and llow it to pack tightly
Why is coupling of protein synthesis important in prokaryotes?
Because it is the basis for attenuation, a mechanism of gene regulation
What is attenuation?
Transcriptional attenuation is a regulatory mechanism that causes premature termination of transcription under certain conditions, thereby preventing the expression of the mRNA required for expression of the corresponding gene products
What are the two principle proteins of the eukaryotic cytoskeleton?
Actin
Tubulin
True of false: Prokaryotes have proteins that are part of a cytoskeleton functionally similar to that of eukaryotes.
True; it was previously believed that prokaryotes lacked any type of cytoskeleton, but we now know this to be incorrect
What maintains prokaryotic cell shape and form the prokaryotic mitotis appartus?
The actin-like cytoskeleton
What type of bacteria have a helical band of filaments made of an actin-like protein called MreB?
Bacillus and spirilla
What is MreB?
An actin-like protein that underlies the cell membrane of prokaryotic cells that has a similar tertiary structure to actin but lacks sequence similarity (only about 10% conservation)
When MreB filaments are disruption, what occurs?
Spherical cells developed; thus it is believed they determine cell shape
In addition to helping determine cell shape, what else are MreB filaments thought to be involved with?
Intracellular motility
What specifically does MreB seem associated with regarding intracellular motility?
They seem to consitute the prokaryotic equivalent of a mitotic apparatus. Soon after chromosome replication, specific sequences near the duplicated origin regions - the prokaryotic equivalent of centromeres - appear to become attached to MreB filaments and are moved rapidly to opposite ends of the cell
What is involved in bacterial cytokinesis?
A tubulin-like protein
What type of filament does the tubulin-like protein form?
A band of filaments around the middle of the prokarytoic cell, which contract and separate the cells
What is the name of the tubulin-like protein found in prokaryotes?
FtsZ
What protein may help FtsZ find the middle of the cell?
The Min proteins prevent the FtsZ ring from being placed anywhere but near the mid cell and are hypothesized to be involved in a spatial regulatory mechanism that links size increases prior to cell division to FtsZ polymerization in the middle of the cell
When chromosome segregation is completed by the actin-like ______ filaments, the _____ ______ contracts, constricting the cell and initiating cytokinesis
MreB filaments
FtsZ ring
What is the cytoplasm?
- A concentrated solution of proteins and small molecules
- A dense suspension of ribosomes
- An area of storage granules of reserve nutrients, gas vacuoles, or other structures
The cytoplasm is a dense suspension of ________________
Ribosomes
There are typically how many ribosomes per prokaryotic cell?
Tens of thousands
Why do prokaryotic cytoplasms have a granular appearance in thin sections in electron microscopy?
Because they have an immense number of ribosomes
Why are there so many ribosomes in the cytoplasm?
Because making proteins is the principal job of a growing cell
What are the sites of protein synthesis?
Ribosomes
In growing cells, where are many ribosomes located?
Bound to the inner surface of the cell membrane
What ribosomes make soluble proteins in the cell?
Cytoplasmic ribosomes
What ribosomes make all of the membrane proteins of the cell as well as the periplasmic and secretory proteins?
Ribosomes bound to the inner surface of the cell membrane
What are storage granules?
Polymeric reserves of nutrients
What are the two most common reserves?
Glycogen
Polyhydroxybutyrate (PHB)
What is glycogen a polymer of?
Glucose
What is polyhydroxybutyrate a polymer of?
Beta-hydroxybutyric acid
Why are reserves stored as polymers?
To prevent turgor pressure from going up too much - 100,000 molecules of surgar in a prokaryotic cell would have a large impact on the osmolarity of the cytoplasm; 100 molecules of polysaccharide, each 1000 sugar residues long, would contain the same amount of sugar but essentially no impact on osmolarity
After carbon, what is the most often countered compound in storage granules?
Sulfur
How is sulfur stored in the cell if it is stored at all?
As elementary sulfure or polysulfide
Where are sulfur deposits usually located in the cell?
These sulfur globules are more liquid than solid and are usually deposited in the periplasm
Where is phosphate stored?
In membrane-bound esicles called acidocalcisomes
Are acidocalcisomes considered organelles?
Yes
How is phosphate stored in acidocalcisomes?
As polyphosphate, long polymers of phosphate up to 700 phosphates in length
Is the interior of an acidocalcisome acidic or basic?
Acidic
How is phosphate transported into an acidocalcisome?
As pyrrophosphate and then polymerized into polyphosphate
What drives the proton pump of the acidocalcisome?
Pyrophosphate hydrolysis
What acidifies the acidocalcisome?
A proton pump in the membrane drive by pyrophosphate hydrolysis
How is the high density of negative charge in the acidocalcisome partially neutralized (e.g., phosphate groups are negatively charged)?
By the pumping in of hydronium ions and partially by divalent cations like calcium for which an ATP-drive pump is embedded in the membrane of acidocalcisomes
What provides buoyancy to aquatic cells?
Gas vacuoles
What are two ways that prokaryotes remain afloat?
Motility
Gas vacuoles
What are gas vacuoles composed of?
Many individual gas vesicles
What are gas vesicles?
Hollow tubular structures of protein with conical ends; the proteins of which have a highly hydrophobic interior surface so that as vesicles form in water, they fill with whatever gas is in solution in the cytoplasm
What gas fills the gas vesciles?
Whatever gas is in solution in the cytoplasm
What allows prokaryotic cells to distinguish north from south?
Magnetosomes
What is a magnetosome?
A row of granules of a magnetic mineral, enclosed in membranous invaginations of the cell membranes; the row of magnetite granules is organized by cytoskeletal filaments of actin-like protein MreB that lie just inside the membrane
How are magnetosomes and sulfur globules similar?
They are both periplasmic - magnetosomes exist within invaginations of the cell membrane and are thus periplasmic, just as sulfur globules are
How are magnetite granules organized?
By actin-like protein MreB
What are bacteria with magnetosomes called?
Microaerophiles
How do microaerophiles use magnetosomes?
To find the right concentration of oxygen
What is magneto-aerotaxis?
Magnetotactic bacteria orient and migrate along geomagnetic field lines. Magneto-aerotaxis increases the efficiency of respiring cells to efficiently find and maintain position at a preferred microaerobic oxygen concentration
What type of bacteria did we look at in class that has magnetosomes?
Magnetospirillum magnetotacticum
Is the bacterial magnetosome a uniquely prokaryotic organelle?
Yes, it is
What is the surface layer of protein external to the envelope called?
An S-layer
What are capsules?
Gelantinous layers of polysaccharide external to the wall and the S-layer, if present
What are the two most important roles of capsules?
Avoiding phagocytosis
Promoting adherence
What can capsules accomplish for pathogens that infect animal tissues?
They can prevent immune system recognition
What is a slime layer?
Excess capsular material that tend to slough off rather than remain with the cell
What is a true capsule?
A discrete detectable layer of polysaccharides deposited outside the cell wall
What is a slime layer?
A less discrete structure or matrix which embeds the cell
What is a glycocalyx?
A thin layer of tangled polysaccharide fibers that occurs on the surface of cells growing in nature
How do dental caries relate to bacterial capsules?
Bacteria in human mouoth must attach to tooth enamel or epithelial cells to avoid being swallowed. They do so by means of their capsules. As these bacteria metabolize nutrients, particularly sugars, they produce acids that dissolve the underlying enamel, forming a cavity or dental caries
Why do we brush and floss our teeth?
To remove adherent bacteria, not to remove food particles
Griffith’s description of “rough” versus “smooth” strains of strep were due to _____________
Capsules
According to Dr. Bates, a glycocalyx can be either a __________ ______________, which is easily washable, or a ______________, which is firmly attached and tends to float
Slime layer
Capsule
What is a biofilm?
An aggregation of cells held together in a common matrix of capsular material
Do biofilms contain a single kind of microbe?
They can contain a single kind of mircobe or they can be mixed
What are the maturation steps of a biofilm?
- Motile cells attach to a surface and begin to produce copious amounts of extracellular polysaccharides
- Cells in the biofilm become immotile and multiply
- Channels are formed through the biofilm to facilitate nutrient diffusion
- Cells of other species may be attracted to form a mixed biofilm; some cells regain motility and leave the biofilm; and
What is a problem that biofilms pose to humans?
Cells in a biofilm are much more resistant to antibiotics
What structures mediate the specific attachment of cells to other cells?
Pili
What are pili?
Pili are fine, hair-like appendages protruding from the cell surface; they are fibers made up of many molecules of globular proteins
What holds together the globular proteins that make up pili?
Quarternary forces
What is the name of the protein monomer that makes up a pilus?
Pilin
The proteins at the very tip of a pilus are often different from the pilins that make up the bulk of the pilus. What are the molecules at the tip called?
Adhesins
What is the role of adhesins?
They have specific binding sites that allow the pilus to attach to specific structures
How are type I pili formed?
- Pilins are synthesized by ribosomes on the cell membrane and then secreted into the periplasm (of gram-negative bacteria)
- Periplasmic chaperone proteins escort the pilins across the periplasm and transfer them to the growing pilus
- Pili are assmbled by the addition of pilins at the base
- Special adhesin proteins are usually at the tip of pili
The pilus is a ______________ thred of pilin molecules.
Helical
What determines the specificity of attachment of a pilus?
The adhesins at the tip of the pilus
Are flagella organelles?
No
What are flagella?
They are rigid, helical structures that rotate and move cells through liquid
What are flagella composed of?
Flagellin
How are flagellin molecules added?
At the tip, not the base
How is the formation of pili and flagella different?
Flagellin molecules are added at the tip of the structure
Pilin molecules are added at the base of the structure
Flagella typically have three distinct structural regions. What are they?
- Filament
- Hook
- Basal body
WhatThere are four types of flagellation discussed in class. What are they?
- Polar
- Bipolar
- Peritrichous
- None
What is polar flagellation?
When one or several flagella are found at the top of rod-shaped or spiral cells
What is peritrichous flagellation?
A pattern of flagellation in which flagella may be distributed over the entire surface
What is bipolar flagellation?
Flagellation on two opposing sides of the cell
What part of the flagellum provides the motive force for swimming?
The filament
What is a flagellum’s filament attached to?
The hook, a flexible region at the surface of the cell
To what does the hook of a flagellum connect?
The basal body
What is the basal body?
It is the part of the flagellum that embeds the flagellum firmly in the cell envelope and membrane and acts as a microscopic rotatry motor to rotate the hook and filament
How do flagella function?
By being rotated; because they are helical, their rotation exerts force on the cell; in most cases, prokaryotic flagella push the cell in only one direction
What do flagellum use as their energy source?
Proton motive force
Flagella are rotated by the entry ions through the _____________ _______.
Basal body
How many disks make up the basal body of prokaryotic flagellum?
Two
The bottom one of which is attached to a rod that penetrates through a hole in the other disk and attaches to the hook; the other disc is attached to the murein
Which disk rotates during prokaryote flagella movement?
The bottom one; the other one remains stationary
What does gliding motility require?
Contact with a surface
In what group of bacteria has gliding been studied best?
Myxobacteria
What two mechanisms seem to be available for prokaryotes that glide?
- Pili-mediated gliding
- Polysaccharide extrusion gliding
What is Brownian motion?
The appearance of movement based upon the random knocking of water into lipid droplets or bacteria; it is completely random and not a type of motility
