Cell Structure and Function Part 2 Flashcards
1
Q
Nutrient Transport
A
- Carrier-mediated transport systems
- Show saturation effect
- Highly specific
2
Q
What are the three transport events?
A
- Uniport
- Symport
- Antiport
3
Q
Uniport
A
uniporters transport in one direction across the membrane
4
Q
Symport
A
Symporters function as co-transporters
5
Q
Antiport
A
Antiporters transport a molecule across the membrane while simultaneously transporting another molecule in the opposite direction
6
Q
Simple Transport
A
- Lac permease of Escherichia coli
- Lactose is transported into E. coli by the simple transporter lac permease, a symporter - two molecules move across membrane in same direction
- Activity of lac permease is energy-driven
- Transports lactose and a H+ into the cell simultaneously
7
Q
ABC (ATP-binding cassette) transport systems
A
- More than 200 ABC transporter
- Involved in uptake of organic compounds (e.g. sugars, amino acids), inorganic nutrients (e.g. sulfate, phosphate), and trace metals
- Typically display high subtrate specificity
- Gram-negatives employ periplasmic-binding proteins and ATP-driven transport proteins
- Gram-positive employ subtrate-binding lipoproteins (anchored to external surface of cell membrane) and ATP-driven transport proteins.
8
Q
Nutrient Transport
A
- ABC transporters (ATP-binding cassette)
- Solute binding protein
- Periplasm
- Binds specific subtrate
- Integral membrane proteins (transporter)
- ATP-hydrolyzing protein
- Supply energy for the transport event
9
Q
Group Translocation
A
- e.g. phosphotransferase system in E. coli
- Sugar is phosphorylated during transport across the membrane
- Moves glucose, fructose, and mannose
- Phosphoenolpyruvate (PEP) donates a P to a phosphorelay system
- P is transferred through a series of carrier proteins and deposited onto the sugar as it is brought into the cell
10
Q
What are the three major classes of transport systems in prokaryotes?
A
- Simple transport
- Group translocation
- ABC system
- All require energy in some form, usually proton motive force or ATP
11
Q
Simple Transport (Simple)
A
- One protein
- Driven by the energy in the proton motive force
12
Q
Group Translocation (Simple)
A
- Two protein
- Chemical modification of the transported substance driven by phosphoenolppyruvate
13
Q
ABC Transporter (Simple)
A
- Three protein
- Periplasmic binding proteins are involved and energy comes from ATP
14
Q
Cell Wall of Bacteria and Archaea
A
- Outside the cell membrane
- Rigid
- Helps determine cell shape
- Rigid
- Not a major permeability barrier
- Porous to most small molecules
- Protects the cell from osmotic changes
15
Q
Function of the cell wall
A
- Cell wall prevents cell expansion - protects against osmotic lysis
- Protects against toxic substances - large hydrophobic molecules
- EX) detergents, antibiotics
- Pathogenicity
- Helps evade host immune system
- Helps bacterium stick to surfaces
- Partly responsible for cell shape
16
Q
Peptidoglycan (PG)
A
- Species of Bacteria separated into two groups based on Gram stain
- Gram-positives and gram-negatives have different cell wall
- Gram-negative cell wall
- Two layers: LPS and peptidoglycan
- Gram-positive cell wall
- one layer: peptidoglycan
- Gram-negative cell wall
17
Q
Peptidoglycan (Part 2)
A
- Rigid layer that provides strength to cell wall
- Polysaccharide composed of:
- N-acetylglucosamine and N-acetylmuramic acid
- Amino acids
- Lysine or diaminopimelic acid (DAP)
- Cross-linked differently in gram-negative bacteria and gram-positive bacteria
- Form glycan tetrapeptide
18
Q
Peptidoglycan (Part 3)
A
- More than 100 different PG structures identified
- Vary in peptide cross-links and/or interbridge
- No interbridge is present in gram-negative Bacteria (e.g. E. coli)
- The interbridge in Staphylococcus aureus (gram positive) is made up of 5 glycine residues
19
Q
Peptidoglycan (Part 4)
A
- Gram-positive cell walls
- Contain up to 90% peptidoglycan
- Common to have teichoic acids (acidic substances) embedded in their cell wall
- Lipoteichoic acids: teichoic acids covalently bound to membrane lipids
20
Q
Model of Peptidoglycan Surrounding the Cell
A
- Backbone formed of NAM and NAG connected by glycosidic bonds
- Crosslinks formed by peptides
- Peptidoglycan strand is helical
- Allows 3-dimensional crosslinking
- E. coli has one layer
- Some cell walls can be 50 - 100 layers thick, e.g. Bacillus species
21
Q
Prokaryotes that lack cell walls?
A
- Mycoplasmas
- Thermoplasma
22
Q
Mycoplasmas
A
- Group of pathogenic bacteria
- Have sterols cytoplasmic membrane - adds strength and rigidity to membrane
23
Q
Thermoplasma
A
- Species of Archaea
- Contain lipoglycans in membrane that have strengthening effect
24
Q
LPS: The Outer Membrane
A
- Total cell wall contains around 10% peptidoglycan
- Most of cell wall composed of outer membrane, aka lipopolysaccharide (LPS) layer
- LPS consists of core polysaccharide and O-polysaccharide
- LPS replaces most of phospholipids in outer half of outer membrane
- Endotoxin (lipid A): the toxic component of LPS
25
LPS: The Outer Membrane (Part 2)
- Periplasm: space located between cytoplasmic and outer membranes
- around 15 nm wide
- Contents have gel-like consistency
- Houses many proteins
- Porins: channels for movement of hydrophilic low-molecular-weight substances
26
Relationship of Cell Wall Structure to Gram Stain
- In Gram stain reaction, insoluble crystal violet-iodine (CV-I) complex forms inside cell
- Complex is extracted by alcohol from gram-negative, not gram-positive bacteria
- Gram-positive bacteria have thick cell walls consisting mainly of
peptidoglycan
- Becomes dehydrated during alcohol step so pores in wall close
- Prevents CV-I complex from escaping
- Gram-negative bacteria – alcohol penetrates OM
- CV-I extracted from cell
- Cells appear nearly invisible until counterstained with second dye (safarin)
27
Archaeal Cell Walls
- No peptidoglycan
- Typically no outer membrane
- Pseudomurein
- Polysaccharide similar to peptidoglycan
- Composed of N-acetylglucosamine and N-acetyltalosaminuronic acid
- Found in cell walls of certain methanogenic Archaea
- Cell walls of some Archaea lack pseudomurein
28
Archaeal Cell Walls (S-layers )
- Most common cell wall type among archaea
- Consist of protein or glycoprotein
- Paracrystalline structure
- Some Archaea have only S-layer (no other cell wall components)
- Most have additional cell wall elements
29
Summary of Archaeal Cell Wall
- Variety of structure possible
- Some closely resemble peptidoglycan
- Others lack polysaccharide completely
- Most Archaea contain some type of cell wall structure - functions to prevent osmotic lysis and give shape
- Because they lack peptidoglycan, Archaea are resistant to lysozyme and penicillin
30
Cytoplasm
Material bounded by plasma membrane (PM)
31
Protoplast
- PM and everything within
- Macromolecules: amino acids, nucleotides, etc
- Soluble proteins
- DNA and RNA (nucleoid)
32
Proteins
- Serve many functions:
- Enzymes: Catalyze chemical reactions
- Transport proteins: Move other molecules across membranes
- Polypeptide: A long polymer of amino acids joined by peptide bonds
- Proteins are made of polypeptides
33
Structural Proteins
- Help determine shape of the cell
- Involved in cell division
34
The Nucleoid
- Region that contains the genome
- The typical bacterial genome:
- Single circular double stranded (ds) DNA chromosome
- May have one or more plasmids
- Smaller circular dsDNA
- Self-replicating
- Carry non-essential genes
- Selective advantage
- EX) Genes for antibiotic resistance
35
DNA
- Carries genetic info of all living cells
- Polymer of deoxyribonucleotides
36
Ribosomes
- Site of protein synthesis
- 70S ribosome
- 2 parts
- 30S subunit (Small subunit)
- Protein
- 16S rRNA
- 50S subunit (Large subunit)
- Protein
- 23S and 5S rRNA
- Cytoplasmic ribosomes
- Cytoplasmic proteins
- PM associated ribosomes
- Membrane proteins
- Proteins to be exported from the cell
37
Cell Surface Structure (Capsule and Slime Layers)
- Polysaccharide / protein layers
- May be thick or thin, rigid or flexible
- Assist in attachment to surfaces
- Protect against phagocytosis
- Resist desiccation
38
Cell Surface Structure (Fimbriae)
- Filamentous protein structures
- Enable organisms to stick to surfaces or form pellicles
39
Cell Surface Structure (Pili)
- Filamentous protein structures
- Typically longer than fimbriae
- Assist in surface attachhment
- Facilitate genetic exchange between cells (conjugation)
- Type IV pili involved in twitching motility
40
Cell Inclusion Bodies
- Visible aggregates in cytoplasm
- Carbon storage polymers
- Poly-B(Beta)-hydroxybutyric acid (PHB): lipid
- Glycogen: glucose polymer
- Polyphosphates: accumulations of inorganic phosphate
- Sulfur globules: composed of elemental sulfur
- Magnetosomes: magnetic storage inclusions
41
Inclusion Bodies
- Carbon storage polymers
- Poly-B(Beta)-hydroxybutyric acid (PHB)
- Lipid storage
- Glycogen granules
- Polymer of glucose
- Inorganic inclusions
- Polyphosphate granules: volutin
- Storage of phosphate and energy
- Sulfur globules
- Storage of sulfur used in energy generation
42
Inclusion Bodies (Magnetosomes)
- Magnetic inclusions
- Intracellular granules of F3O4 or Fe3S4
- Gives the cell magnetic properties
- Allows it to orient itself in a magnetic field
- Bacteria migrate along Earth's magnetic magnetotaxis
43
Gas Vesicles
- Confer buoyancy in planktonic cells
- Spindle-shaped, gas-filled structures made of protein
- Function by decreasing cell density
- Impermeable to water
44
Endospores
- Highly differentiated cells resistant to heat, harsh chemicals, and radiation
- "Dormant" stage of bacterial life cycle
- Ideal for dispersal via wind, water, or animal gut
45
Endospores (Part 2)
- Produced only by some gram positives
- EX) Bacillus sp. - aerobic Gram + rods
- Clostridium sp. - anaerobic Gram + rods
- Vegetative cell - capable of normal growth
- Metabolically active
- Endospore - dormant cell, formed inside of a mother cell
- Metabolically inactive
- Triggered by lack of nutrients
- Takes about 8 - 10 hours
46
Endospores (Layers + Core)
- Layers:
- Spore coat and cortex - protect against chemicals, enzymes, physical damage, and heat
- Two membranes - permeability barriers against chemicals
- Core:
- Dehydrated - protects against heat
- Ca-dipicolinic acid and SASPs (small acid soluble proteins)
- Protect against DNA damage
47
Endospores can resist to?
- Boiling for hours
- UV, y radiation
- Chemical disinfectants
- Dessication
- Age
48
Lifecycle of a spore forming bacterium
- Stage I: Asymmetric Cell Division
- Stage II: Septation
- Stage III: Mother Cell engulfs the Forespore
- Stage IV: Formation of the Cortex
- Stage V: Coat synthesis
- Stage VI: Endospore matures
- Stage VII: Mother cell is lysed
49
Stage I: Asymmetric Cell Division
- DNA replicates
- Identical chromosomes pulled to opposite ends of the cell
50
Stage II: Septation
- Divides cell into 2 unequal compartments:
- Forespore (prespore)
- Mother cell
51
Stage III: Mother Cell engulfs the Forespore
- Forespore surrounded by two membranes
52
Stage IV: Formation of the Cortex
- Thick layers of peptidoglycan form between two membranes
- Highly cross-linked layer - core wall
- Loosely cross-linked layer - cortex (around 1/2 of spore volume)
53
Stage V: Coat synthesis
- Protein layers surround the core wall
- Spore coat
- Exosporium (found in some G+)
- Protect the spore from chemicals and enzymes
54
Stage VI: Endospore matures
- Core is dehydrated
- around 10 - 30 % of a vegetative cell's water content
55
Stage VII: Mother cell is lysed
- Mother cell disintegrates
- Mature spore is released
56
Flagella and Swimming Motility
- Hollow protein filaments
- Impact motility
- Must be stained to view
- Flagella stain
- Can be used for identification:
- Monotrichous - single flagellum
- Polar or subpolar
- Amphitrichous: Flagella at opposite ends
- Lophotrichous: Multiple flagella in a single tuft
- Peritrichous: Flagella distributed around cell
57
Flagellar Structure
1. Filament
2. Hook
3. Basal Body (motor)
58
Filament
- Rigid helical protein (around 20µm long)
- Composed of identical protein subunits - flagellin
59
Hook
Flexible coupling between filament and basal body
60
Basal Body (motor)
- Consists of central rod that passes through series of rings:
- L ring - LPS layer
- P ring - Peptidoglycan
- MS ring - Membrane
- C - ring - Cytoplasm (associated with membrane)
61
Flagella Movement
- Energy to turn the flagella comes from the proton motive force (PMF)
- Gradient of protons (H+) across the cytoplasmic membrane
- High [H+] outside
- Low [H+] inside
- Mot proteins form a channel that allows H+ to move into the cytoplasm
- Provides the energy to turn the flagellum
- Flagellum turns like a propeller to drive the cell forward
62
Flagellar Synthesis
- Several genes are required for flagellar synthesis and motility
- MS ring is made first
- Other proteins and hook are made next
- Filament grows from tip
63
Differences in swimming motions
- Peritrichously flagellated cells move slowly in a straight line
- Polarly flagellated cells move more rapidly and typically spin around
64
Gliding Motility
- Flagella-independent motility
- Slower and smoother than swimming
- Requires surface contact
- Mechanisms
- Excretion of polysaccharide slime
- Type IV pili
- Gliding-specific proteins
65
Chemotaxis and Other Taxis
- Taxis: directed movement in response to chemical or physical gradients
- Chemotaxis: response to chemical
- Phototaxis: response to light
- Aerotaxis: response to oxygen
- Osmotaxis: response to ionic strength
- Hydrotaxis: response to water
66
Chemotaxis
- Best studied in E. coli
- Bacteria respond to temporal, not spatial, difference in chemical concentration
- "Run and tumble" behavior
- Attractants and repellants sensed by chemoreceptors
- Directed movement toward an attractant or away from a repellent
- Biased random walk
- EX) E. coli shows biased random walk toward glucose when there is a concentration gradient
- The cell still exhibits a series of runs and tumbles
- If it senses that the [glucose] is increasing:
- The tumble is delayed
- The run lasts longer
67
Measuring Chemotaxis
- Measured by inserting a capillary tube containing an attractant or a repellent in a medium of motile bacteria
- Can also be seen under a microscope
68
Cell Size (Eukaryotes)
- Lower surface area to volume ratio
- Need more sophisticated transport mechanisms
- Grow slower
69
The Nucleus and Cell Division (Eukaryotes)
- Genetic material is housed in a nucleus
- Generally larger than prokaryotes
- Complex internal structure
- Membrane bound organelles
- Intra-cytolplastic membranes used for transport
- Cytoskeleton
- Divide by mitosis and meiosis
70
Key Differences Between Prokaryotic & Eukaryotic Cells
- Prokaryotic
- Size of Cell: Typically 0.2 - 2.0 um diam
- Nucleus: No nuclear membrane or nucleolus (nucleoid)
- Membrane-enclosed organelles: Absent
- Ribosomes: Smaller size (70S)
- Chromosomal DNA: Singular, circular
- Cell Division: Binary fission
- Eukaryotic
- Size of Cell: Typically 10 - 100 um diam
- Nucleus: True nucleus with nuclear membrane and nucleolus
- Membrane-enclosed organelles: Present (e.g. Golgi, mitochondria, chloroplasts, etc.)
- Ribosomes: Larger Size (80S)
- Chromosomal DNA: Multiple linear chromosomes with histones
- Cell Division: Mitosis
71
The Nucleus
- The nucleus holds the genetic information
- Multiple linear dsDNA chromosomes
72
Chloroplasts
- Site of photosynthesis
- Chlorophyll
- Surrounded by 2 membranes
- DNA and ribosomes (70S)
- Most closely related to Cyanobacteria
- Blue-green algae
73
Mitochondria
- Site of respiration and oxidative phosphorylation
- Surrounded by 2 membranes
- DNA and ribosomes (70S)
- Most closely related to Rickettsia
- Proteobacteria
- Obligate intracellular pathogens
- EX) Rocky-mountain spotted fever
74
The Endosymbiotic Hypothesis
- Mitochondria and Chloroplasts evolved from bacteria
- Evidence
- Semi-autonomous
- Circular chromosomes
- Lack histones
- 70S ribosomes
- Two membranes
- Outer membrane has porins
75
Viruses
- Acellular infectious particles
- Obligate intracellular pathogens
- Reproduce only inside of living cells
- Lack independent metabolism
- Composed of at least 2 parts:
- Nucleic acid genome (DNA or RNA) and protein coat (capsid)
- Together = Nucleocapsid
- Some viruses have an envelope – layer of lipid surrounding the
nucleocapsid
76
Viral Genomes
- DNA or RNA - never both (at the same time)
- Single stranded or double stranded
- Circular or linear
- Can be in several pieces - segmented
- Genome size
- Smallest: around 3.6 kb (3,600) for some ssRNA viruses (3 genes)
- Largest > 150 kb (150,000) for some dsDNA viruses (> 100 genes)
77
Virion Structure (Capsid)
- Protein coat that surronds the genome
- Allows transfer of viral genome between host cells
- Made of identical polypeptides - Protomer
- Helical capsids
- Protomers form a spiral cylinder
- Nucleic acid genome coiled inside
- Ex. Tobacco mosaic virus capsid is made of around 2100 identical protomers
78
Structure of the Virion (Icosahedral capsids)
- Regular geometric shape with 20 triangular faces
- Exhibit symmetry
- Protomers aggregate to form capsomeres
- Ex. Human papillomaviruses have form their capsids from pentamers (clusters of 5)
79
Structure of the Virion (Binal capsids)
- Geometric head with an attached helical tail
- Ex. T4 bacteriophage of E. coli
- Genome is carried in a polyhedral head, helical tail is used to inject DNA into a host cell
80
Nucleocytoplasmic Large DNA Viruses
- Viruses with complex multi-layered structure
- Ex. Mimivirus (infects amoebae)
- 0.75 µm in diameter, 1200 kbp DNA
- Larger than some bacteria
81
Structure of the Virion (Envelope)
- A lipid bilayer surrounding the nucleocapsid that was acquired from the host membrane
- Consists of host lipids and viral proteins - spikes
- Ex. Influenza virus
- Flexible helical capsid, surrounded by an envelope
- Two major spikes: hemagglutinin (H) and neuraminidase (N)
82
Host Range
- Viruses infect all domains of life
- Bacteriophage (phage) - viruses that infect bacteria
- Ex) T4 Phage - infects E. coli
- Animal viruses - infect and multiply only inside of animal cells
- Ex) Human papillomavirus - infects human epithelial cells
- Causes benign tumors (warts)
83
Host Range (Part 2)
- Most viruses are specific to a single host species
- Virus must attach to specific receptors on the host cell surface
- Ex) HIV binds to CD4
- Chemoreceptor on surface of some human
immune system cells
- HIV infects only humans
- Some viruses infect more than one species
- Ex) Influenza attaches to a glycoprotein found on
surface of several animal cells
- Infects humans, pigs, chickens, seals etc.
84
Viral Replication Cycle
1. Adsorption - attachment to the host cell
2. Penetration and uncoating - entry into the host cell
3. Synthesis of viral nucleic acids and protein
4. Assembly of new virions
5. Release of new virions
85
1. Adsorption - attachment to the host cell
- Involves specific receptors on the host cell surface
- Ex) LPS, outer membrane proteins or glycoproteins
86
2. Penetration and uncoating - entry into the host cell
- Bacteriophage - usually inject their nucleic acid into the cell
- Leave the capsid outside the cell as a "ghost"
87
Entry by Animal Viruses
- Fusion with the plasma membrane (Only possible with an envelop viruses)
- Endocytosis
- BInding to specific receptors triggers normal endocytic activity (Naked viruses and Envelop Viruses)
- In either case, once inside:
- The capsid is removed
- Viral genome is released into the cell
88
3. Synthesis of viral nucleic acids and protein
- Viral genes are expressed and viral proteins are synthesized (by the host's own ribosomes)
- Viral genome is replicated (by the host's replication machinery)
89
4. Assembly of new virions
- Viral proteins are assembled into capsids, and then genomes are packaged into nucleocapsids
- Viruses do not reproduce by division
90
5. Release of new virions
- Two basic strategies:
- Naked viruses usually accumulate, eventually lysing the host cell to release progeny - lytic infection
- Enveloped viruses are usually released by budding
- Virions push through the cytoplasmic membrane without killing the host cell - persistent infection
91
Viral Replication Cycle (Budding)
- Release of enveloped viruses
- Viral proteins inserted into the host membrane - Spikes
- Nucleocapsid associates with the spikes, and buds through the membrane to form the envelope
- Ex) Influenza
- Neuraminidase allows new virions to exit the host cell
- Hemagglutanin allows viruses to absorb to the next host
