MBIO 1010 > Cell Structure and Function Part 2 > Flashcards

Cell Structure and Function Part 2 Flashcards

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1
Q

Nutrient Transport

A
  • Carrier-mediated transport systems
    • Show saturation effect
    • Highly specific
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2
Q

What are the three transport events?

A
  1. Uniport
  2. Symport
  3. Antiport
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3
Q

Uniport

A

uniporters transport in one direction across the membrane

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4
Q

Symport

A

Symporters function as co-transporters

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5
Q

Antiport

A

Antiporters transport a molecule across the membrane while simultaneously transporting another molecule in the opposite direction

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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
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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.
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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
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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
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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
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11
Q

Simple Transport (Simple)

A
  • One protein
  • Driven by the energy in the proton motive force
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12
Q

Group Translocation (Simple)

A
  • Two protein
  • Chemical modification of the transported substance driven by phosphoenolppyruvate
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13
Q

ABC Transporter (Simple)

A
  • Three protein
  • Periplasmic binding proteins are involved and energy comes from ATP
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14
Q

Cell Wall of Bacteria and Archaea

A
  • Outside the cell membrane
    • Rigid
      • Helps determine cell shape
  • Not a major permeability barrier
  • Porous to most small molecules
  • Protects the cell from osmotic changes
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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
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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
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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
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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
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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
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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
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21
Q

Prokaryotes that lack cell walls?

A
  • Mycoplasmas
  • Thermoplasma
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22
Q

Mycoplasmas

A
  • Group of pathogenic bacteria
  • Have sterols cytoplasmic membrane - adds strength and rigidity to membrane
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23
Q

Thermoplasma

A
  • Species of Archaea
  • Contain lipoglycans in membrane that have strengthening effect
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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
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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
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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)
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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
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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
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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
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Cytoplasm
Material bounded by plasma membrane (PM)
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Protoplast
- PM and everything within - Macromolecules: amino acids, nucleotides, etc - Soluble proteins - DNA and RNA (nucleoid)
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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
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Structural Proteins
- Help determine shape of the cell - Involved in cell division
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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
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DNA
- Carries genetic info of all living cells - Polymer of deoxyribonucleotides
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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
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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
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Cell Surface Structure (Fimbriae)
- Filamentous protein structures - Enable organisms to stick to surfaces or form pellicles
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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
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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
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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
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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
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Gas Vesicles
- Confer buoyancy in planktonic cells - Spindle-shaped, gas-filled structures made of protein - Function by decreasing cell density - Impermeable to water
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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
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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
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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
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Endospores can resist to?
- Boiling for hours - UV, y radiation - Chemical disinfectants - Dessication - Age
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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
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Stage I: Asymmetric Cell Division
- DNA replicates - Identical chromosomes pulled to opposite ends of the cell
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Stage II: Septation
- Divides cell into 2 unequal compartments: - Forespore (prespore) - Mother cell
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Stage III: Mother Cell engulfs the Forespore
- Forespore surrounded by two membranes
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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)
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Stage V: Coat synthesis
- Protein layers surround the core wall - Spore coat - Exosporium (found in some G+) - Protect the spore from chemicals and enzymes
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Stage VI: Endospore matures
- Core is dehydrated - around 10 - 30 % of a vegetative cell's water content
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Stage VII: Mother cell is lysed
- Mother cell disintegrates - Mature spore is released
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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
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Flagellar Structure
1. Filament 2. Hook 3. Basal Body (motor)
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Filament
- Rigid helical protein (around 20µm long) - Composed of identical protein subunits - flagellin
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Hook
Flexible coupling between filament and basal body
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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)
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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
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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
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Differences in swimming motions
- Peritrichously flagellated cells move slowly in a straight line - Polarly flagellated cells move more rapidly and typically spin around
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Gliding Motility
- Flagella-independent motility - Slower and smoother than swimming - Requires surface contact - Mechanisms - Excretion of polysaccharide slime - Type IV pili - Gliding-specific proteins
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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
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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
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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
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Cell Size (Eukaryotes)
- Lower surface area to volume ratio - Need more sophisticated transport mechanisms - Grow slower
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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
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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
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The Nucleus
- The nucleus holds the genetic information - Multiple linear dsDNA chromosomes
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Chloroplasts
- Site of photosynthesis - Chlorophyll - Surrounded by 2 membranes - DNA and ribosomes (70S) - Most closely related to Cyanobacteria - Blue-green algae
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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
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The Endosymbiotic Hypothesis
- Mitochondria and Chloroplasts evolved from bacteria - Evidence - Semi-autonomous - Circular chromosomes - Lack histones - 70S ribosomes - Two membranes - Outer membrane has porins
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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
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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)
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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
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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)
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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
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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
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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)
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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)
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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.
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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
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1. Adsorption - attachment to the host cell
- Involves specific receptors on the host cell surface - Ex) LPS, outer membrane proteins or glycoproteins
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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"
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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
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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)
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4. Assembly of new virions
- Viral proteins are assembled into capsids, and then genomes are packaged into nucleocapsids - Viruses do not reproduce by division
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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
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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
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