Download 2.7 Membrane Structure

Chapter 2
Microbial Cell Structure and Function
I. Microscopy
• 2.1 Discovering Cell Structure: Light Microscopy
• 2.2 Improving Contrast in Light Microscopy
• 2.3 Imaging Cells in Three Dimensions
• 2.4 Probing Cell Structure: Electron Microscopy
2.1 Some Principles of Light Microscopy
• Compound light microscope uses visible light to
illuminate cells
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Many different types of light microscopy:
• Bright-field
• Phase-contrast
• Dark-field
• Fluorescence
2.1 Some Principles of Light Microscopy
• Bright-field scope (Figure 2.1a)
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Specimens are visualized because of differences in contrast
(density) between specimen and surroundings (Figure 2.2)
Two sets of lenses form the image (Figure 2.1b)
• Objective lens and ocular lens
• Total magnification = objective magnification ocular
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magnification
Maximum magnification is ~2,000
2.1 Some Principles of Light Microscopy
• Magnification: the ability to make an object larger
• Resolution: the ability to distinguish two adjacent objects
as separate and distinct
• Resolution is determined by the wavelength of light used
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and numerical aperture of lens
Limit of resolution for light microscope is about 0.2 μm
2.2 Improving Contrast in Light Microscopy
• Improving contrast results in a better final image
• Staining improves contrast
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Dyes are organic compounds that bind to specific cellular
materials
Examples of common stains are methylene blue, safranin,
and crystal violet
2.2 Improving Contrast in Light Microscopy
• Differential stains: the Gram stain
• Differential stains separate bacteria into groups
• The Gram stain is widely used in microbiology
(Figure 2.4a)
• Bacteria can be divided into two major groups:
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gram-positive and gram-negative
• Gram-positive bacteria appear purple, and gram-negative
bacteria appear red after staining (Figure 2.4b)
2.2 Improving Contrast in Light Microscopy
• Phase-contrast microscopy
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Invented in 1936 by Frits Zernike
Phase ring amplifies differences in the refractive index of
cell and surroundings
Improves the contrast of a sample without the use of a stain
Allows for the visualization of live samples
Resulting image is dark cells on a light background (Figure
2.5)
2.2 Improving Contrast in Light Microscopy
• Dark-field microscopy
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Light reaches the specimen from the sides
Light reaching the lens has been scattered by specimen
Image appears light on a dark background (Figure 2.5)
Excellent for observing motility
2.2 Improving Contrast in Light Microscopy
• Fluorescence microscopy
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Used to visualize specimens that fluoresce
• Emit light of one color when illuminated with another color of
light (Figure 2.6)
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Cells fluoresce naturally (autofluorescence) or after they
have been stained with a fluorescent dye such as DAPI
Widely used in microbial ecology for enumerating bacteria
in natural samples
2.3 Imaging Cells in Three Dimensions
• Differential interference contrast (DIC) microscopy
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Uses a polarizer to create two distinct beams of polarized
light
Gives structures such as endospores, vacuoles, and
granules a three-dimensional appearance (Figure 2.7)
Structures not visible by bright-field microscopy are
sometimes visible by DIC
2.3 Imaging Cells in Three Dimensions
• Confocal scanning laser microscopy (CSLM)
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Uses a computerized microscope coupled with a laser
source to generate a three-dimensional image (Figure 2.8)
Computer can focus the laser on single layers of the
specimen
Different layers can then be compiled for a threedimensional image
Resolution is 0.1 μm for CSLM
2.4 Probing Cell Structure: Electron Microscopy
• Electron microscopes use electrons instead of photons
to image cells and structures (Figure 2.9)
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Two types of electron microscopes:
• Transmission electron microscopes (TEM)
• Scanning electron microscopes (SEM)
2.4 Probing Cell Structure: Electron Microscopy
• Transmission electron microscopy (TEM)
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Electromagnets function as lenses
System operates in a vacuum
High magnification and resolution (0.2 nm)
Enables visualization of structures at the molecular level
(Figure 2.10 a and b)
Specimen must be very thin (20–60 nm) and be stained
2.4 Probing Cell Structure: Electron Microscopy
• Scanning electron microscopy (SEM)
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Specimen is coated with a thin film of heavy metal (e.g.,
gold)
An electron beam scans the object
Scattered electrons are collected by a detector, and an
image is produced (Figure 2.10c)
Even very large specimens can be observed
Magnification range of 15 –100,000
II. Cells of Bacteria and Archaea
• 2.5 Cell Morphology
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2.6 Cell Size and the Significance of Being Small
2.5 Cell Morphology
• Morphology = cell shape
• Major cell morphologies (Figure 2.11)
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Coccus (pl. cocci): spherical or ovoid
Rod: cylindrical shape
Spirillum: spiral shape
Cells with unusual shapes
• Spirochetes, appendaged bacteria, and filamentous
bacteria
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Many variations on basic morphological types
2.5 Cell Morphology
• Morphology typically does not predict physiology,
ecology, phylogeny, etc. of a prokaryotic cell
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May be selective forces involved in setting the
morphology
• Optimization for nutrient uptake (small cells and those with
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high surface-to-volume ratio)
Swimming motility in viscous environments or near surfaces
(helical or spiral-shaped cells)
Gliding motility (filamentous bacteria)
2.6 Cell Size and the Significance of Being
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Small
• Size range for prokaryotes: 0.2 µm to >700 µm in
diameter
• Most cultured rod-shaped bacteria are between 0.5 and 4.0
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µm wide and < 15 µm long
Examples of very large prokaryotes
• Epulopiscium fishelsoni (Figure 2.12a)
• Thiomargarita namibiensis (Figure 2.12b)
Size range for eukaryotic cells: 10 to >200 µm in
diameter
2.6 Cell Size and the Significance of Being
Small
• Surface-to-volume ratios, growth rates, and evolution
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Advantages to being small (Figure 2.13)
• Small cells have more surface area relative to cell volume than
large cells (i.e., higher S/V)
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Support greater nutrient exchange per unit cell volume
Tend to grow faster than larger cells
2.6 Cell Size and the Significance of Being
Small
• Lower limits of cell size
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Cellular organisms <0.15 µm in diameter are unlikely
Open oceans tend to contain small cells (0.2–0.4 µm in
diameter)
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III. The Cytoplasmic Membrane and Transport
• 2.7 Membrane Structure
• 2.8 Membrane Functions
• 2.9 Nutrient Transport
2.7 Membrane Structure
• Cytoplasmic membrane
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Thin structure that surrounds the cell
Vital barrier that separates cytoplasm from environment
Highly selective permeable barrier; enables concentration of
specific metabolites and excretion of waste products
2.7 Membrane Structure
• Composition of membranes
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General structure is phospholipid bilayer (Figure 2.14)
• Contain both hydrophobic and hydrophilic components
Can exist in many different chemical forms as a result of
variation in the groups attached to the glycerol backbone
Fatty acids point inward to form hydrophobic environment;
hydrophilic portions remain exposed to external
environment or the cytoplasm
2.7 Membrane Structure
• Cytoplasmic membrane (Figure 2.15)
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8–10 nm wide
Embedded proteins
Stabilized by hydrogen bonds and hydrophobic interactions
Mg2+ and Ca2+ help stabilize membrane by forming ionic
bonds with negative charges on the phospholipids
Somewhat fluid
2.7 Membrane Structure
• Membrane proteins
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Outer surface of cytoplasmic membrane can interact with a
variety of proteins that bind substrates or process large
molecules for transport
Inner surface of cytoplasmic membrane interacts with
proteins involved in energy-yielding reactions and other
important cellular functions
Integral membrane proteins
• Firmly embedded in the membrane
Peripheral membrane proteins
• One portion anchored in the membrane
2.7 Membrane Structure
• Archaeal membranes
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Ether linkages in phospholipids of Archaea (Figure 2.16)
Bacteria and Eukarya that have ester linkages in
phospholipids
Archaeal lipids lack fatty acids; have isoprenes instead
Major lipids are glycerol diethers and tetraethers (Figure
2.17a and b)
Can exist as lipid monolayers, bilayers, or mixture (Figure
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2.17)
2.8 Membrane Function
• Permeability barrier (Figure 2.18)
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Polar and charged molecules must be transported
Transport proteins accumulate solutes against the
concentration gradient
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Protein anchor
• Holds transport proteins in place
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Energy conservation
• Generation of proton motive force
2.9 Nutrient Transport
• Carrier-mediated transport systems (Figure 2.19)
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Show saturation effect
Highly specific
2.9 Nutrient Transport
• Three major classes of transport systems in prokaryotes
(Figure 2.20)
• Simple transport
• Group translocation
• ABC system
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All require energy in some form, usually proton motive
force or ATP
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2.9 Nutrient Transport
• Three transport events are possible: uniport, symport,
and antiport (Figure 2.21)
• Uniporters transport in one direction across the membrane
• Symporters function as co-transporters
• Antiporters transport a molecule across the membrane
while simultaneously transporting another molecule in the
opposite direction
2.9 Nutrient Transport
• Simple transport:
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Lac permease of Escherichia coli
• Lactose is transported into E. coli by the simple transporter lac
permease, a symporter
• Activity of lac permease is energy-driven
2.9 Nutrient Transport
• The phosphotransferase system in E. coli (Figure 2.22)
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Type of group translocation: substance transported is
chemically modified during transport across the membrane
Best-studied system
Moves glucose, fructose, and mannose
Five proteins required
Energy derived from phosphoenolpyruvate
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2.9 Nutrient Transport
• ABC (ATP-binding cassette) systems (Figure 2.23)
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>200 different systems identified in prokaryotes
Often involved in uptake of organic compounds (e.g.,
sugars, amino acids), inorganic nutrients (e.g., sulfate,
phosphate), and trace metals
Typically display high substrate specificity
Gram-negatives employ periplasmic-binding proteins and
ATP-driven transport proteins
Gram-positives employ substrate-binding proteins and
membrane transport proteins
IV. Cell Walls of Bacteria and Archaea
• 2.10 Peptidoglycan
• 2.11 LPS: The Outer Membrane
• 2.12 Archaeal Cell Walls
2.10 Peptidoglycan
• Species of Bacteria separated into two groups based on
Gram stain
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Gram-positives and gram-negatives have different cell
wall structure (Figure 2.24)
• Gram-negative cell wall
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• Two layers: LPS and peptidoglycan
Gram-positive cell wall
• One layer: peptidoglycan
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2.10 Peptidoglycan
• Peptidoglycan (Figure 2.25)
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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 grampositive bacteria (Figure 2.26)
2.10 Peptidoglycan
• Gram-positive cell walls (Figure 2.27)
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Can 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
2.10 Peptidoglycan
• Prokaryotes that lack cell walls
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Mycoplasmas
• Group of pathogenic bacteria
Thermoplasma
• Species of Archaea
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2.11 LPS: The Outer Membrane
• Total cell wall contains ~10% peptidoglycan
• Most of cell wall composed of outer membrane, aka
lipopolysaccharide (LPS) layer
• LPS consists of core polysaccharide and O-polysaccharide
(Figure 2.28)
• LPS replaces most of phospholipids in outer half of outer
membrane (Figure 2.29)
• Endotoxin: the toxic component of LPS
2.11 LPS: The Outer Membrane
• Periplasm: space located between cytoplasmic and outer
membranes (Figure 2.29)
• ~15 nm wide
• Contents have gel-like consistency
• Houses many proteins
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Porins: channels for movement of hydrophilic lowmolecular-weight substances (Figure 2.29c)
2.11 LPS: The Outer Membrane
• Structural differences between cell walls of gram-positive
and gram-negative Bacteria are responsible for
differences in the Gram stain reaction
2.12 Archaeal Cell Walls
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No peptidoglycan
Typically no outer membrane
Pseudomurein
• Polysaccharide similar to peptidoglycan (Figure 2.30)
• Composed of N-acetylglucosamine and N•
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acetylalosaminuronic acid
Found in cell walls of certain methanogenic Archaea
Cell walls of some Archaea lack pseudomurein
2.12 Archaeal Cell Walls
• S-Layers
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Most common cell wall type among Archaea
Consist of protein or glycoprotein
Paracrystalline structure (Figure 2.31)
V. Other Cell Surface Structures and Inclusions
• 2.13 Cell Surface Structures
• 2.14 Cell Inclusions
• 2.15 Gas Vesicles
• 2.16 Endospores
2.13 Cell Surface Structures
• Capsules and slime layers
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Polysaccharide layers (Figure 2.32)
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• May be thick or thin, rigid or flexible
Assist in attachment to surfaces
Protect against phagocytosis
Resist desiccation
2.13 Cell Surface Structures
• Fimbriae
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Filamentous protein structures (Figure 2.33)
Enable organisms to stick to surfaces or form pellicles
2.13 Cell Surface Structures
• Pili
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Filamentous protein structures (Figure 2.34)
Typically longer than fimbriae
Assist in surface attachment
Facilitate genetic exchange between cells (conjugation)
Type IV pili involved in twitching motility
2.14 Cell Inclusions
• Carbon storage polymers
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Poly-β-hydroxybutyric acid (PHB): lipid (Figure 2.35)
Glycogen: glucose polymer
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Polyphosphates: accumulations of inorganic phosphate
(Figure 2.36a)
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Sulfur globules: composed of elemental sulfur (Figure
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2.36b)
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Carbonate minerals: composed of barium, strontium, and
magnesium (Figure 2.37)
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Magnetosomes: magnetic storage inclusions (Figure
2.38)
2.15 Gas Vesicles
• Gas vesicles
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Confer buoyancy in planktonic cells (Figure 2.39)
Spindle-shaped, gas-filled structures made of protein
(Figure 2.40)
Impermeable to water
2.15 Gas Vesicles
• Molecular structure of gas vesicles
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Gas vesicles are composed of two proteins, GvpA and
GvpC (Figure 2.41)
Function by decreasing cell density
2.16 Endospores
• Endospores
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Highly differentiated cells resistant to heat, harsh chemicals,
and radiation (Figure 2.42 )
“Dormant” stage of bacterial life cycle (Figure 2.43)
Ideal for dispersal via wind, water, or animal gut
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Present only in some gram-positive bacteria
2.16 Endospores
• Endospore structure (Figure 2.45)
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Structurally complex
Contains dipicolinic acid
Enriched in Ca2+
Core contains small acid-soluble spore proteins (SASP)
2.16 Endospores
• The sporulation process
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Complex series of events (Figure 2.47)
Genetically directed
VI. Microbial Locomotion
• 2.17 Flagella and Swimming Motility
• 2.18 Gliding Motility
• 2.19 Chemotaxis and Other Taxes
2.17 Flagella and Swimming Motility
• Flagella: structure that assists in swimming
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Different arrangements: peritrichous, polar, lophotrichous
(Figure 2.48)
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Helical in shape
2.17 Flagella and Swimming Motility
• Flagellar structure of Bacteria
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Consists of several components (Figure 2.51)
Filament composed of flagellin
Move by rotation
Flagellar structure of Archaea
• Half the diameter of bacterial flagella
• Composed of several different proteins
• Move by rotation
2.17 Flagella and Swimming Motility
• Flagellar synthesis
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Several genes are required for flagellar synthesis and
motility (Figure 2.53)
MS ring is made first
Other proteins and hook are made next
Filament grows from tip
2.17 Flagella and Swimming Motility
• Flagella increase or decrease rotational speed in relation
to strength of the proton motive force
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Differences in swimming motions (Figure 2.54)
• Peritrichously flagellated cells move slowly in a straight line
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Polarly flagellated cells move more rapidly and typically spin
around
2.18 Gliding Motility
• Gliding motility
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Flagella-independent motility (Figure 2.56)
Slower and smoother than swimming
Movement typically occurs along long axis of cell
Requires surface contact
Mechanisms
• Excretion of polysaccharide slime
• Type IV pili
• Gliding-specific proteins
2.19 Chemotaxis and Other Taxes
• Taxis: directed movement in response to chemical or
physical gradients
• Chemotaxis: response to chemicals
• Phototaxis: response to light
• Aerotaxis: response to oxygen
• Osmotaxis: response to ionic strength
• Hydrotaxis: response to water
2.19 Chemotaxis and Other Taxes
• Chemotaxis
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Best studied in E. coli
Bacteria respond to temporal, not spatial, difference in
chemical concentration
“Run and tumble” behavior (Figure 2.57)
Attractants and receptors sensed by chemoreceptors
2.19 Chemotaxis and Other Taxes
• Measuring chemotaxis (Figure 2.58)
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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
VII. Eukaryotic Microbial Cells
• 2.20 The Nucleus and Cell Division
• 2.21 Mitochondria, Hydrogenosomes, and
Chloroplast
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2.22 Other Major Eukaryotic Cell Structures
2.20 The Nucleus and Cell Division
• Eukaryotes
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Contain a membrane-enclosed nucleus and other
organelles (e.g., mitochondria, Golgi complex, endoplasmic
reticula, microtubules, and microfilaments (Figure 2.60)
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2.20 The Nucleus and Cell Division
• Nucleus: contains the chromosomes (Figure 2.61)
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DNA is wound around histones (Figure 2.61b)
Visible under light microscope without staining
Enclosed by two membranes
Within the nucleus is the nucleolus
• Site of ribosomal RNA synthesis
2.20 The Nucleus and Cell Division
• Cell division
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Mitosis
• Normal form of nuclear division in eukaryotic cells
• Chromosomes are replicated and partitioned into two nuclei
(Figure 2.62)
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• Results in two diploid daughter cells
Meiosis
• Specialized form of nuclear division
• Halves the diploid number to the haploid number
• Results in four haploid gametes
2.21 Mitochondria, Hydrogenosomes, and
Chloroplast
• All specialize in energy metabolism
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Mitochondria (Figure 2.63)
• Respiration and oxidative phosphorylation
• Bacterial dimensions (rod or spherical)
• Over 1,000 per animal cell
• Surrounded by two membranes
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• Folded internal membranes called cristae
• Contain enzymes needed for respiration and ATP production
• Innermost area of mitochondrion called matrix
• Contains enzymes for the oxidation of organic compounds
2.21 Mitochondria, Hydrogenosomes, and
Chloroplast
• Hydrogenosome (Figure 2.64)
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Similar size to mitochondria; however, lack TCA cycle
enzymes and cristae
Oxidation of pyruvate to H2, CO2, and acetate
Trichomonas and various protists have hydrogenosomes
2.21 Mitochondria, Hydrogenosomes, and
Chloroplast
• Chloroplast (Figure 2.65)
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Chlorophyll-containing organelle found in phototrophic
eukaryotes
Size, shape, and number of chloroplasts vary
Flattened membrane discs are thylakoids (Figure 2.65c)
Lumen of the chloroplast is called the stroma
• Stroma contains large amounts of RubisCO
• RubisCO is key enzyme in Calvin cycle
2.21 Mitochondria, Hydrogenosomes, and
Chloroplast
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Chloroplasts and mitochondria suggested as
descendants of ancient prokaryotic cells (endosymbiosis)
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Evidence that supports idea of endosymbiosis
• Mitochondria and chloroplasts contain DNA
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• DNA is circular
Mitochondria and chloroplasts contain own ribosomes
Eukaryotic nuclei contain genes derived from bacteria
2.22 Other Major Eukaryotic Cell Structures
• Endoplasmic reticulum (ER)
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Two types of ER (smooth and rough)
• Rough contains attached ribosomes; smooth does not
• Smooth ER participates in the synthesis of lipids
• Rough ER is a major producer of glycoproteins
Golgi complex (Figure 2.66): stacks of membrane distinct
from, but functioning in concert with, the ER
• Modifies products of the ER destined for secretion
2.22 Other Major Eukaryotic Cell Structures
• Lysosomes
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Membrane-enclosed compartments
Contain digestive enzymes used for hydrolysis
Allow for lytic activity to occur within the cell without
damaging other cellular components
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2.22 Other Major Eukaryotic Cell Structures
• Microtubules (Figure 2.67a and b)
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25 nm in diameter; composed of α- and β-tubulin
Function in maintaining cell shape, in motility, in
chromosome movement, and in movement of organelles
Microfilaments (Figure 2.67c)
• 7 nm in diameter; polymers of actin
• Function in maintaining cell shape, motility by pseudopodia,
and cell division
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Intermediate filaments
• 8–12 nm in diameter; keratin proteins
• Function in maintaining cell shape and positioning of
organelles in cell
2.22 Other Major Eukaryotic Cell Structures
• Flagella and cilia (Figure 2.68)
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Organelles of motility allowing cells to move by swimming
Cilia are short flagella
Structurally distinct from prokaryotic flagella
Bundle of nine pairs of microtubules surrounding the central
pair
Dynein is attached to the microtubules and uses ATP
Propel the cell using a whiplike motion
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