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Chapter 6
• The Cell
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• All organisms are made of cells
• The cell is the simplest collection of matter
that can live
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• Cell structure correlated to cellular function
Figure 6.1
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10 µm
• Biologists use microscopes and the tools of
biochemistry to study cells
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• Microscopes visualize cells too small to see
with the naked eye
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• Light microscopes (LMs)
– Pass visible light through a specimen
– Magnify with lenses
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1m
Human height
Length of some
nerve and
muscle cells
0.1 m
Chicken egg
1 cm
Light microscope
10 m
Unaided eye
• Types of microscopes
Most plant
and Animal cells
10 µ m
Nucleus
Most bacteria
Mitochondrion
1µm
100 nm
10 nm
Smallest bacteria
Viruses
Ribosomes
Proteins
1 nm
Lipids
Small molecules
Figure 6.2
0.1 nm
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Atoms
Electron microscope
100 µm
Electron microscope
Frog egg
1 mm
– Visualization methods
TECHNIQUE
RESULT
(a) Brightfield (unstained specimen).
Passes light directly through specimen.
Unless cell is naturally pigmented or
artificially stained, image has little
contrast. [Parts (a)–(d) show a
human cheek epithelial cell.]
50 µm
(b) Brightfield (stained specimen).
Staining with various dyes enhances
contrast, but most staining procedures
require that cells be fixed (preserved).
(c)
Phase-contrast. Enhances contrast
in unstained cells by amplifying
variations in density within specimen;
especially useful for examining living,
unpigmented cells.
Figure 6.3
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(d) Differential-interference-contrast (Nomarski).
Like phase-contrast microscopy, it uses optical
modifications to exaggerate differences in
density, making the image appear almost 3D.
(e) Fluorescence. Shows the locations of specific
molecules in the cell by tagging the molecules
with fluorescent dyes or antibodies. These
fluorescent substances absorb ultraviolet
radiation and emit visible light, as shown
here in a cell from an artery.
50 µm
(f) Confocal. Uses lasers and special optics for
“optical sectioning” of fluorescently-stained
specimens. Only a single plane of focus is
illuminated; out-of-focus fluorescence above
and below the plane is subtracted by a computer.
A sharp image results, as seen in stained nervous
tissue (top), where nerve cells are green, support
cells are red, and regions of overlap are yellow. A
standard fluorescence micrograph (bottom) of this
relatively thick tissue is blurry.
50 µm
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• Electron microscopes (EMs)
– Focus beam of e- through a specimen (TEM)
or onto its surface (SEM)
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• Scanning electron microscope (SEM)
– surface of a specimen
TECHNIQUE
RESULTS
1 µm
Cilia
(a) Scanning electron microscopy (SEM). Micrographs taken
with a scanning electron microscope show a 3D image of the
surface of a specimen. This SEM
shows the surface of a cell from a
rabbit trachea (windpipe) covered
with motile organelles called cilia.
Beating of the cilia helps move
inhaled debris upward toward
the throat.
Figure 6.4 (a)
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• Transmission electron microscope (TEM)
– internal ultrastructure of cells
Longitudinal
section of
cilium
(b)
Transmission electron microscopy (TEM). A transmission electron
microscope profiles a thin section of a
specimen. Here we see a section through
a tracheal cell, revealing its ultrastructure.
In preparing the TEM, some cilia were cut
along their lengths, creating longitudinal
sections, while other cilia were cut straight
across, creating cross sections.
Figure 6.4 (b)
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Cross section
of cilium
1 µm
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• Cell fractionation
– Takes cells apart and separates the major
organelles
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• Centrifuge
– fractionates cells into their components
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Cell fractionation
Homogenization
Tissue
cells
1000 g
Homogenate
(1000 times the
force of gravity)
Differential centrifugation
10 min
Supernatant poured
into next tube
20,000 g
20 min
Pellet rich in
nuclei and
cellular debris
Figure 6.5
80,000 g
60 min
150,000 g
3 hr
Pellet rich in
mitochondria
(and chloroplasts if cells
are from a Pellet rich in
plant)
“microsomes”
(pieces of
plasma membranes and
Pellet rich in
cells’ internal ribosomes
membranes)
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• 2 types of cells
– Prokaryotic
– Eukaryotic
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• Cells basic features
– Bounded by a plasma membrane
– Semifluid cytosol
– Chromosome(s)
– Ribosomes
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• Prokaryotic cells
– Do not contain a nucleus
– DNA in a region called
the nucleoid
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Pili: attachment structures on
the surface of some prokaryotes
Nucleoid: region where the
cell’s DNA is located (not
enclosed by a membrane)
Ribosomes: organelles that
synthesize proteins
Bacterial
chromosome
(a) A typical
rod-shaped bacterium
Plasma membrane: membrane
enclosing the cytoplasm
Cell wall: rigid structure outside
the plasma membrane
Capsule: jelly-like outer coating
of many prokaryotes
0.5 µm
Flagella: locomotion
organelles of
some bacteria
Figure 6.6 A, B
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(b) A thin section through the
bacterium Bacillus coagulans
(TEM)
• Eukaryotic cells
– True nucleus, bounded by nuclear envelope
– Generally quite a bit bigger than prokaryotic
cells
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•Eukaryotic cells have internal membranes that
compartmentalize their functions
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• Logistics of cellular metabolism sets limits on
the size of cells
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• Smaller cell =
– Higher surface to volume ratio, facilitates
exchange of materials
Surface area increases while
total volume remains constant
5
1
1
Total surface area
(height  width 
number of sides 
number of boxes)
6
150
750
Total volume
(height  width  length
 number of boxes)
1
125
125
Surface-to-volume
ratio
(surface area  volume)
6
12
6
Figure 6.7
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• Plasma membrane
– Selective barrier
– Passage of nutrients
and waste
Outside of cell
Carbohydrate side chain
Hydrophilic
region
Inside of cell
0.1 µm
Hydrophobic
region
Figure 6.8 A, B
(a) TEM of a plasma
membrane. The
plasma membrane,
here in a red blood
cell, appears as a
pair of dark bands
separated by a
light band.
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Hydrophilic
region
Phospholipid
Proteins
(b) Structure of the plasma membrane
• Plant and animal cells
– Have most of the same organelles
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• Animal cell
ENDOPLASMIC RETICULUM (ER)
Rough ER
Smooth ER
Nuclear envelope
Nucleolus
NUCLEUS
Chromatin
Flagelium
Plasma membrane
Centrosome
CYTOSKELETON
Microfilaments
Intermediate filaments
Ribosomes
Microtubules
Microvilli
Golgi apparatus
Peroxisome
Figure 6.9
Mitochondrion
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Lysosome
In animal cells but not plant cells:
Lysosomes
Centrioles
Flagella (in some plant sperm)
• Plant cell
Nuclear envelope
Nucleolus
Chromatin
NUCLEUS
Centrosome
Rough
endoplasmic
reticulum Smooth
endoplasmic
reticulum
Ribosomes (small brwon dots)
Central vacuole
Tonoplast
Golgi apparatus
Microfilaments
Intermediate
filaments
CYTOSKELETON
Microtubules
Mitochondrion
Peroxisome
Plasma membrane
Chloroplast
Cell wall
Plasmodesmata
Wall of adjacent cell
Figure 6.9
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In plant cells but not animal cells:
Chloroplasts
Central vacuole and tonoplast
Cell wall
Plasmodesmata
Eukaryotic cell’s genetic instructions housed in
the nucleus and carried out to ribosomes
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• Nucleus
– Contains most of the genes in the
eukaryotic cell
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• Nuclear envelope
Nucleus
Nucleus
1 µm
Nucleolus
Chromatin
Nuclear envelope:
Inner membrane
Outer membrane
Nuclear pore
Pore
complex
Rough ER
Surface of nuclear
envelope.
1 µm
Ribosome
0.25 µm
Close-up of
nuclear
envelope
Figure 6.10
Pore complexes (TEM).
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Nuclear lamina (TEM).
• Ribosomes
– Made of rRNA and protein
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– Carry out protein synthesis
Ribosomes
ER
Cytosol
Endoplasmic reticulum (ER)
Free ribosomes
Bound ribosomes
Large
subunit
0.5 µm
TEM showing ER and ribosomes
Figure 6.11
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Small
subunit
Diagram of a ribosome
Endomembrane system regulates protein traffic
and performs metabolic functions
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• Endoplasmic reticulum (ER)
– 1/2 total membrane in eukaryotic cells
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• ER membrane
– Continuous w/ nuclear envelope
Smooth ER
Rough ER
Nuclear
envelope
ER lumen
Cisternae
Ribosomes
Transitional ER
Transport vesicle
Smooth ER
Rough ER 200 µm
Figure 6.12
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• 2 regions of ER
– Smooth ER, lacks ribosomes
– Rough ER, contains ribosomes
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• Smooth ER
– Synthesizes lipids
– Metabolizes carbohydrates
– Stores calcium
– Detoxifies poison
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• Rough ER
– Bound ribosomes
– Produces proteins & membranes, which are
distributed by transport vesicles
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• Golgi apparatus
– Receives transport vesicles produced in the
rough ER
– Flattened sacs (cisternae)
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• Golgi apparatus:
– Modification of the products of rough ER
– Manufacture of macromolecules
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• Golgi apparatus
Golgi
apparatus
cis face
(“receiving” side of
Golgi apparatus)
1 Vesicles move
2 Vesicles coalesce to
6 Vesicles also
form new cis Golgi cisternae
from ER to Golgi
transport certain
Cisternae
proteins back to ER
3 Cisternal
maturation:
Golgi cisternae
move in a cisto-trans
direction
Figure 6.13
5 Vesicles transport specific
proteins backward to newer
Golgi cisternae
4 Vesicles form and
leave Golgi, carrying
specific proteins to
other locations or to
the plasma membrane for secretion
trans face
(“shipping” side of
Golgi apparatus)
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0.1 0 µm
TEM of Golgi apparatus
• Lysosome
– Membranous sac of hydrolytic enzymes
– Digests macromolecules
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• Lysosomes carry out intracellular digestion by
– Phagocytosis
Nucleus
1 µm
Lysosome
Lysosome contains
active hydrolytic
enzymes
Food vacuole
fuses with
lysosome
Hydrolytic
enzymes digest
food particles
Digestive
enzymes
Lysosome
Plasma membrane
Digestion
Food vacuole
Figure 6.14 A
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(a) Phagocytosis: lysosome digesting food
• Autophagy
Lysosome containing
two damaged organelles
1µm
Mitochondrion
fragment
Peroxisome
fragment
Lysosome fuses with
vesicle containing
damaged organelle
Hydrolytic enzymes
digest organelle
components
Lysosome
Figure 6.14 B
Vesicle containing
damaged
mitochondrion
(b) Autophagy:
lysosome breaking down damaged organelle
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Digestion
Vacuoles
• (plant or fungal cells)
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• Food vacuoles
– Formed by phagocytosis
• Contractile vacuoles
– Pump excess water out of protist cells
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• Central vacuoles (plants)
– Hold organic cmpds and water
Central vacuole
Cytosol
Tonoplast
Nucleus
Central
vacuole
Cell wall
Chloroplast
Figure 6.15
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5 µm
• Endomembrane system
– Compartmental organization
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• Organelles of the endomembrane system
1 Nuclear envelope is
connected to rough ER,
which is also continuous
with smooth ER
Nucleus
Rough ER
2
Membranes and proteins
produced by the ER flow in
the form of transport vesicles
to the Golgi
Smooth ER
cis Golgi
Nuclear envelop
3
Golgi pinches off transport
Vesicles and other vesicles
that give rise to lysosomes and
Vacuoles
Plasma
membrane
trans Golgi
4
Lysosome available
for fusion with another
vesicle for digestion
Figure 6.16
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5 Transport vesicle carries 6
proteins to plasma
membrane for secretion
Plasma membrane expands
by fusion of vesicles; proteins
are secreted from cell
• Mitochondria and chloroplasts change E from
one form to another
• Mitochondria
– cellular respiration
• Chloroplasts
– photosynthesis
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• Mitochondria
– Found in nearly all eukaryotic cells
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• Mitochondria enclosed by 2 membranes
– Smooth outer membrane
– Inner membrane folded into cristae
Mitochondrion
Intermembrane space
Outer
membrane
Free
ribosomes
in the
mitochondrial
matrix
Inner
membrane
Cristae
Matrix
Figure 6.17
Mitochondrial
DNA
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100 µm
• Chloroplast (type of plastid)
– Plants and algae
– Contains chlorophyll
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• Chloroplasts
– In leaves and other green organs
Chloroplast
Ribosomes
Stroma
Chloroplast
DNA
Inner and outer
membranes
Granum
1 µm
Figure 6.18
Thylakoid
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• Chloroplast structure:
– Thylakoids, membranous sacs
– Stroma, the internal fluid
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• Peroxisomes
– Produce H2O2 and convert it to water
Chloroplast
Peroxisome
Mitochondrion
Figure 6.19
1 µm
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• Cytoskeleton
– Network of fibers
– Organizes functions throughout cell
Microtubule
Figure 6.20
Figure 6.20
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0.25 µm
Microfilaments
– Cell motility, utilizes motor proteins
ATP
Vesicle
Receptor for
motor protein
Motor protein
Microtubule
(ATP powered)
of cytoskeleton
(a) Motor proteins that attach to receptors on organelles can “walk”
the organelles along microtubules or, in some cases, microfilaments.
Vesicles
Microtubule
Figure 6.21 A, B
(b) Vesicles containing neurotransmitters migrate to the tips of nerve cell
axons via the mechanism in (a). In this SEM of a squid giant axon, two
vesicles can be seen moving along a microtubule. (A separate part of the
experiment provided the evidence that they were in fact moving.)
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0.25 µm
Components of the Cytoskeleton
• 3 main types of fibers of cytoskeleton
Table 6.1
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• Microtubules
– Shape the cell
– Guide movement of organelles
– Separate the chromosomes in dividing cells
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• Centrosome
–
“microtubule-organizing center”
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Centrosome
– a pair of centrioles
Centrosome
Microtubule
Centrioles
0.25 µm
Figure 6.22
Longitudinal section
of one centriole
Microtubules
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Cross section
of the other centriole
• Cilia and flagella
– Specialized arrangements of microtubules
– Locomotor appendages
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• Flagella beating pattern
(a) Motion of flagella. A flagellum
usually undulates, its snakelike
motion driving a cell in the same
direction as the axis of the
flagellum. Propulsion of a human
sperm cell is an example of
flagellatelocomotion (LM).
Direction of swimming
Figure 6.23 A
1 µm
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• Ciliary motion
(b) Motion of cilia. Cilia have a backand-forth motion that moves the
cell in a direction perpendicular
to the axis of the cilium. A dense
nap of cilia, beating at a rate of
about 40 to 60 strokes a second,
covers this Colpidium, a
freshwater protozoan (SEM).
Figure 6.23 B
15 µm
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• Cilia and flagella: common ultrastructure
Outer microtubule
doublet
Dynein arms
0.1 µm
Central
microtubule
Outer doublets
cross-linking
proteins inside
Microtubules
Radial
spoke
Plasma
membrane
Basal body
(b)
0.5 µm
(a)
0.1 µm
Triplet
(c)
Figure 6.24 A-C
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Cross section of basal body
Plasma
membrane
• The protein dynein
– Bending cilia and flagella
Microtubule
doublets
ATP
Dynein arm
(a) Powered by ATP, the dynein arms of one microtubule doublet
grip the adjacent doublet, push it up, release, and then grip again.
If the two microtubule doublets were not attached, they would slide
relative to each other.
Figure 6.25 A
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ATP
Outer doublets
cross-linking
proteins
Anchorage
in cell
(b) In a cilium or flagellum, two adjacent doublets cannot slide far because
they are physically restrained by proteins, so they bend. (Only two of
the nine outer doublets in Figure 6.24b are shown here.)
Figure 6.25 B
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1
3
2
(c) Localized, synchronized activation of many dynein arms
probably causes a bend to begin at the base of the Cilium or
flagellum and move outward toward the tip. Many successive
bends, such as the ones shown here to the left and right,
result in a wavelike motion. In this diagram, the two central
microtubules and the cross-linking proteins are not shown.
Figure 6.25 C
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• Microfilaments
– Made of protein actin
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– Found in microvilli
Microvillus
Plasma membrane
Microfilaments (actin
filaments)
Intermediate filaments
Figure 6.26
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0.25 µm
• Microfilaments
– Myosin and actin
Muscle cell
Actin filament
Myosin filament
Myosin arm
Figure 6.27 A
(a) Myosin motors in muscle
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cell contraction.
• Amoeboid movement
– Contraction of actin & myosin
Cortex (outer cytoplasm):
gel with actin network
Inner cytoplasm: sol
with actin subunits
Extending
pseudopodium
Figure 6.27 B
(b) Amoeboid movement
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• Cytoplasmic streaming
Nonmoving
cytoplasm (gel)
Chloroplast
Streaming
cytoplasm
(sol)
Parallel actin
filaments
Figure 6.27 C (b) Cytoplasmic streaming in plant cells
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Cell wall
• Intermediate filaments
– Control cell shape
– Fix organelles in place
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Extracellular components and connections
between cells coordinate cellular activities
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• Cell wall
– Extracellular structure of plant cells
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• Plant cell walls
– Cellulose + other polysaccharides and protein
– May have multiple layers
Central
vacuole
of cell
Plasma
membrane
Secondary
cell wall
Primary
cell wall
Central
vacuole
of cell
Middle
lamella
1 µm
Central vacuole
Cytosol
Plasma membrane
Plant cell walls
Figure 6.28
Plasmodesmata
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The Extracellular Matrix (ECM) of Animal Cells
• Animal cells
– Lack cell walls
– Covered by matrix, (the ECM)
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• ECM
– Glycoproteins and other macromolecules
EXTRACELLULAR FLUID
Collagen
A proteoglycan
complex
Polysaccharide
molecule
Carbohydrates
Core
protein
Fibronectin
Plasma
membrane
Integrin
Integrins
Microfilaments
Figure 6.29
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CYTOPLASM
Proteoglycan
molecule
• Functions of ECM
– Support
– Adhesion
– Movement
– Regulation
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Intercellular Junctions
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Plants:
• Plasmodesmata
– Channels, perforate cell walls
Cell walls
Interior
of cell
Interior
of cell
Figure 6.30
0.5 µm
Plasmodesmata
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Plasma membranes
• Animals, (3 types of intercellular junctions)
– Tight junctions
– Desmosomes
– Gap junctions
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• Intercellular junctions in animals
TIGHT JUNCTIONS
Tight junction
Tight junctions prevent
fluid from moving
across a layer of cells
0.5 µm
At tight junctions, the membranes of
neighboring cells are very tightly pressed
against each other, bound together by
specific proteins (purple). Forming continuous seals around the cells, tight junctions
prevent leakage of extracellular fluid across
A layer of epithelial cells.
DESMOSOMES
Desmosomes (also called anchoring
junctions) function like rivets, fastening cells
Together into strong sheets. Intermediate
Filaments made of sturdy keratin proteins
Anchor desmosomes in the cytoplasm.
Tight junctions
Intermediate
filaments
Desmosome
Gap
junctions
Space
between Plasma membranes
cells
of adjacent cells
1 µm
Extracellular
matrix
Gap junction
Figure 6.31
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0.1 µm
GAP JUNCTIONS
Gap junctions (also called communicating
junctions) provide cytoplasmic channels from
one cell to an adjacent cell. Gap junctions
consist of special membrane proteins that
surround a pore through which ions, sugars,
amino acids, and other small molecules may
pass. Gap junctions are necessary for communication between cells in many types of tissues,
including heart muscle and animal embryos.
The Cell: A Living Unit Greater Than the Sum of Its Parts
5 µm
• Cells rely on the integration of structures and
organelles in order to function
Figure 6.32
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