Download Chapter 6 A Tour of the Cell

Chapter 6 A Tour of the Cell
The Fundamental Units of Life
Concept 6.1: Biologists use microscopes and
the tools of biochemistry to study cells
 Cells are usually too small to be seen by the naked eye
 All organisms are made of cells
 The cell is the simplest collection of matter
that can be alive
Electron microscopy
Light microscopy
 All cells are related by their descent from earlier
cells
Unaided eye
 Cells can differ substantially from one another but
share common features
Human
height
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Microscopy
 Microscopes are used to
visualize cells
 In a light microscope
(LM), visible light is passed
through a specimen and
then through glass lenses
 Lenses refract (bend) the
light, so that the image is
magnified
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Superresolution
microscopy
Length
of some
nerve
and
muscle Chicken
egg
cells
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Education,
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m
1 m Inc.0.1
m
1 cm
Nucleus
Most
bacteria
Frog Human
egg egg
1 mm
Most
plant
and
animal
cells
100 µm
10 µm
Smallest
bacteria
Small
Viruses Proteins molecules
Mitochondrion
1 µm
Ribosomes
100 µm
Lipids
10 nm
Atoms
1 nm
0.1 nm
 Three important parameters of
microscopy
 Magnification, the ratio of an object’s
image size to its real size
 Resolution, the measure of the clarity
of the image, or the minimum distance
of two distinguishable points
 Contrast, visible differences in
brightness between parts of the sample
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Brightfield
(unstained
specimen)
50 µm
Brightfield
(stained specimen)
Fluorescence
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Phase-contrast
Differentialinterference-contrast
(Nomarski)
Confocal (without)
10 µm
 Confocal microscopy
and deconvolution
microscopy provide
sharper images of
three-dimensional
tissues and cells
Deconvolution
 New techniques for
labeling cells
improve resolution
10 µm
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Figure 6.3c
Confocal (with)
Super-resolution
(without)
Super-resolution
(with)
1 µm
 Various techniques enhance contrast and enable
cell components to be stained or labeled
 Recent advances in
light microscopy
50 µm
 Light microscopes can magnify effectively to about
1,000 times the size of the actual specimen
Light Microscopy (LM)
 Two basic types of electron
microscopes (EMs) are used to
study subcellular structures

Scanning electron microscopes
(SEMs) focus a beam of electrons
onto the surface of a specimen,
providing images that look 3-D

Transmission electron
microscopes (TEMs) focus a
beam of electrons through a
specimen

TEMs are used mainly to study
the internal structure of cells
Electron Microscopy (EM)
Scanning
electron
microscopy (SEM)
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2 µm
Transmission
2 µm
electron
microscopy (TEM)
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Figure 6.4
Cell Fractionation
Homogenization
Tissue
cells
 Cell fractionation takes cells apart and
separates the major organelles from one another
Homogenate
Centrifugation
 Centrifuges fractionate cells into their
component parts
1,000 g
10 min
Supernatant poured into next tube
20,000 g
20 min
 Cell fractionation enables scientists to determine
the functions of organelles
Pellet rich in
nuclei and
cellular debris
 Biochemistry and cytology help correlate cell
function with structure
80,000 g
60 min
150,000 g
3 hr
Pellet rich in
mitochondria
and chloroplasts
Differential
centrifugation
Pellet rich in
“microsomes”
Pellet rich in
ribosomes
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Concept 6.2: Eukaryotic cells have
internal membranes that compartmentalize
their functions
Comparing Prokaryotic and Eukaryotic Cells
 The basic structural and functional unit of every
organism is one of two types of cells: prokaryotic
or eukaryotic
 Only organisms of the domains Bacteria and
Archaea consist of prokaryotic cells
 Basic features of all cells
 Plasma membrane
 Semifluid substance called cytosol
 Chromosomes (carry genes)
 Ribosomes (make proteins)
 Protists, fungi, animals, and plants all consist of
eukaryotic cells
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Figure 6.5
Fimbriae
 Prokaryotic cells are characterized by having
Nucleoid
 No nucleus
Ribosomes
 DNA in an unbound region called the nucleoid
Plasma membrane
 No membrane-bound organelles
Cell wall
Bacterial
chromosome
Capsule
0.5 µm
 Cytoplasm bound by the plasma membrane
Flagella
(a) A typical
rod-shaped
bacterium
(b) A thin section through
the bacterium Bacillus
coagulans (TEM)
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 Eukaryotic cells are characterized by having
 DNA in a nucleus that is bounded by a
membranous nuclear envelope
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 The plasma membrane is a selective barrier that
allows sufficient passage of oxygen, nutrients, and
waste to service the volume of every cell
 Membrane-bound organelles
 Cytoplasm in the region between the plasma
membrane and nucleus
 Eukaryotic cells are generally much larger than
prokaryotic cells
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Figure 6.6
Outside of cell
(a) TEM of a plasma membrane
 Metabolic requirements set upper limits on the
size of cells
Inside
of cell
0.1 µm
Carbohydrate side chains
 The surface area to volume ratio of a cell is critical
 As a cell increases in size, its volume grows
proportionately more than its surface area
Hydrophilic
region
Hydrophobic
region
Hydrophilic
region
Phospholipid
Proteins
(b) Structure of the plasma membrane
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Figure 6.8a
ENDOPLASMIC
RETICULUM (ER)
A Panoramic View of the Eukaryotic Cell
Rough ER Smooth ER
 A eukaryotic cell has internal membranes that
partition the cell into organelles
 The basic fabric of biological membranes is a
double layer of phospholipids and other lipids
 Plant and animal cells have most of the same
organelles
Flagellum
Nuclear
envelope
Nucleolus NUCLEUS
Chromatin
Centrosome
Plasma
membrane
CYTOSKELETON:
Microfilaments
Intermediate filaments
Microtubules
Ribosomes
Microvilli
Golgi apparatus
Peroxisome
Mitochondrion
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Lysosome
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Figure 6.8c
Ribosomes
Human cells from lining
of uterus (colorized TEM)
Central vacuole
Mitochondrion
Peroxisome
Chloroplast
Plasma
membrane
Cell wall
Nucleus
Mitochondrion
A single yeast cell
(colorized TEM)
Cell wall
Mitochondrion
Nucleus
Nucleolus
Cells from duckweed
(colorized TEM)
1 μm
Cell
Chloroplast
8 μm
CYTOSKELETON
Plant Cells
Microtubules
Cell wall
Vacuole
Yeast cells budding
(colorized SEM)
Unicellular
Eukaryotes
Microfilaments
Nucleus
Nucleolus
5 μm
Golgi
apparatus
Cell
1 μm
Buds
5 μm
Smooth ER
Parent
cell
Fungal Cells
Rough ER
Animal Cells
Nuclear
envelope
NUCLEUS
Nucleolus
Chromatin
10 μm
Figure 6.8b
Flagella
Nucleus
Nucleolus
Vacuole
Chlamydomonas
(colorized SEM)
Chloroplast
Cell wall
Chlamydomonas
(colorized TEM)
Plasmodesmata
Wall of adjacent cell
Concept 6.3: The eukaryotic cell’s genetic
instructions are housed in the nucleus and
carried out by the ribosomes
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The Nucleus: Information Central
Nucleus
1 μm
Nucleus
Nucleolus
Chromatin
 The nucleus contains most of the DNA in a
eukaryotic cell
Nuclear envelope:
Inner membrane
Outer membrane
Nuclear pore
 Ribosomes use the information from the DNA to
make proteins
Rough
ER
Surface of
nuclear envelope
(TEM)
Pore
complex
Ribosome
0.25 μm
Close-up
of nuclear
envelope
Pore complexes (TEM)
Chromatin
0.5 μm
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Nuclear lamina (TEM)
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Ribosomes: Protein Factories
 Ribosomes are complexes made of ribosomal RNA and
protein
 Ribosomes carry out protein synthesis in two locations
Concept 6.4: The endomembrane system
regulates protein traffic and performs metabolic
functions in the cell
 The endomembrane system consists of
 In the cytosol (free ribosomes)
 Nuclear envelope
 On the outside of the endoplasmic reticulum or the nuclear
envelope (bound ribosomes)
 Endoplasmic reticulum
 Golgi apparatus
0.25 μm
Ribosomes
Free ribosomes in cytosol
Endoplasmic
reticulum (ER)
ER
 Lysosomes
 Vacuoles
Ribosomes bound to ER
Large
subunit
 Plasma membrane
Small
subunit
TEM showing ER
and ribosomes
Diagram of a
ribosome
Computer model
of a ribosome
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 These components are either continuous or
connected via transfer by vesicles
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The Endoplasmic Reticulum: Biosynthetic
Factory
Functions of ER
 The smooth ER
 Synthesizes lipids
 Metabolizes carbohydrates
Smooth ER
Rough ER
Nuclear
envelope
Smooth ER
Rough ER
 Detoxifies drugs and poisons
 Stores calcium ions
 The rough ER
ER lumen
Cisternae
Ribosomes
Transport vesicle
 Has bound ribosomes, which secrete glycoproteins
(proteins covalently bonded to carbohydrates)
Transitional
ER
0.20 μm
 Distributes transport vesicles, secretory proteins
surrounded by membranes
 Is a membrane factory for the cell
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The Golgi Apparatus: Shipping and Receiving Center
 The Golgi apparatus consists of flattened membranous
sacs called cisternae
Golgi
apparatus
cis face
(“receiving” side of
Golgi apparatus)
The Golgi Apparatus: Shipping and
Receiving Center
 Functions of the Golgi apparatus
 Modifies products of the ER
0.1 μm
 Manufactures certain macromolecules
Cisternae
 Sorts and packages materials into transport
vesicles
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trans face
(“shipping” side of
Golgi apparatus)
TEM of Golgi apparatus
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Lysosomes: Digestive Compartments
 A lysosome is a membranous sac of hydrolytic
enzymes that can digest macromolecules
 Lysosomal enzymes work best in the acidic
environment inside the lysosome
 Hydrolytic enzymes and lysosomal membranes
are made by rough ER and then transferred to the
Golgi apparatus for further processing
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 Some types of cell can engulf smaller organisms
or food particles by phagocytosis; this forms a
food vacuole
 A lysosome fuses with the food vacuole and
digests the molecules
 Lysosomes also use enzymes to recycle the
cell’s own organelles and macromolecules,
a process called autophagy
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Figure 6.13
Vacuoles: Diverse Maintenance Compartments
Vesicle containing
two damaged
organelles
1 μm
Nucleus
1 μm
Mitochondrion
fragment
 Vacuoles perform a variety of functions in different
kinds of cells
Peroxisome
fragment
Lysosome
Digestive
enzymes
 Food vacuoles are formed by phagocytosis
Lysosome
Lysosome
Plasma
membrane
Peroxisome
Digestion
Food
vacuole
Mitochondrion
Vesicle
(a) Phagocytosis: lysosome digesting food
 Vacuoles are large vesicles derived from the ER
and Golgi apparatus
Digestion
(b) Autophagy: lysosome breaking down
damaged organelles
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 Contractile vacuoles, found in many freshwater
protists, pump excess water out of cells
 Central vacuoles, found in many mature plant
cells, hold organic compounds and water
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Figure 6.14
The Endomembrane System: A Review
 The endomembrane system is a complex and dynamic
player in the cell’s compartmental organization
Central vacuole
Nucleus
Cytosol
Rough ER
Nucleus
Central
vacuole
Smooth ER
cis Golgi
Cell wall
Chloroplast
5 μm
trans Golgi
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Plasma
membrane
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Concept 6.5: Mitochondria and chloroplasts
change energy from one form to another
 Mitochondria are the sites of cellular respiration,
a metabolic process that uses oxygen to
generate ATP
The Evolutionary Origins of Mitochondria and
Chloroplasts
 Mitochondria and chloroplasts have similarities
with bacteria
 Enveloped by a double membrane
 Chloroplasts, found in plants and algae, are the
sites of photosynthesis
 Contain free ribosomes and circular DNA molecules
 Peroxisomes are oxidative organelles
 Grow and reproduce somewhat independently
in cells
 These similarities led to the endosymbiont
theory
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Figure 6.16
 The endosymbiont theory suggests that an early
ancestor of eukaryotes engulfed an oxygen-using
nonphotosynthetic prokaryotic cell
 The engulfed cell formed a relationship with the
host cell, becoming an endosymbiont
 The endosymbionts evolved into mitochondria
Endoplasmic
reticulum
Nucleus
Nuclear
envelope
Ancestor of
eukaryotic cells (host cell)
Mitochondrion
Engulfing of
photosynthetic
prokaryote
Chloroplast
At least
Mitochondrion
one cell
 At least one of these cells may have then taken up
a photosynthetic prokaryote, which evolved into a
chloroplast
Engulfing of oxygenusing nonphotosynthetic
prokaryote, which
becomes a mitochondrion
Nonphotosynthetic
eukaryote
Photosynthetic eukaryote
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Figure 6.17b
Mitochondria: Chemical Energy Conversion
Mitochondria
Mitochondrion
10 μm
Intermembrane
space
Outer
membrane
DNA
Mitochondrial
DNA
Inner
membrane
Free
ribosomes
in the
mitochondrial
matrix
Nuclear DNA
Cristae
Matrix
(b) Network of mitochondria in
Euglena (LM)
0.1 μm
(a) Diagram and TEM of mitochondrion
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Figure 6.18b
Chloroplasts: Capture of Light Energy
Ribosomes
50 μm
Stroma
Inner
and outer
membranes
Granum
DNA
Intermembrane space
Thylakoid
(a) Diagram and TEM of chloroplast
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Chloroplasts
(red)
1 μm
(b) Chloroplasts in an
algal cell
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Figure 6.19
Peroxisomes: Oxidation
Peroxisome
 Peroxisomes are specialized metabolic
compartments bounded by a single membrane
Mitochondrion
 Peroxisomes produce hydrogen peroxide and
convert it to water
 Peroxisomes perform reactions with many
different functions
 How peroxisomes are related to other organelles
is still unknown
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Chloroplasts
1 μm
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Concept 6.6: The cytoskeleton is a network of
fibers that organizes structures and activities in the
cell
Roles of the Cytoskeleton: Support and Motility
 The cytoskeleton is a network of fibers extending
throughout the cytoplasm
 The cytoskeleton helps to support the cell and
maintain its shape
 It organizes the cell’s structures and activities, anchoring
many organelles
 It interacts with motor proteins to produce motility
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10 μm
 Inside the cell, vesicles can travel along tracks
provided by the cytoskeleton
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Figure 6.21
ATP
Components of the Cytoskeleton
Vesicle
Receptor for
motor protein
 Three main types of fibers make up the
cytoskeleton
Motor protein Microtubule
(ATP powered) of cytoskeleton
(a) Motor proteins “walk” vesicles along cytoskeletal
fibers.
Microtubule
Vesicles
0.25 μm
 Microtubules are the thickest of the three
components of the cytoskeleton
 Microfilaments, also called actin filaments, are the
thinnest components
 Intermediate filaments are fibers with diameters in
a middle range
(b) SEM of a squid giant axon
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Table 6.1
Microtubules
 Microtubules are hollow rods about 25 nm in
diameter and about 200 nm to 25 microns long
 Functions of microtubules
 Shaping the cell
 Guiding movement of organelles
 Separating chromosomes during cell division
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Centrosomes and
Centrioles
Centrosome
Cilia and Flagella
 In animal cells,
microtubules grow out
from a centrosome near
the nucleus
 In animal cells, the
centrosome has a pair of
centrioles, each with nine
triplets of microtubules
arranged in a ring
Longitudinal
section of one
centriole
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Figure 6.23
Microtubule
Centrioles
0.25 μm
Microtubules
Cross section
of the other centriole
 Microtubules control the beating of flagella and
cilia, microtubule-containing extensions that
project from some cells
 Cilia and flagella differ in their beating patterns
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 Cilia and flagella share a common structure
(a) Motion of flagella
Direction of swimming
0.1 μm
Outer microtubule
doublet
Motor proteins
(dyneins)
Central
microtubule
5 μm
Radial spoke
Microtubules
(b) Motion of cilia
Direction of organism’s movement
Power
stroke
Plasma
membrane
Plasma
membrane
Basal
body
Recovery
stroke
(b) Cross section of
motile cilium
0.1 μm
Cross-linking
proteins between
outer doublets
Triplet
0.5 μm
(a) Longitudinal section
of motile cilium
15 μm
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(c) Cross section of basal body
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 Cilia and flagella share a common structure
 How dynein “walking” moves flagella and cilia
 A core of microtubules sheathed by the
plasma membrane
 Dynein arms alternately grab, move, and release
the outer microtubules
 A basal body that anchors the cilium or flagellum
 Cross-linking proteins limit sliding
 A motor protein called dynein, which drives the
bending movements of a cilium or flagellum
 Forces exerted by dynein arms cause doublets
to curve, bending the cilium or flagellum
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 Microfilaments are solid rods about 7 nm in
diameter, built as a twisted double chain of
actin subunits
 The structural role of microfilaments is to bear
tension, resisting pulling forces within the cell
 They form a 3-D network called the cortex just
inside the plasma membrane to help support the
cell’s shape
Microvillus
0.25 µm
Figure 6.25
Microfilaments (Actin Filaments)
Plasma membrane
Microfilaments (actin
filaments)
Intermediate filaments
 Bundles of microfilaments make up the core of
microvilli of intestinal cells
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 Microfilaments that function in cellular motility contain the
protein myosin in addition to actin
 In muscle cells, thousands of actin filaments are arranged
parallel to one another
 Thicker filaments composed of myosin interdigitate with the
thinner actin fibers
Figure 6.26
Cortex (outer cytoplasm):
gel with actin network
100 µm
Inner cytoplasm
(more fluid)
Extending
pseudopodium
Muscle cell
0.5 µm
Amoeboid movement
30 µm
Chloroplast
Cytoplasmic streaming in
plant cells
Actin
filament
Myosin
filament
Myosin
head
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Intermediate Filaments
Concept 6.7: Extracellular components and
connections between cells help coordinate
cellular activities
 Intermediate filaments range in diameter from 8–12
nanometers, larger than microfilaments but smaller than
microtubules
 They support cell shape and fix organelles in place
 Most cells synthesize and secrete materials that
are external to the plasma membrane
 Intermediate filaments are more permanent cytoskeleton
fixtures than the other two classes
 These extracellular structures are involved in a
great many cellular functions
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Cell Walls of Plants
 Plant cell walls may have
multiple layers
 The cell wall is an extracellular structure that
distinguishes plant cells from animal cells
 Primary cell wall: Relatively
thin and flexible
 Prokaryotes, fungi, and some unicellular
eukaryotes also have cell walls
 Middle lamella: Thin layer
between primary walls of
adjacent cells
 The cell wall protects the plant cell, maintains its
shape, and prevents excessive uptake of water
 Plant cell walls are made of cellulose fibers
embedded in other polysaccharides and protein
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 Secondary cell wall (in some
cells): Added between the
plasma membrane and the
primary
cell wall
Secondary
cell wall
Primary
cell wall
Middle
lamella
1 μm
Central vacuole
Cytosol
Plasma membrane
 Plasmodesmata are
channels between adjacent
plant cells
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Plant cell walls
Plasmodesmata
Figure 6.28
The Extracellular Matrix (ECM) of Animal Cells
 Animal cells lack cell walls but are covered by an
elaborate extracellular matrix (ECM)
 The ECM is made up of glycoproteins such as
collagen, proteoglycans, and fibronectin
 ECM proteins bind to receptor proteins in the
plasma membrane called integrins
EXTRACELLULAR FLUID
Collagen
A proteoglycan
complex
Polysaccharide
molecule
Carbohydrates
Fibronectin
Core
protein
Plasma
membrane
Proteoglycan
molecule
Microfilaments
CYTOPLASM Integrins
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Cell Junctions
 The ECM has an influential role in the lives of cells
 ECM can regulate a cell’s behavior by
communicating with a cell through integrins
 Neighboring cells in tissues, organs, or organ
systems often adhere, interact, and communicate
through direct physical contact
 The ECM around a cell can influence the activity
of gene in the nucleus
 Mechanical signaling may occur through
cytoskeletal changes, that trigger chemical signals
in the cell
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Plasmodesmata in Plant Cells
Tight Junctions, Desmosomes, and Gap
Junctions in Animal Cells
 Plasmodesmata are channels that perforate plant
cell walls
 Through plasmodesmata, water and small solutes
(and sometimes proteins and RNA) can pass from
cell to cell
Cell walls
Interior
of cell
 At tight junctions, membranes of neighboring cells
are pressed together, preventing leakage of
extracellular fluid
 Desmosomes (anchoring junctions) fasten cells
together into strong sheets
 Gap junctions (communicating junctions) provide
cytoplasmic channels between adjacent cells
Interior
of cell
0.5 μm
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 Three types of cell junctions are common in
epithelial tissues
Plasmodesmata
Plasma membranes
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Figure 6.30
Tight junctions prevent
fluid from moving
across a layer of cells.
The Cell: A Living Unit Greater Than the Sum of Its
Parts
Tight
junction
 Cells rely on the integration of structures and organelles in
order to function
TEM
0.5 μm
Tight junction
Intermediate
filaments
 For example, a macrophage’s ability to destroy bacteria
involves the whole cell, coordinating components such as
the cytoskeleton, lysosomes, and plasma membrane
Desmosome
Gap
junction
Desmosome 1 μm
(TEM)
Plasma
membranes of
adjacent cells
Extracellular
matrix
Space
between cells
TEM
Ions or small
molecules
0.1 μm
Gap junctions
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