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Chapter 6
A Tour of the Cell
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
10 µm
Overview: The Importance of Cells
• All organisms are made of cells
• The cell is the simplest collection of matter
that can live
How old is the earth?
How do we know?
What was it like back then (young planet)?
How did life 1st come about?
The probable evolution of cells was posited by A.
Oparin (1920’s): “chemical evolution”
Inorganic compounds (CHONPS) organic molecules primitive cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The chemical evolution of cells
•
Had to start with building blocks
•
The Miller-Urey experiment
•
The experiment used water (H2O),
methane (CH4), ammonia (NH3), and
hydrogen (H2).
•
The chemicals were all sealed inside a
sterile array of glass tubes and flasks
connected in a loop, with one flask halffull of liquid water and another flask
containing a pair of electrodes.
•
The liquid water was heated to induce
evaporation, sparks were fired between
the electrodes to simulate lightning
through the atmosphere and water
vapor, and then the atmosphere was
cooled again so that the water could
condense and trickle back into the first
flask in a continuous cycle.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The result:
• At the end of one week of continuous operation,
Miller and Urey observed that as much as 10–15%
of the carbon within the system was now in the
form of organic compounds.
• Two percent of the carbon had formed amino
acids that are used to make proteins in living cells,
with glycine as the most abundant.
• Sugars, lipids, and some of the building blocks for
nucleic acids were also formed. (Wikipedia)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
From organic molecules to cells…
• It’s proposed that this “primordial soup” of
organics in the young oceans may have
puddled together, condensed/evaporated into
self-sustaining “globs” = primitive cells?
Some labs have since been able to replicate this
process (make primitive “beginner-level”
protocells)
• Microspheres
• Coacervates
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Once upon a time, about 3.5 billion years ago…
Self-replicating molecules
First to appear were self-replicating molecules.
Then these molecules became more complicated and evolved a
membrane inside which they were protected from the changing
external environment. These were the first cells that became primitive
bacteria, termed Prokaryotes.
•
Archaebacteria
•
“Extremophiles”
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Then what happened?
• According to some more recent findings, many scientists
have pushed back the origin of cells to around 3.8 billion
years!
• The first cells were likely heterotrophic and anaerobic
• According to the heterotroph hypothesis, autotrophs
developed after the evolution of heterotrophs partly
because the primitive atmosphere of the earth lacked
oxygen.
• Oxygen built up as a result of photosynthesis. There is no
evidence of plants or cyanobacteria in the fossil record
before about 2 billion years ago. Also rocks containing
oxides (elements combined with oxygen) don't appear until
then either.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
What about eukaryotic cells?
• Evolved around 1.75 – 2 billion years ago
• “The Endosymbiont Hypothesis” (Lynn Margulis)
• narrated animation
Supporting evidence:
•
Both mitochondria and plastids contain
DNA that is different from that of the cell
nucleus and that is similar to that of
bacteria (in being circular and in its
size).
•
They are surrounded by two or more
membranes, and the innermost of these
shows differences in composition from
the other membranes of the cell. The
composition is like that of a prokaryotic
cell membrane.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Multicellularity
• 1st multicellular eukaryotes seem to have
evolved about a billion years ago
• Algae, kelp, fungi
• May have come about as colonial groups of
individual cells that have division of labor
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Cell anatomy reflects function
• Concept 6.1: To study cells, biologists use
microscopes and the tools of biochemistry
1m
0.1 m
Human height
Length of some
nerve and
muscle cells
Chicken egg
1 cm
Light microscope
• Scientists use microscopes to
visualize cells too small to see
with the naked eye
10 m
Measurements
1 centimeter (cm) = 102 meter (m) = 0.4 inch
1 millimeter (mm) = 10–3 m
1 micrometer (µm) = 10–3 mm = 10–6 m
1 nanometer (nm) = 10–3 mm = 10–9 m
– Can be used to
visualize different
sized cellular
structures
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
10 µ m
1µm
100 nm
Most plant
and Animal
cells
Nucleus
Most bacteria
Mitochondrion
Smallest bacteria
Viruses
10 nm
Ribosomes
Proteins
1 nm
Lipids
Small molecules
Figure 6.2
0.1 nm
Atoms
Electron microscope
• Different types of
microscopes
100 µm
Electron microscope
Frog egg
1 mm
• Light microscopes (LMs)
– Pass visible light through
a specimen
– Magnify cellular
structures with lenses
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• Electron microscopes
(EMs)
– Focus a beam of
electrons through a
specimen (TEM) or
onto its surface
(SEM)
• The scanning electron microscope (SEM)
– Provides for detailed study of the 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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• The transmission electron microscope (TEM)
– Provides for detailed study of the 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)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Cross section
of cilium
1 µm
Isolating Organelles by Cell Fractionation
• Cell fractionation
– Takes cells apart and separates the major
organelles from one another
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• The centrifuge
– Is used to fractionate cells into their
component parts
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Comparing Prokaryotic and Eukaryotic Cells
• All cells have several basic features in common
• They are bounded by a plasma membrane
• They contain a semifluid substance called the
cytosol
• They contain chromosomes
• They all have ribosomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Two types of cells make up every organism
Prokaryotic cells
Eukaryotic cells
• Do not contain a
nucleus
• Have a membranebound nucleus
• Have their DNA
located in a region
called
the nucleoid
• Have membrane-bound
organelles that
compartmentalize their
functions
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
– Contain a true nucleus, bounded by a
membranous nuclear envelope
– Are generally quite a bit bigger than
prokaryotic cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Limitations of Cell Size: Surface Area to Volume Ratio
• The logistics of carrying out cellular metabolism
sets limits on the size of cells
Surface area increases while
total volume remains constant
• A smaller cell has a
higher surface to
volume ratio, which
facilitates the
exchange of
materials into and
out of the cell
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Cell Membrane
• The plasma membrane
– Functions as a selective barrier
– Allows sufficient 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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Hydrophilic
region
Phospholipid
Proteins
(b) Structure of the plasma membrane
A Panoramic View of the Eukaryotic Cell
• Eukaryotic cells
– Have extensive and elaborately arranged
internal membranes, which form organelles
• Plant and
animal cells
have most of
the same
organelles
•
Tutorial on parts of
a typical animal cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• A 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Lysosome
In animal cells but not plant cells:
Lysosomes
Centrioles
Flagella (in some plant sperm)
• A 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
In plant cells but not animal cells:
Chloroplasts
Central vacuole and tonoplast
Cell wall
Plasmodesmata
The Nucleus: Genetic Library of the Cell
• The nucleus
– Contains most of the genes in the
eukaryotic cell (where are the rest?)
The eukaryotic
cell’s genetic
instructions are
housed in the
nucleus and
carried out by the
ribosomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Nuclear membrane
• The nuclear envelope
– Encloses the nucleus, separating its contents
from the cytoplasm
Nucleus
1 µm
Nucleolus
Chromatin
Nucleus
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).
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Nuclear lamina (TEM).
Ribosomes: Protein Factories in the Cell
•
Ribosomes
–
Are particles made of ribosomal RNA
and protein
–
Carry out protein synthesis
Endoplasmic reticulum (ER)
Ribosomes
Cytosol
Free ribosomes
Bound ribosomes
Large
subunit
0.5 µm
Figure 6.11
TEM showing ER and ribosomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Small
subunit
Diagram of a ribosome
Concept 6.4: The endomembrane system
regulates protein traffic and performs metabolic
functions in the cell
• The endomembrane system
– Includes many different structures
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Endoplasmic Reticulum: Biosynthetic Factory
• The endoplasmic reticulum (ER)
– Accounts for more than half the total
membrane in many eukaryotic cells
Smooth ER
• The ER
membrane is
continuous
with the
nuclear
envelope
Rough ER
ER lumen
Cisternae
Ribosomes
Transitional ER
Transport vesicle
Smooth ER
Rough ER 200 µm
Figure 6.12
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Nuclear
envelope
• There are two distinct regions of ER
– Smooth ER, which lacks ribosomes
– Rough ER, which contains ribosomes
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Functions of Smooth ER
• The smooth ER
– Synthesizes lipids
– Metabolizes carbohydrates
– Stores calcium
– Detoxifies poison
Ex: liver cells
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Functions of Rough ER
• The rough ER
– Has bound ribosomes
– Produces proteins and membranes, which are
distributed by transport vesicles
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The Golgi Apparatus: Shipping and
Receiving Center
• The Golgi apparatus
– Receives many of the transport vesicles
produced in the rough ER
– Consists of flattened membranous sacs called
cisternae
• Functions of the Golgi apparatus include
– Modification of the products of the rough ER
– Manufacture of certain macromolecules
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Functions of the 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
Lysosomes: Digestive Compartments
• A lysosome
– Is a membranous
sac of hydrolytic
enzymes
– Can digest all
kinds of
macromolecules
• Lysosomes carry out
intracellular digestion
by phagocytosis
Nucleus
Lysosome
Lysosome contains
active hydrolytic
enzymes
Food vacuole
fuses with
lysosome
Hydrolytic
enzymes digest
food particles
Digestive
enzymes
Lysosome
Plasma membrane
Digestion
Food vacuole
(a) Phagocytosis: lysosome digesting food
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1 µm
Lysosomes
• Autophagy
Lysosome containing
two damaged organelles
Animation of
how lysosomes
work
http://highered.mcgrawhill.com/sites/0072495855/student_vi
ew0/chapter2/animation__lysosomes
.html
1µm
Mitochondrion
fragment
Peroxisome
fragment
Lysosome fuses with
vesicle containing
damaged organelle
Hydrolytic enzymes
digest organelle
components
Lysosome
Vesicle containing
damaged mitochondrion
Figure 6.14 B
Digestion
(b) Autophagy: lysosome breaking down damaged organelle
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Vacuoles: Diverse Maintenance Compartments
• A plant or fungal cell
– May have one or several vacuoles
– Water, food, or waste storage
• In animal cells, these are called “vesicles”
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Food vacuoles
– Are formed by phagocytosis
– animation
• Contractile vacuoles
– Pump excess water out of protist cells
link
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• Central vacuoles
– Are found in plant cells
– Hold reserves of important organic
compounds and water
Central vacuole
Cytosol
Tonoplast
Nucleus
Central
vacuole
Cell wall
Chloroplast
Figure 6.15
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5 µm
• Relationships among 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
• Concept 6.5: Mitochondria and chloroplasts
change energy from one form to another
• Mitochondria
– Are the sites of cellular respiration
• Chloroplasts
– Found only in plants, are the sites of
photosynthesis
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Mitochondria: Chemical Energy Conversion
• Mitochondria are enclosed by two membranes
– Are found in nearly all eukaryotic cells
– A smooth outer membrane
– An 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
Chloroplasts: Capture of Light Energy
• The chloroplast
– Is a specialized member
of a family of closely
related plant organelles
called plastids
– Choroplasts specifically
contain chlorophyll
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Chloroplasts
Are found in leaves and other green organs of
plants and in algae
•
Chloroplast structure includes
–
Thylakoids, membranous
sacs
–
Stroma, the internal fluid
Chloroplast
Ribosomes
Stroma
Chloroplast
DNA
Inner and outer
membranes
Granum
1 µm
Figure 6.18
Thylakoid
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Peroxisomes: Oxidation
• Peroxisomes
– Produce hydrogen peroxide and convert it to
water
Chloroplast
Peroxisome
Mitochondrion
Figure 6.19
1 µm
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Cytoskeleton
Concept 6.6: The cytoskeleton is a network of
fibers that organizes structures and activities in
the cell
Microtubule
Figure 6.20
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0.25 µm
Microfilaments
Roles of the Cytoskeleton: Support, Motility, and Regulation
• The cytoskeleton
– Is a network of fibers extending throughout the
cytoplasm
• Gives mechanical support to the cell
• Is involved in cell motility, which utilizes
motor proteins
Animated, narrated tutorial
http://www.wiley.com/college/pratt/0471393878/student/anim
ations/actin_myosin/index.html
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Components of the Cytoskeleton
• There are three main types of fibers that make
up the cytoskeleton
Table 6.1
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Microtubules
• Microtubules
– Shape the cell
– Guide movement of organelles
– Help separate the chromosome copies in
dividing cells
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Centrosomes and Centrioles
• The centrosome
– Is considered to be a “microtubule-organizing
Centrosome
center”
•
•
Contains a pair of
centrioles
Microtubule
These form the
spindle for
chromosome
division in
mitosis/meiosis
Centrioles
0.25 µm
Figure 6.22
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Longitudinal section
of one centriole
Microtubules
Cross section
of the other centriole
Cilia and Flagella
• Cilia and flagella
– Contain specialized arrangements of
microtubules
– Are locomotor appendages of some cells
– Animated tutorial
http://programs.northlandcollege.edu/biology/Biology1111/animations/flagellum.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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 share a 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
– Is responsible for the bending movement of
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 (Actin Filaments)
• Microfilaments are built from molecules of the
protein actin
• Are found in microvilli
Microvillus
Plasma membrane
Microfilaments (actin
filaments)
Intermediate filaments
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 6.26
0.25 µm
• Microfilaments that function in cellular motility
– Contain the protein myosin in addition to actin
Muscle cell
Actin filament
Myosin filament
Myosin arm
Figure 6.27 A
(a) Myosin motors in muscle cell contraction.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Amoeboid movement
– Involves the contraction of actin and myosin
filaments
Cortex (outer cytoplasm):
gel with actin network
Inner cytoplasm: sol
with actin subunits
Extending
pseudopodium
Figure 6.27 B
(b) Amoeboid movement
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Cytoplasmic streaming
– Is another form of locomotion created by
microfilaments
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
• Intermediate filaments
– Support cell shape
– Fix organelles in place
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Extracellular components and connections between
cells help coordinate cellular activities
• The cell wall
– Is an extracellular structure of plant cells that
distinguishes them from animal cells
– Give support, but limit mobility
• Plant cell walls = cellulose
• Fungal cell walls = chitin
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Plant cell walls
– Are made of cellulose fibers embedded in
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
– Are covered by an elaborate matrix, the ECM
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The ECM
– Is made up of 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 the ECM include
– Support
– Adhesion
– Movement
– Regulation
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Intercellular Junctions
• A cell junction is a structure within a
tissue of a multicellular organism.
• Cell junctions are especially abundant in
epithelial tissues. They consist of protein
complexes and provide contact between
neighboring cells, between a cell and the
extracellular matrix, or they built up the
paracellular barrier of epithelia and control
the paracellular transport.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Plants: Plasmodesmata
• Plasmodesmata
– Are channels that perforate plant cell walls
Cell walls
Interior
of cell
Interior
of cell
Figure 6.30
0.5 µm
Plasmodesmata
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Plasma membranes
Animals: Tight Junctions, Desmosomes, and Gap Junctions
• In animals, there are three types of intercellular
junctions
– Tight junctions
– Desmosomes
– Gap junctions
•Invertebrates have several other types of specific
junctions, for example Septate junctions or the CeAJ
(C. elegans apical junction).
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Types of 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings