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Cells Part 2
© 2014 Pearson Education, Inc.
Cytoskeleton
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Network of fibers extending throughout the cytoplasm.
It organizes the cell’s structures and activities, anchoring many
organelles.
Helps to support the cell and maintain the shape.
Interacts with motor proteins to produce motility.
Inside the cell, vesicles can travel along tracks provided by the
cytoskeleton
It is composed of three types of molecular structures:
Microtubules, Microfilaments, Intermediate Filaments
•
Microtubules: the thickest of the three components of the
cytoskeleton. Are hollow rods about 25 nm in diameter and
about 200 nm to 25 microns long. Rolled into tubes called
tubulin. They are stiff to provide rigidity and shape. They also
provide an essential role in the equal separation of
chromosomes to daughter cells in mitosis (centrioles). Also give
organelles tracks for movement.
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Centrosomes and
Centrioles
(Microtubules)
• 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.
Found near the
nucleus at right
angles to each
other. They produce
spindles during cell
division.
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Figure 6.22
Centrosome
Microtubule
Centrioles
0.25 μm
Longitudinal
section of one
centriole
Microtubules
Cross section
of the other centriole
ATP
Vesicle
Receptor for
motor protein
Motor protein Microtubule
(ATP powered) of cytoskeleton
(a) Motor proteins “walk” vesicles along cytoskeletal
fibers.
Microtubule
Vesicles
(b) SEM of a squid giant axon
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0.25 μm
Microfilaments (Actin Filaments)
• 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.
• Bundles of microfilaments make up the core of
microvilli of intestinal cells.
• 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.
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Microfilaments
 Localized contraction brought about by actin and
myosin also drives amoeboid movement.
 Cells crawl along a surface by extending pseudopodia
(cellular extensions) and moving toward them.
 Cytoplasmic streaming is a circular flow of cytoplasm
within cells.
 This streaming speeds distribution of materials within
the cell.
 In plant cells, actin-myosin interactions drive
cytoplasmic streaming.
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Muscle cell
0.5 µm
Actin
filament
Myosin
filament
Myosin
head
(a) Myosin motors in muscle cell contraction
Cortex (outer cytoplasm):
gel with actin network
100 µm
Inner cytoplasm
(more fluid)
Extending
pseudopodium
(b) Amoeboid movement
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Chloroplast
30 µm
(c) Cytoplasmic streaming in
plant cells
Plasma membrane
Microfilaments (actin
filaments)
Intermediate filaments
Figure 6.25
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0.25 µm
Microvillus
Intermediate Filaments
 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.
 Intermediate filaments are more permanent
cytoskeleton fixtures than the other two classes.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Cilia and Flagella
• Microtubules control the beating of flagella and cilia,
microtubule-containing extensions that project from
some cells.
• Cilia and flagella differ in their beating patterns.
• They grow out of basal bodies that anchor the cilia or
flagellum. A motor protein called dynein, drives the
bending movements.
• Length of whip like extension determines the name.
Cilia are short, many. Used to move the organism or
move materials past the cell.
• Flagella are long and usually only one or two are
present in any individual cell.
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0.1 μm
Outer microtubule
doublet
Motor proteins
(dyneins)
Central
microtubule
Radial spoke
Microtubules
Plasma
membrane
Basal
body
(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
(c) Cross section of basal body
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Plasma
membrane
(a) Motion of flagella
Direction of swimming
5 μm
(b) Motion of cilia
Direction of organism’s movement
Power
stroke
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Recovery
stroke
15 μm
Animation: Cilia and Flagella
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Extracellular components: between cell coordination of
cellular activities
• Most cells synthesize and secrete materials that are
external to the plasma membrane. These extracellular
structures are involved in a great many cellular
functions. Includes Cell Wall in Plants, Extracellular
Matrix in Animal Cells, Cell Junctions, and
Plasmodesmata in Plant Cells.
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Cell Walls of Plants
 The cell wall is an extracellular structure that distinguishes
plant cells from animal cells
 Prokaryotes, fungi, and some unicellular eukaryotes also
have cell walls
 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
 Plant cell walls may have multiple layers
 Primary cell wall: Relatively thin and flexible
 Middle lamella: Thin layer between primary walls of
adjacent cells
 Secondary cell wall (in some cells): Added between the
plasma membrane and the primary cell wall
 Plasmodesmata are channels between adjacent plant cells
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Secondary
cell wall
Primary
cell wall
Middle
lamella
1 μm
Central vacuole
Cytosol
Plasma membrane
Plant cell walls
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Plasmodesmata
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
 The ECM has an influential role in the lives of cells.
The ECM can regulate a cell’s behavior by
communicating with a cell through integrins. 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.
© 2014 Pearson Education, Inc.
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 in Animals
 Neighboring cells in tissues, organs, or organ systems
often adhere, interact, and communicate through direct
physical contact
 Three types of cell junctions are common in epithelial
tissues
 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
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Tight junctions prevent
fluid from moving
across a layer of cells.
Tight
junction
TEM
0.5 μm
Tight junction
Intermediate
filaments
Desmosome
Gap
junction
Desmosome 1 μm
(TEM)
Plasma
membranes of
adjacent cells
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Space
between cells
Extracellular
matrix
TEM
Ions or small
molecules
0.1 μm
Gap junctions
Plasmodesmata in Plant 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
Interior
of cell
0.5 μm
Figure 6.29
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Plasmodesmata
Plasma membranes
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.