10 Flashcards
(226 cards)
1
Q
Cell Biology 222
A
2
Q
Lecture10
A
3
Q
Cytoskeleton
A
4
Q
Created by Dr. Najah AL-Baqami
A
5
Q
1/16/2025 1
A
6
Q
➢ The cytoskeleton is a network of different protein fibers that provides many
A
7
Q
functions: it maintains or changes the shape of the cell; it secures some organelles in
A
8
Q
specific positions; it enables movement of cytoplasm and vesicles within the cell;
A
9
Q
and it enables the cell to move in response to stimuli.
A
10
Q
➢ There are three types of fibers within the cytoskeleton: microfilaments,
A
11
Q
intermediate filaments, and microtubules. Some of the cytoskeletal fibers work in
A
12
Q
conjunction with molecular motors which move along the fibers within the cell to
A
13
Q
carry out a diverse set of functions. There are two main families of cytoskeletally-
A
14
Q
associated molecular motors: dyneines and kinesins.
A
15
Q
1/16/2025 2
A
16
Q
➢ The cytoskeleton is a highly dynamic
A
17
Q
structure that is continuously
A
18
Q
reorganized as a cell changes shape,
A
19
Q
divides, and responds to its
A
20
Q
environment.
A
21
Q
➢ The cytoskeleton controls the location
A
22
Q
of the organelles that conduct these
A
23
Q
specialized functions.
A
24
Q
➢ It is also responsible for the segregation
A
25
of chromosomes into daughter cells and
26
the pinching apart of cells at cell
27
division.
28
Page571
29
1/16/2025 3
30
➢ The cytoskeleton is built on a framework of three types
31
of protein filaments: intermediate filaments,
32
microtubules, and actin filaments.
33
➢ Each type of filament has distinct mechanical
34
properties and is formed from a different protein
35
subunit
36
Page572
37
1/16/2025 4
38
Page572
39
1/16/2025 5
40
Page573
41
1/16/2025 6
42
Page573
43
1/16/2025 7
44
Page573
45
1/16/2025 8
46
➢ Intermediate filaments strengthen animal cells (epithelial cells )
47
➢ Intermediate filaments are particularly prominent in the cytoplasm of cells that are
48
subject to mechanical stress. They are present in large numbers, for example, along
49
the length of nerve cell axons by stretching and distributing the effect of locally
50
applied forces, keep cells and their membranes from breaking in response to
51
mechanical shear.
52
➢ The nuclear envelope is supported by a meshwork of Intermediate Filaments.
53
Page574-576
54
1/16/2025 9
55
Figure 17–7 Intermediate filaments support and strengthen the nuclear envelope.
56
(A) Schematic cross section through the nuclear envelope. The intermediate filaments of
57
the nuclear lamina line the inner face of the nuclear envelope and are thought to provide
58
attachment sites for the DNA-containing chromatin. Page574-576
59
1/16/2025 10
60
Figure 17–4 Intermediate filaments
61
strengthen animal cells. If a sheet of
62
epithelial cells is stretched by external
63
forces (due to the growth or
64
movements of the surrounding tissues,
65
for example), then the network of
66
intermediate filaments and
67
desmosomal junctions that extends
68
through the sheet develops tension
69
and limits the extent of stretching. If
70
the junctions alone were present, then
71
the same forces would cause a major
72
deformation of the cells, even to the
73
extent of causing their plasma
74
membranes to rupture.
75
Page574-576
76
1/16/2025 11
77
➢ Microtubules usually grow out of an organizing structure (a centrosome,
78
spindle pole, the basal body of a cilium).
79
➢ The centrosome is the major microtubule-organizing center in animal
80
cells.
81
➢ Microtubules are maintained by a balance of assembly and disassembly.
82
➢ Microtubules organize the interior of the cell.
83
➢ Motor Proteins drive intracellular transport.
84
➢ Organelles move along microtubules.
85
1/16/2025 12
86
Figure 17–8 Microtubules usually grow out of an organizing structure. Unlike
87
intermediate filaments, microtubules (dark green) extend from an organizing center such as
88
(A) a centrosome, (B) a spindle pole, or (C) the basal body of a cilium. Page 577-584
89
1/16/2025 13
90
Figure 17–10 Tubulin
91
polymerizes from nucleation
92
sites on a centrosome. (A)
93
Schematic drawing showing that
94
a centrosome consists of an
95
amorphous matrix of protein
96
containing the -tubulin rings
97
that nucleate microtubule
98
growth. In animal cells, the
99
centrosome contains a pair of
100
centrioles, each made up of a
101
cylindrical array of short
102
microtubules.
103
(B) A centrosome with attached microtubules. The minus end of each microtubule is embedded in the centrosome, having
104
grown from a nucleating ring, whereas the plus end of each microtubule is free in the cytoplasm. (C) A reconstructed image
105
shows a dense thicket of microtubules emanating from the centrosome of a C. elegans cell. (C, from E.T. O’Toole et al., J. Cell Biol. 163:451–
106
456, 2003. With permission from The Rockefeller University Press.)
107
Page 577-584
108
1/16/2025 14
109
Figure 17–13 The selective
110
stabilization of microtubules can
111
polarize a cell. A newly formed
112
microtubule will persist only if both
113
its ends are protected from
114
depolymerization. In cells, the minus
115
ends of microtubules are generally
116
protected by the organizing centers
117
from which the filaments grow. The
118
plus ends are initially free but can be
119
stabilized by other proteins.
120
Here, for example, a nonpolarized cell is depicted in (A) with new microtubules growing from and shrinking
121
back to a centrosome in many directions randomly. Some of these microtubules happen by chance to encounter
122
proteins (capping proteins) in a specific region of the cell cortex that can bind to and stabilize the free plus ends
123
of microtubules (B). This selective stabilization will lead to a rapid reorientation of the microtubule arrays (C)
124
and convert the cell to a strongly polarized form (D). Page 577-584
125
1/16/2025 15
126
Figure 17–14 Microtubules transport
127
cargo along a nerve cell axon. In nerve
128
cells, all the microtubules in the axon
129
point in the same direction, with their
130
plus ends toward the axon terminal. The
131
oriented microtubules serve as tracks for
132
the directional transport of materials
133
synthesized in the cell body but required
134
at the axon terminal (such as membrane
135
proteins required for growth).
136
For an axon passing from your spinal cord to a muscle in your shoulder, say, the journey takes about two days.
137
In addition to this outward traffic of material (red circles) driven by one set of motor proteins, there is inward
138
traffic (blue circles) in the reverse direction driven by another set of motor proteins. The inward traffic carries
139
materials ingested by the tip of the axon or produced by the breakdown of proteins and other molecules back
140
toward the cell body.
141
Page 577-584
142
1/16/2025 16
143
Figure 17–16 Motor proteins move along
144
microtubules using their globular heads. (A)
145
Kinesins and cytoplasmic dyneins are
146
microtubule motor proteins that generally move
147
in opposite directions along a microtubule. Each
148
of these proteins (drawn here to scale) is a dimer
149
composed of two identical molecules. Each
150
protein has two globular heads that interact with
151
microtubules at one end and a single tail at the
152
other. (B) Schematic diagram of a motor protein
153
showing ATP-dependent “walking” along a
154
filament.
155
Page 577-584
156
1/16/2025 17
157
Figure 17–17 Different motor proteins transport cargo along microtubules. Most kinesins move toward the
158
plus end of a microtubule, whereas dyneins move toward the minus end. Both types of microtubule motor
159
proteins exist in many forms, each of which is thought to transport a different cargo. The tail of the motor
160
Page 577-584
161
protein determines what cargo the protein transports.
162
1/16/2025 18
163
➢ Actin filaments can grow by the addition of actin monomers at either end.
164
➢ ATP hydrolysis decreases the stability of the actin polymer.
165
➢ Forces generated in the actin-rich cortex move a cell forward.
166
➢ Actin associates with myosin to Form contractile structures.
167
Page 590-597
168
1/16/2025 19
169
Figure 17–28 Actin filaments allow eucaryotic cells to adopt a variety of shapes and perform a variety
170
of functions. Various actin-containing structures are shown here in red: (A) microvilli; (B) contractile
171
bundles in the cytoplasm; (C) sheetlike (lamellipodia) and fingerlike (filopodia) protrusions from the
172
leading edge of a moving cell; (D) contractile ring during cell division. Page 590-597
173
1/16/2025 20
174
Figure 18–33 The contractile ring divides the cell in two. (A) Scanning electron
175
micrograph of an animal cell in culture in the last stages of cytokinesis. (B) Schematic
176
diagram of the midregion of a similar cell showing the contractile ring beneath the plasma
177
Page 636
178
membrane and the remains of the two sets of interpolar microtubules.
179
1/16/2025 21
180
Figure 17–30 ATP hydrolysis
181
decreases the stability of the
182
actin polymer. Actin monomers in
183
the cytosol carry ATP, which is
184
hydrolyzed to ADP soon after
185
assembly into a growing filament.
186
The ADP molecules remain
187
trapped within the actin filament,
188
unable to exchange with ATP until
189
the actin monomer that carries
190
them dissociates from the filament.
191
Page 590-597
192
1/16/2025 22
193
Figure 17–32 Forces generated in the actin-rich
194
cortex move a cell forward. In this proposed
195
mechanism for cell movement, actin polymerization at
196
the leading edge of the cell pushes the plasma membrane
197
forward (protrusion) and forms new regions of actin
198
cortex, shown here in red. New points of anchorage are
199
made between the actin filaments and the surface on
200
which the cell is crawling (attachment). Contraction at
201
the rear of the cell then draws the body of the cell
202
forward (traction). New anchorage points are established
203
at the front, and old ones are released at the back as the
204
cell crawls forward. The same cycle is repeated over and
205
over again, moving the cell forward in a stepwise
206
fashion.
207
Page 590-597
208
1/16/2025 23
209
Figure 17–36 The short tail of a
210
myosin-I molecule contains sites
211
that bind to various components of
212
the cell, including membranes. (A)
213
Myosin-I has a single globular head
214
and a tail that attaches to another
215
molecule or organelle in the cell. This
216
arrangement allows the head domain
217
to move a vesicle relative to an actin
218
filament (B), or an actin filament and
219
the plasma membrane relative to each
220
other (C). Note that the head group of
221
the myosin always walks toward the
222
plus end of the actin filament it
223
contacts.
224
Page 590-597
225
1/16/2025 24
226
1/16/2025 25
