Chapter 4:
4.4-4.5 Mini Lecture
Concept 4.4 The Cytoskeleton Provides Strength and Movement
The cytoskeleton:
• Supports and maintains cell shape
• Holds organelles in position
• Moves organelles
• Is involved in cytoplasmic streaming
• Interacts with extracellular structures to
anchor cell in place
Concept 4.4 The Cytoskeleton Provides Strength and Movement
The cytoskeleton has three components
with very different functions:
• Microfilaments
• Intermediate filaments
• Microtubules
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Microfilaments:
• Help a cell or parts of a cell to move
• Determine cell shape
• Are made from the protein actin—which attaches to
the “plus end” and detaches at the “minus end” of the
filament
• The filaments can be made shorter (more
detachment) or longer (more assembly)
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Intermediate
filaments:
• At least 50 different kinds
• Have tough, ropelike
protein assemblages
• more permanent than
other filaments
• Anchor cell structures in
place
• Resist tension, maintain
rigidity
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Microtubules:
The largest diameter
components, with two
roles:
• Form rigid internal
skeleton for some cells
or regions
• Act as a framework for
motor proteins to
move structures in the
cell
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Microtubules line
movable cell
appendages.
Cilia—short,
usually many
present, move
with stiff power
stroke and flexible
recovery stroke
Flagella—longer,
usually one or two
present,
movement is
snakelike
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Cilia and flagella appear in
a “9 + 2” arrangement:
• Doublets—nine fused
pairs of microtubules form
a cylinder
• One unfused pair in
center
Motion occurs as doublets
slide past each other.
Figure 4.13 A Motor Protein Drives Vesicles along Microtubules
Figure 4.15 The Plant Cell Wall
The plant cell wall has
three major roles:
•Provides support for
the cell and limits
volume by remaining
rigid
•Acts as a barrier to
infection
•Contributes to form
during growth and
development
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
Adjacent plant cells
are connected by
plasmodesmata.
These channels allow
movement of water,
ions, small
molecules,
hormones, and
some RNA and
proteins.
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
Many animal cells are surrounded by an
extracellular matrix.
The fibrous component is the protein
collagen.
The gel-like matrix consists of
proteoglycans.
A third group of proteins links the collagen
and the matrix together.
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
Role of extracellular matrices in animal
cells:
• Hold cells together in tissues
• Contribute to physical properties of
cartilage, skin, and other tissues
• Filter materials
• Orient cell movement during growth and
repair
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
Cell junctions are specialized structures
that protrude from adjacent cells and
“glue” them together—seen often in
epithelial cells:
• Tight junctions
• Desmosomes
• Gap junctions
Figure 4.18 Junctions Link Animal Cells (Part 1)
Tight junctions prevent
substances from moving
through spaces between
cells.
Desmosomes hold cells together
but allow materials to move in
the matrix.
Gap junctions are channels that run
between membrane pores in adjacent
cells, allowing substances to pass
between the cells.