1. Cells Part 2

2. Cytoskeleton 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.

3. 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.
Figure 6.22
Centrosome
Microtubule
Centrioles
0.25 μm
Longitudinal
section of one
centriole
Cross section
of the other centriole
Microtubules

4. Motor proteins “walk” vesicles along cytoskeletal fibers.
ATP
Receptor for
motor protein
Motor protein
(ATP powered)
Microtubule
of cytoskeleton
(a)
(a)
Motor proteins “walk” vesicles along cytoskeletal
fibers.
0.25 μm
Microtubule
Vesicles
Figure 6.21 Motor proteins and the cytoskeleton
(b)
SEM of a squid giant axon

5. 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.

6. 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.

7. Myosin motors in muscle cell contraction (c) Cytoplasmic streaming in
Actin
filament
Myosin
filament
Myosin
head
Chloroplast
30 µm
(a)
Myosin motors in muscle cell contraction
(c)
Cytoplasmic streaming in
plant cells
Cortex (outer cytoplasm):
gel with actin network
100 µm
Figure 6.26 Microfilaments and motility
Inner cytoplasm
(more fluid)
Extending
pseudopodium
(b)
Amoeboid movement

8. Microfilaments (actin filaments)
Microvillus
0.25 µm
Plasma membrane
Microfilaments (actin
filaments)
Figure 6.25 A structural role of microfilaments
Intermediate filaments
Figure 6.25

9. 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.

10. Table 6.1 The structure and function of the cytoskeleton

11. 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.

12. Cross section of basal body
Plasma
membrane
0.1 μm
Outer microtubule
doublet
Motor proteins
(dyneins)
Central
microtubule
Radial spoke
Microtubules
Cross-linking
proteins between
outer doublets
(b)
Cross section of
motile cilium
Plasma
membrane
0.1 μm
Basal
body
Triplet
Figure 6.24 Structure of a flagellum or motile cilium
0.5 μm
(a)
Longitudinal section
of motile cilium
(c)
Cross section of basal body

13. Direction of organism’s movement
Motion of flagella
Direction of swimming
5 μm
(b)
Motion of cilia
Direction of organism’s movement
Figure 6.23 A comparison of the beating of flagella and motile cilia
Power
stroke
Recovery
stroke
15 μm

14. Animation: Cilia and Flagella

15. 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.

16. 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

17. Secondary cell wall Primary cell wall Middle lamella 1 μm
Central vacuole
Figure 6.27 Plant cell walls
Cytosol
Plasma membrane
Plant cell walls
Plasmodesmata

18. 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.

19. A proteoglycan EXTRACELLULAR FLUID complex Polysaccharide Collagen
molecule
Collagen
Carbo-
hydrates
Fibronectin
Core
protein
Plasma
membrane
Figure 6.28 Extracellular matrix (ECM) of an animal cell
Proteoglycan
molecule
Microfilaments
Integrins
CYTOPLASM

20. 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

21. Tight junctions prevent fluid from moving across a layer of cells.
TEM
0.5 μm
Tight junction
Intermediate
filaments
Desmosome
Figure 6.30 Exploring cell junctions in animal tissues
Desmosome
(TEM)
1 μm
Gap
junction
Ions or small
molecules
Plasma
membranes of
adjacent cells
Extracellular
matrix
TEM
Space
between cells
0.1 μm
Gap junctions

22. 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
Plasmodesmata
Plasma membranes
Figure 6.29

23. Figure 6.UN03 Summary of key concepts: nucleus and ribosomes

24. Figure 6.UN04 Summary of key concepts: endomembrane system

25. Figure 6.UN05 Summary of key concepts: mitochondria, chloroplasts and peroxisomes