Cytoskeleton, Centrosomes & Microtubules - Ninth Grade Science
Early light microscopists viewed the cytoplasm of a cell as a formless, fluid soup of protoplasm within which organelles floated like fruit in gelatin. However, the advent of high-resolution electron microscopy and fluorescent antibody tagging revealed that the eukaryotic cytoplasm is structured by an internal architectural framework: the cytoskeleton. Spanning from the nuclear envelope to the plasma membrane, the cytoskeleton is an interconnected network of filamentous protein polymers that provides structural shape, enables mechanical motility, orchestrates organelle transport, and segregates chromosomes during cell division. Anchoring and organizing this network in animal cells is the centrosome. In this chapter, you will investigate the three filament systems of the cytoskeleton, analyze the architecture of centrosomes and centrioles, and explore cilia and flagella.
The Cytoskeleton: The Structural Framework of the Cell
The cytoskeleton is a dynamic three-dimensional network of protein filaments and tubules extending throughout the cytoplasm of eukaryotic cells.
+--------------------------------------------------------------------------+
| Three Cytoskeletal Filament Systems |
+--------------------------------------------------------------------------+
| 1. Microtubules (25 nm): Hollow tubes of tubulin protein. |
| Spindle fibers, cilia, organelle rails|
| 2. Microfilaments (7 nm): Thin twisted chains of actin protein. |
| Cell shape, muscle contraction, pseud.|
| 3. Intermediate Filaments (10 nm): Rope-like fibrous keratin/lamin cords.|
| Mechanical tensile strength, nucleus |
+--------------------------------------------------------------------------+
Comparison of the Three Cytoskeletal Filaments
Microtubule (25 nm) Microfilament (7 nm) Intermediate Fil. (10 nm)
Hollow Cylindrical Tube Twisted Actin Double Helix Rope-like Fibrous Cables
+---------------+ o-o-o-o-o-o ===============
| o o o o o o o | o-o-o-o-o-o ===============
| (Tubulin Dimer)| (Tensile Anchor)
+---------------+
1. Microtubules (Diameter $\approx 25 ext{ nm}$)
Microtubules are the thickest cytoskeletal filaments, constructed as hollow cylinders composed of repeating heterodimers of alpha-tubulin and beta-tubulin proteins. Thirteen protofilaments align side-by-side to form a cylinder with a 25 nm outer diameter. Microtubules exhibit structural polarity, possessing a rapidly growing plus ($+$) end and a slowly growing minus ($-$) end. They function as intracellular railway tracks along which motor proteins (kinesin and dynein) transport vesicles and organelles. Microtubules also form the mitotic spindle apparatus during cell division and construct the structural core of cilia and flagella.
2. Microfilaments (Actin Filaments, Diameter $\approx 7 ext{ nm}$)
Microfilaments are the thinnest cytoskeletal elements, composed of globular actin (G-actin) monomers polymerized into two intertwined helical strands of filamentous actin (F-actin). Concentrated beneath the plasma membrane as the cell cortex, microfilaments provide structural support. Combined with motor proteins like myosin, actin microfilaments drive cellular motility, muscle contraction, cytoplasmic streaming (cyclosis), and the formation of pseudopodia in Amoeba. During animal cell division, a contractile ring of actin microfilaments constricts the plasma membrane, cleaving the daughter cells in cytokinesis.
3. Intermediate Filaments (Diameter $\approx 8 ext{ to } 12 ext{ nm}$)
Intermediate in diameter between microfilaments and microtubules, intermediate filaments are tough, rope-like fibers constructed from various fibrous proteins (including keratins in epithelial tissues, vimentin in connective tissues, and lamins lining the inner nuclear membrane). Unlike microtubules and microfilaments, intermediate filaments do not participate in cell motility; their primary role is mechanical tensile strength, anchoring the nucleus in place and preventing cells from rupturing when subjected to physical pulling forces.
The Centrosome and Centrioles: The MTOC
The centrosome is the primary Microtubule Organizing Center (MTOC) in animal cells, coordinating the nucleation, anchoring, and spatial orientation of microtubules during interphase and cell division.
The Centrosome Architecture (Animal Cell)
Pericentriolar Material (PCM Protein Cloud)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ ~
~ Centriole 1 ~
~ +=========+ ~
~ | 9 Triplet| ~
~ | Pinwheel | ~
~ +=========+ ~
~ + ~
~ | Oriented at 90 deg! ~
~ +---+ | ~
~ | 9 | | Centriole 2 ~
~ +---+ v ~
~ ~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Centriole Structure: The $9 + 0$ Triplet Pinwheel
The centrosome contains a pair of cylindrical, barrel-shaped structures called centrioles, oriented mutually perpendicular to one another at a ninety-degree angle ($90^\circ$). The centrioles are surrounded by an amorphous protein matrix called the pericentriolar material (PCM). Under electron microscopy, each centriole reveals a "$9 + 0$" triplet pinwheel architecture:
- Nine sets of symmetrically arranged peripheral microtubule triplets (labeled A, B, and C tubules) form the outer circular rim of the cylinder.
- Zero microtubules occupy the center ($9 + 0$ arrangement).
- Radial protein spokes connect each peripheral triplet to a central hub in a cartwheel configuration.
Centrosomes in Plants vs Animals:
Centrosomes with centrioles are characteristic features of animal cells.
Higher plant cells lack centrioles; instead, plants organize mitotic spindle
fibers using diffuse, non-centriolar microtubule organizing centers distributed
throughout the peripheral cytoplasm.
The Role of Centrosomes in Cell Division
During the synthesis (S) phase of the cell cycle, the single centrosome duplicates, producing two centrosomes. When the cell enters mitosis, the two centrosomes separate, migrating to opposite poles of the cell while polymerizing microtubules to construct the mitotic spindle apparatus. Spindle fibers attach to the kinetochores of chromosomes, orchestrating their alignment along the metaphase plate and pulling sister chromatids apart toward opposite poles during anaphase.
Cilia and Flagella: Motility Organelles
Cilia and flagella are hair-like, membrane-bound cellular appendages that protrude from the cell surface to provide swimming locomotion or sweep fluids across tissue surfaces.
+------------------------------------+------------------------------------+
| Cilia (Singular: Cilium) | Flagella (Singular: Flagellum) |
+------------------------------------+------------------------------------+
| Short hair-like projections | Long whip-like projections |
| (typically 5 to 10 um long) | (typically 50 to 150 um long) |
| Present in large numbers | Present singly or in small numbers |
| (hundreds to thousands per cell) | (one to four per cell) |
| Coordinated oar-like beating motion| Undulatory, sinusoidal wave motion |
| Examples: Paramecium, human | Examples: Human sperm cell tail, |
| respiratory epithelium | Euglena swimming tail |
+------------------------------------+------------------------------------+
The Axoneme: The 9 + 2 Microtubule Architecture
Doublet 1 Doublet 2
(A)(B) (A)(B)
\ /
\ /
+------------+
/ (O) (O) \
Doublet 9 --| Central |-- Doublet 3
(Outer Rim) | Singlets | (Dynein Arms attached)
\ (Tubules) /
+------------+
/ \
/ \
(A)(B) (A)(B)
Doublet 6 Doublet 5
* 9 Peripheral Doublet Microtubules surrounding 2 Central Singlets!
The Axoneme: The $9 + 2$ Pattern
Both cilia and flagella share an internal structural core called the axoneme, enclosed by an extension of the plasma membrane. The axoneme exhibits a "$9 + 2$" microtubule arrangement:
- Nine peripheral doublet microtubules (each consisting of an A tubule fused to a B tubule) arranged in an outer circle.
- Two central single microtubules running down the axis, encased in a central protein sheath.
- Radial spokes extend inward from each outer doublet toward the central sheath, while nexin protein bridges link adjacent outer doublets together.
- Motor Dynein Arms: Projecting from the A tubule of each outer doublet toward the neighboring B tubule are pairs of inner and outer arms composed of the motor protein dynein. Powered by ATP hydrolysis, the dynein arms walk along the adjacent microtubule doublet. Because the doublets are cross-linked by flexible nexin bridges, the sliding movement is converted into a localized bending curve, generating rhythmic beating motions.
The Basal Body
At the intracellular base of every cilium and flagellum sits a cylindrical anchor structure called the basal body (kinetosome). The basal body possesses a "$9 + 0$" triplet architecture identical to a centriole, from which the axoneme's microtubules nucleate and grow outward.
Chapter Exercises
Exercise 1: Compare the three major filament systems of the eukaryotic cytoskeleton—microtubules, microfilaments, and intermediate filaments—in terms of their protein monomer composition, physical diameters, and primary cellular functions.
Exercise 2: Contrast the structural architecture of an animal centriole with the internal axoneme of a swimming cilium, stating their respective microtubule arrangements using numerical notation.
Exercise 3: Explain the molecular mechanism by which dynein motor arms produce the rhythmic beating motion of cilia and flagella, and describe how human respiratory epithelial cilia protect the lungs from particulate pollution.
Solutions and Explanations
Solution 1: Comparing the three cytoskeletal filament systems: Microtubules: Diameter of 25 nanometers; composed of alpha-tubulin and beta-tubulin heterodimers assembled into hollow cylindrical tubes. Primary functions include forming the mitotic spindle during cell division, serving as internal railway tracks for kinesin and dynein vesicle transport, and providing the structural core of cilia and flagella. Microfilaments: Diameter of 7 nanometers; composed of globular actin monomers polymerized into two intertwined helical chains. Primary functions include providing structural support to the cell cortex, driving muscle contraction via myosin interactions, generating cytoplasmic streaming, producing pseudopodia during amoeboid cell crawling, and constricting the cleavage furrow during cytokinesis. Intermediate Filaments: Diameter of 8 to 12 nanometers; composed of fibrous structural proteins (including keratins, vimentins, and nuclear lamins) assembled into rope-like cords. Primary functions include providing mechanical tensile strength to prevent cellular tearing under physical stress, maintaining nuclear shape, and anchoring the nucleus and organelles in place.
Solution 2: An animal centriole has a "$9 + 0$" triplet pinwheel architecture with no central tubules: it consists of nine sets of peripheral microtubule triplets arranged in a circle, joined to a central hub via radial spokes in a cartwheel configuration. In contrast, the internal axoneme of a cilium or flagellum has a "$9 + 2$" doublet architecture: it consists of nine sets of peripheral doublet microtubules linked by nexin bridges surrounding two central singlet microtubules enclosed in a central sheath. The centriole has no membrane, whereas the cilium's axoneme is enclosed by an extension of the plasma membrane.
Solution 3: The rhythmic beating of cilia and flagella is driven by dynein motor arms attached to the peripheral microtubule doublets of the axoneme. Powered by the hydrolysis of ATP, the dynein arms on one doublet reach out, bind to the adjacent doublet, and attempt to walk along its length, creating an active sliding force between neighboring doublets. However, because adjacent doublets are tethered together by elastic nexin protein cross-links, they cannot slide freely past one another. The sliding force is converted into mechanical bending along the length of the shaft. In the human respiratory tract, millions of ciliated epithelial cells lining the trachea and bronchi beat in a synchronized, wave-like metachronal rhythm toward the pharynx. Mucus secreted by goblet cells traps inhaled dust, pollen, and bacteria, and the beating cilia sweep this contaminated mucus carpet upward away from the delicate lungs toward the throat, where it is swallowed and sterilized by stomach acid, protecting the respiratory system.