The Cytoskeleton
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You should be familiar with the basic eukaryotic cell architecture, including the nucleus, endomembrane system, and the concept that cellular structures are not static but dynamic. Prior knowledge of protein structure and basic biochemistry is assumed.
Why this matters
The cytoskeleton is the dynamic internal framework that gives the cell its shape, enables movement, and organises internal traffic.
Without the cytoskeleton, eukaryotic cells would be shapeless sacs of cytoplasm, unable to divide, move, or transport materials efficiently. It acts as a structural scaffold, a motor track, and a signalling platform, integrating mechanical forces with biochemical pathways. Understanding the cytoskeleton is essential for grasping how cells maintain integrity, respond to their environment, and execute complex processes like mitosis and intracellular transport.
The Three Main Components
The cytoskeleton is composed of three distinct types of protein filaments: microtubules, microfilaments (actin filaments), and intermediate filaments. Each has a unique diameter, structure, and set of functions. Microtubules are the thickest, hollow tubes that serve as tracks for motor proteins and form the mitotic spindle. Microfilaments are the thinnest, solid rods of actin that drive cell motility and shape changes. Intermediate filaments are rope-like structures that provide tensile strength and anchor organelles in place. Unlike the other two, intermediate filaments are far less dynamic and serve mainly in structural support (though nuclear lamins are taken apart and rebuilt at each mitosis).
Microtubules: Polar Tracks for Transport
Microtubules are hollow cylinders approximately 25 nm in diameter, composed of 13 protofilaments made of \(\alpha\)- and \(\beta\)-tubulin dimers. They exhibit polarity, with a "plus" end that grows faster and a "minus" end that is typically anchored at the centrosome. This polarity is crucial for directional transport. Motor proteins, such as kinesin and dynein, use ATP hydrolysis to move cargo along the microtubule. Kinesin generally moves cargo toward the cell periphery (plus end), while dynein moves cargo toward the centre (minus end). The dynamic instability of microtubules, where they rapidly grow and shrink, allows the cell to reorganise its internal architecture quickly in response to stimuli.
Actin Filaments: The Engine of Motility
Actin filaments, or microfilaments, are solid rods approximately 7 nm in diameter, composed of globular actin (G-actin) monomers polymerized into filamentous actin (F-actin). Like microtubules, actin filaments are polar, with a barbed (plus) end and a pointed (minus) end. The rapid turnover of actin filaments, driven by the addition of G-actin at the plus end and depolymerization at the minus end, allows for processes like cell crawling, cytokinesis, and muscle contraction. In muscle cells, actin filaments interact with myosin motor proteins to generate force. In non-muscle cells, actin networks form the cortical layer beneath the plasma membrane, providing structural support and facilitating endocytosis and exocytosis.
Intermediate Filaments: Structural Anchors
Intermediate filaments are approximately 10 nm in diameter and are composed of various fibrous proteins, such as keratin, vimentin, and lamin. Unlike microtubules and actin filaments, intermediate filaments are not polar and do not use ATP for assembly or disassembly. They are highly stable and provide tensile strength to the cell, resisting mechanical stress. In the nucleus, nuclear lamins form a meshwork beneath the nuclear envelope, maintaining nuclear shape and integrity. Intermediate filaments also anchor organelles, such as the Golgi apparatus and mitochondria, in specific locations within the cytoplasm. Their stability makes them ideal for long-term structural support rather than dynamic processes.
The concept
Organelles and the structure-function principle
Each organelle's form is matched to its role.
A common misconception is that the cytoskeleton is a static structure. In reality, it is highly dynamic, with microtubules and actin filaments constantly assembling and disassembling. This dynamic instability is not a flaw but a feature, allowing the cell to rapidly reorganise in response to signals. For example, during mitosis, the interphase microtubule network is disassembled and reassembled into the mitotic spindle. Confusing the cytoskeleton with a rigid skeleton like that of animals leads to misunderstandings of cell motility and division. The cytoskeleton is a fluid, responsive network, not a fixed frame. C
Worked example
Problem. A kinesin motor protein moves a vesicle along a microtubule at a speed of 800 nm/s. If the vesicle must travel a distance of 10 μm to reach its destination, how long will the journey take? Express your answer in seconds.
- Convert the distance from micrometres to nanometres: \( 10\ \mu\mathrm{m} \times 1000\ \mathrm{nm}/\mu\mathrm{m} = 10000\ \mathrm{nm} \).
- Use the formula \( \text{time} = \frac{\text{distance}}{\text{speed}} \).
- Substitute the values: \( \text{time} = \frac{10000\ \mathrm{nm}}{800\ \mathrm{nm/s}} \).
- Calculate the result: \( \text{time} = 12.5\ \mathrm{s} \).
Units check. Nanometres divided by nanometres per second yields seconds, which is the correct unit for time.
Three quick tests
- Which cytoskeletal component is non-polar and does not use ATP for assembly?
Answer
Intermediate filaments. - What is the primary function of microtubules in cell division?
Answer
To form the mitotic spindle, which separates chromosomes. - Which motor protein moves cargo toward the cell periphery along microtubules?
Answer
Kinesin.
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Every lecture and every concept in this unit, with prerequisites and threads.
Lecture notes drafted with a local model and checked line by line by the editor; every number in the worked example was recomputed.