Difficulty: Intermediate | Prerequisites: Basic cell biology (cell membrane, organelles, ATP as energy currency).
This topic sits at the heart of how cells move things around, change shape, and power animal locomotion. Every process in your body that involves physical movement, from transporting a vesicle down an axon to dividing a cell in two, relies on the same basic toolkit: a protein scaffold (the cytoskeleton) and molecular motors that walk along it. You should already be comfortable with basic cell structure, membrane proteins, and how ATP works as an energy source. If you understand why cells need to shuttle cargo and generate force, the rest of this chapter clicks into place.
Cells build two main track systems (microtubules and microfilaments) from protein subunits that assemble and disassemble dynamically. Motor proteins (kinesin, dynein, myosin) walk along these tracks using ATP, carrying cargo or generating force. The balance between growth and shrinkage of these filaments is tightly regulated and is essential for everything from cell division to muscle contraction.
Cytoskeleton
A protein-based intracellular network of filaments that provides structural support, enables transport, and generates movement. Think of it as the cell's internal scaffolding and road system combined.
Microtubule
Hollow, tube-like polymer made of tubulin dimers, used for long-distance intracellular transport and cell division. In simple terms, these are the motorways of the cell.
Microfilament (actin filament)
Solid polymer made of the protein actin, involved in cell shape, local movement, and muscle contraction. Think of it as the cell's local road network and rope system.
Tubulin
The protein subunit of microtubules. A dimer of alpha-tubulin and beta-tubulin that assembles spontaneously when activated by GTP.
G-actin (globular actin)
The monomeric form of actin. When G-actin polymerises, it forms F-actin (filamentous actin), the functional strand of a microfilament.
F-actin (filamentous actin)
The polymerised strand of actin monomers that makes up a microfilament. In simple terms, this is what you get when you string G-actin beads together into a chain.
Motor protein
An enzyme (ATPase) that converts chemical energy from ATP into mechanical movement along a cytoskeletal track.
Kinesin
Motor protein that moves toward the plus (+) end of microtubules (anterograde transport, away from the nucleus). Think of it as the outbound delivery truck.
Dynein
Motor protein that moves toward the minus (-) end of microtubules (retrograde transport, toward the nucleus). Larger than kinesin and roughly five times faster.
Myosin
Motor protein that moves along microfilaments (actin). The key motor behind muscle contraction and most actin-based cellular movements.
MTOC (microtubule organising centre)
Structure near the nucleus where the minus (-) ends of microtubules are anchored. In simple terms, it is the hub from which the microtubule network radiates outward.
Dynamic instability
The property of microtubules by which they alternate between phases of growth and rapid shrinkage, driven by whether the beta-tubulin at the tip is GTP-bound (growth) or GDP-bound (shrinkage).
MAPs (microtubule-associated proteins)
Proteins that bind to microtubules and regulate their stability, dynamics, and interactions. They are targets of hormonal regulation.
GTP cap
A region of GTP-bound tubulin at the growing end of a microtubule that promotes continued assembly. Loss of the cap triggers rapid disassembly.
Treadmilling
Simultaneous assembly at the (+) end and disassembly at the (-) end of a filament, so the filament appears to move without changing overall length. In simple terms, the filament is a treadmill belt, with new subunits added at one end and removed at the other.
Capping proteins
Proteins that bind to the ends of microfilaments and block further addition or removal of actin monomers. Capping the (+) end causes net shrinkage; capping the (-) end causes net growth.
All physiological processes depend on movement: intracellular transport, changes in cell shape, cell motility, and whole-animal locomotion. Every one of these uses the same basic cellular machinery.
The cytoskeleton is a protein-based intracellular network with two main elements:
Microtubules (long-distance tracks)
Microfilaments (local tracks and force generators)
Motor proteins are ATPase enzymes that convert ATP energy into mechanical movement:
Kinesin and dynein walk along microtubules
Myosin walks along microfilaments
Three ways cells use the cytoskeleton for movement:
Cytoskeleton as a "road" with motor protein carriers -- motor proteins carry cargo along filament tracks (e.g. vesicle transport in neurons)
Reorganisation of the cytoskeletal network (amoeboid movement) -- the network itself reshapes, with no motor protein required (e.g. white blood cell crawling)
Motor proteins pull on the cytoskeletal "rope" -- when motor proteins are anchored, they pull the filament toward them, generating force (e.g. muscle contraction)
Microtubules are hollow, tube-like polymers of the protein tubulin.
Tubulin is a dimer of alpha-tubulin and beta-tubulin
Assembly is spontaneous (no enzyme required) and driven by GTP binding
13 protofilaments line up side by side to form a sheet, which rolls into a tube
Dimers can be added or removed from both ends, but growth is asymmetric, faster at the (+) end
The two ends of a microtubule are structurally and functionally different.
Minus (-) end: anchored at the MTOC (microtubule organising centre), near the nucleus
Plus (+) end: attached to integral proteins in the plasma membrane; this is the fast-growing end
Whether a microtubule grows or shrinks depends on the nucleotide state of the beta-tubulin at its tip.
GTP-bound beta-tubulin = GROWTH. New dimers add easily and the microtubule elongates.
GDP-bound beta-tubulin = SHRINKAGE. Dimers dissociate and the microtubule disassembles.
Once beta-tubulin is incorporated into the microtubule, its GTP may or may not hydrolyse to GDP. This stochastic hydrolysis is what makes microtubules dynamically unstable.
Four factors regulate the balance:
Local tubulin concentration -- high [tubulin] promotes growth
Dynamic instability -- the GTP hydrolysis cycle on beta-tubulin drives alternating grow/shrink phases
MAPs (microtubule-associated proteins) -- regulate dynamics and are targets of hormonal regulation
Chemical disruptors -- plant poisons such as taxol and colchicine freeze microtubule dynamics, which blocks cell division (this is the basis of some anti-cancer drugs)
Cells can also lock tubulin in the GTP or GDP state using sulfate, preventing phosphate hydrolysis and fixing the microtubule in either a growing or shrinking state.
Motor proteins walk along microtubules, powered by ATP hydrolysis.
Kinesin moves toward the (+) end (anterograde, away from nucleus)
Dynein moves toward the (-) end (retrograde, toward nucleus)
Dynein is larger than kinesin and moves roughly five times faster
Each step consumes 1 ATP
The rate of movement depends on the ATPase domain of the motor protein and on regulatory proteins
A classic example: in neurons, kinesin carries filled vesicles from the cell body down the axon to the synapse, while dynein returns empty vesicles back to the cell body.
Cellular process | Physiological function |
|---|---|
Cytokinesis | Microtubules ensure chromosomes are equally divided after mitosis |
Axon structure | Microtubules support the long axons of the nervous system |
Vesicle transport | Microtubules carry hormones from sites of synthesis to sites of release |
Pigment dispersion | Microtubules control the distribution of pigment granules to affect animal colour |
Flagellar movement | Flagella (built from tubulin) allow sperm to swim toward the egg |
Ciliary movement | Cilia propel mucus and other fluids over the epithelial surface |
Sensory reception | Olfactory receptors and photoreceptors use cilia for signal detection |
Microfilaments are solid polymers composed of the protein actin. They tend to operate locally within the cell and often work with the motor protein myosin.
G-actin monomers polymerise to form a strand called F-actin
Assembly is spontaneous and 6 to 10 times faster at the (+) end than the (-) end
Actin joins together to form microfilaments, which are thinner than microtubules
Actin polymerisation alone can generate movement by reorganising the cytoskeleton, with no motor protein needed (e.g. amoeboid movement by white blood cells)
Sliding filaments using myosin are the more common mechanism; the interaction between a microfilament and myosin generates force
When assembly at the (+) end and disassembly at the (-) end happen at the same rate, the filament maintains a constant length while appearing to move through space. No new actin monomers are needed for this process. Treadmilling is sufficient for amoeba-like movement, such as when white blood cells change shape.
During treadmilling the filament length stays constant, but sometimes the cell needs net growth or net shrinkage. Capping proteins control this.
Cap the (+) end -- blocks assembly, so disassembly at the (-) end dominates = net shrinkage
Cap the (-) end -- blocks disassembly, so assembly at the (+) end dominates = net growth
Capping proteins are frequent targets of hormones that regulate microfilament dynamics
Taxol (paclitaxel) and colchicine are plant-derived poisons that freeze microtubule dynamics. Because dividing cells depend on microtubule assembly and disassembly to pull chromosomes apart, these drugs halt cell division. Taxol is one of the most widely used chemotherapy agents in cancer treatment. Kinesin and dynein-based transport is the reason neurons, which can be over a metre long, are able to ship neurotransmitter-filled vesicles from the cell body to distant synapses and bring recycling material back.
Students often think microtubules and microfilaments are interchangeable. They are not: microtubules are long-distance highways made of tubulin; microfilaments are local tracks made of actin, used mainly for shape changes and force generation.
Students often assume that treadmilling means the filament is physically moving through the cell. The filament stays in place; subunits are added at one end and removed at the other, creating an illusion of movement.
Students often confuse the direction of kinesin and dynein. Kinesin = (+) end = anterograde (away from nucleus). Dynein = (-) end = retrograde (toward nucleus).
Students sometimes think GTP hydrolysis on tubulin is enzymatic. Tubulin assembly is spontaneous and does not require an enzyme; GTP hydrolysis happens intrinsically within the incorporated beta-tubulin.
⚠️ Know the mnemonic: GTP-bound = GROWTH, GDP-bound = SHRINKAGE. This is heavily tested.
⚠️ Be able to explain dynamic instability in your own words and link it to the GTP state of beta-tubulin.
⚠️ Know which motor protein goes which direction: kinesin = (+) = anterograde; dynein = (-) = retrograde.
⚠️ Understand why taxol and colchicine are used as anti-cancer drugs (they disrupt microtubule dynamics, blocking cell division).
⚠️ Be able to distinguish treadmilling from net growth/shrinkage and explain the role of capping proteins.
⚠️ Know that capping the (+) end = net shrinkage, capping the (-) end = net growth.
True or false: Microtubule assembly requires an enzyme. (False, it is spontaneous.)
Fill in the blank: Kinesin moves toward the ______ end of a microtubule. (Plus / +)
True or false: Capping the minus end of a microfilament causes net shrinkage. (False, capping the minus end causes net growth.)
Fill in the blank: The protein subunit of microtubules is a dimer of ______ and ______. (Alpha-tubulin and beta-tubulin.)
True or false: Dynein is smaller and slower than kinesin. (False, dynein is larger and roughly five times faster.)
Q: Name the two main cytoskeletal elements and the motor proteins associated with each.
A: Microtubules (motor proteins: kinesin and dynein) and microfilaments (motor protein: myosin).
Q: A microtubule has GTP-bound beta-tubulin at its plus end. Will it grow or shrink? Why?
A: It will grow. GTP-bound beta-tubulin promotes the addition of new dimers. If the GTP hydrolyses to GDP, the dimers dissociate and the microtubule shrinks.
Q: Explain why taxol is effective as a chemotherapy drug.
A: Taxol stabilises microtubules and prevents their dynamic disassembly. Since cell division depends on the controlled assembly and disassembly of the mitotic spindle (made of microtubules), freezing the dynamics blocks mitosis and prevents cancer cell proliferation.
Q: What is treadmilling and why does it not require new actin monomers?
A: Treadmilling is simultaneous assembly at the (+) end and disassembly at the (-) end. The subunits removed from one end are recycled to the other, so the filament maintains constant length without needing a fresh supply of monomers.
Q: A capping protein binds to the (+) end of a microfilament. What happens to its overall length and why?
A: The filament undergoes net shrinkage. Assembly at the (+) end is blocked, but disassembly at the (-) end continues, so the filament gets shorter over time.
This material connects directly to the next section on myosin and muscle contraction (Part 2), where the actin-myosin sliding filament model depends on everything covered here about microfilaments, motor proteins, and ATP-driven movement. It also ties back to cell signalling: hormones regulate MAPs and capping proteins, linking signal transduction to cytoskeletal dynamics. If you go on to study neuroscience, axonal transport by kinesin and dynein is central to understanding how neurons function and how neurodegenerative diseases arise.
cytoskeleton, microtubule, microfilament, actin filament, tubulin, alpha-tubulin, beta-tubulin, kinesin, dynein, myosin, motor protein, ATPase, MTOC, centrosome, dynamic instability, GTP cap, GDP, treadmilling, capping protein, CapZ, tropomodulin, G-actin, F-actin, protofilament, MAPs, microtubule-associated proteins, taxol, paclitaxel, colchicine, anti-cancer drugs, axonal transport, anterograde, retrograde, vesicle transport, cell motility, amoeboid movement, cilia, flagella, cell division, cytokinesis, polarity