Microfilaments vs Microtubules

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Microfilaments and microtubules are key components of the cytoskeleton in eukaryotic cells. A cytoskeleton provides structure to the cell and connects to every part of the cell membrane and every organelle. Microtubules and microfilaments together allow the cell to hold its shape, and move itself and its organelles.

Comparison chart

Microfilaments versus Microtubules comparison chart
	Microfilaments	Microtubules
Structure	Double Helix	Helical lattice
Size	7 nm in diameter	20-25 nm in diameter
Composition	Predominantly composed of contractile protein called actin.	Composed of subunits of protein tubulin. These subunits are termed as alpha and beta.
Strength	Flexible and relatively strong. Resist buckling due to compressive forces and filament fracture by tensile forces.	Stiff and resist bending forces.
Function	Micro-filaments are smaller and thinner and mostly help cells move	Microtubules are shaped similarly but are larger, and help with cell functions such as mitosis and various cell transport functions.

Formation and Structure

Structure of Microtubules

Microtubules are composed of globular proteins called tubulin. Tubulin molecules are bead like structures. They form heterodimers of alpha and beta tubulin. A protofilament is a linear row of tubulin dimers. 12-17 protofilaments associate laterally to form a regular helical lattice.

Fluorescence double staining of a fibroblast. Red: Vinculin; and Green: Actin, the individual subunit of microfilament.

Microtubules constructed from alpha and beta tubulin

Formation of Microfilaments

Actin, the individual subunit of Microfilament

Individual subunits of microfilaments are known as globular actin (G-actin). G-actin subunits assemble into long filamentous polymers called F-actin. Two parallel F-actin strands must rotate 166 degrees to layer correctly on top of each other to form the double helix structure of microfilaments. Microfilaments measure approximately 7 nm in diameter with a loop of the helix repeating every 37 nm.

Biological Role of Microtubules and Microfilaments

Functions of Microfilaments

Microfilaments form the dynamic cytoskeleton, which gives structural support to cells and links the interior of the cell with the surroundings to convey information about the external environment.
Microfilaments provide cell motility. e.g., Filopodia, Lamellipodia.
During mitosis, intracellular organelles are transported by motor proteins to the daughter cells along actin cables.
In muscle cells, actin filaments are aligned and myosin proteins generate forces on the filaments to support muscle contraction.
In non-muscle cells, actin filaments form a track system for cargo transport that is powered by non-conventional myosins such as myosin V and VI. Non-conventional myosins use the energy from ATP hydrolysis to transport cargo (such as vesicles and organelles) at rates much faster than diffusion.

Functions of Microtubules

Microtubules determine the cell structure.
Microtubules form the spindle apparatus to divide the chromosome directly during cell division (mitosis).
Microtubules provide transport mechanism for vesicles containing essential materials to the rest of the cell.
They form a rigid internal core that is used by microtubule-associated motor proteins (MAPs) such as Kinesin and Dyenin to generate force and movement in motile structures such as cilia and flagella. A core of microtubules in the neural growth cone and axon also imparts stability and drives neural navigation and guidance.