Difference Between Microtubules and Microfilaments

The key difference between microtubules and microfilaments is that the tubulin is the protein that makes microtubules while actin is the protein that makes microfilaments. Also, microtubules are long hollow tube-like structures while microfilaments are linear polymers of the globular protein actin. 

Protein fibers are essential to carry out many functions in the living cells. Microtubules and microfilaments are two types of fibers that occur in the cytoplasm of every eukaryotic cell. These fibers are basically responsible for making the criss-cross networks known as ‘cytoskeleton’ of the cytoplasm. The cytoskeleton is a dynamic system which helps to maintain the shape of the cell and anchor the cell organelles in the cytoplasm. Apart from the above mentioned two types of fibers, intermediate fibers are also important in making the cytoskeleton. Certain fibers also make locomotion structures (flagella, cilia, etc.), which are usually present in certain prokaryotes.

CONTENTS

1. Overview and Key Difference
2. What are Microtubules
3. What are Microfilaments
4. Similarities Between Microtubules and Microfilaments
5. Side by Side Comparison – Microtubules vs Microfilaments in Tabular Form
6. Summary

What are Microtubules?

Microtubules are long, hollow cylindrical structures made up of globular proteins consisting of dimers of α- and β-tubulin subunits arranged side by side around a core. They are the largest elements of the cytoskeleton. Each tube has a diameter of 25 nm, and contains a ring of 13 protein protofilaments. Each protofilament constitutes of α- and β-tubulin globular protein subunits through the polymerization process. The functions of microtubules are governing intracellular transport, separation of chromosomes during mitosis, movement of flagella and cilia, and positioning of cellulose molecules during cell wall synthesis in plants.

In many cells, the formation of microtubules starts from the centre of the cell and radiates towards the periphery. The ends away from the centre are plus (+) ends while the ends towards the center are minus (-) ends. Microtubules have a constant flux of continual polymerization and depolymerization; hence, they have a very short half-life ranging from 20 seconds to 10 minutes.

What are Microfilaments?

Microfilaments are the thinnest fibers in the cytoskeleton. They are made of globular actin protein subunits. Each filament has two protein chains loosely twisted together. Each chain is made of ‘pearl’ like globular protein subunits. The diameter of a microfilament is about 7 nm. Moreover, microfilaments possess polarity, so they have plus (+) and minus (-) ends and they represent the growth & direction of microfilaments.

In addition to making the cytoskeleton, some microfilaments also play a role in the contraction of the cell. Those filaments usually exist as bundles below the plasma membrane. We call these microtubules ‘stress fibers’ in a cell.

What are the Similarities Between Microtubules and Microfilaments?

• Both microtubules and microfilaments are fibers of the cytoskeleton of the eukaryotic cells.
• They are long fibers.
• Furthermore, they are polymers.
• Also, both are able to dissolve and reform quickly.

What is the Difference Between Microtubules and Microfilaments?

Microtubules are long hollow tube-like structures composed of tubulin protein. On the other hand, microfilaments are linear polymers of the globular protein actin. Therefore, this is the key difference between microtubules and microfilaments. When considering their sizes, microtubules are the largest fibers while microfilaments are the thinnest fibers present in the cytoskeleton. Thus, size is a significant difference between microtubules and microfilaments.

Furthermore, 13 protofilaments are arrayed side by side around a central core to form microtubules while two actin strands are twisted to form microfilaments. So, this is a structural difference between microtubules and microfilaments. Moreover, microtubules are stiffer than microfilaments. Also, they help in centriole formation, unlike microfilaments. Therefore, this is another difference between microtubules and microfilaments.

The below infographic presents more details on the difference between microtubules and microfilaments comparatively.

Summary – Microtubules vs Microfilaments

Microtubules and microfilaments are two types of long fibers that make the cytoskeleton. Microtubules are long hollow cylindrical structures made of dimers of α- and β-tubulin subunits arranged side by side around a core. On the other hand, microfilaments are the thinnest fibers made of two intertwined actin strands. Microtubules have a larger diameter than microfilaments. Hence, microtubules are the largest component while microfilaments are the thinnest component of the cytoskeleton. Furthermore, microtubules are stiffer than microfilaments. Thus, this is a summary of the difference between microtubules and microfilaments.

Reference:

1. “Microfilament.” NeuroImage, Academic Press, Available here.
2. Boundless. “Boundless Biology.” Lumen Learning, Lumen, Available here.

Image Courtesy:

1. “Microtubules in the leading edge of a cell” By Y tambe – Y tambe’s file (CC BY-SA 3.0) via Commons Wikimedia
2. “MEF microfilaments” By Y tambe – Y tambe’s file (CC BY-SA 3.0) via Commons Wikimedia