Microtubules vs Microfilaments
Protein fibers are essential to carry out many functions in the smallest unit, in the biological system called ‘cell’. Microtubules and microfilaments are two types of fibers found in the cytoplasm of every eukaryotic cell. These fibers are basically responsible for making the crisscrossed network known as ‘cytoskeleton’ which is found in the cytoplasm of cells. This cytoskeleton is a dynamic system which supports to maintain the shape of the cell and anchors 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 found in certain prokaryotes.
Microtubules are made up of globular proteins consisting of dimers of α- and β-tubulin subunits arranged side by side around a core to form a hollow tube. They are the largest elements of the cytoskeleton. Each tube has a diameter of 25 nm, and it’s composed of a ring of 13 protein protofilaments. Each protofilament is made up 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 toward the periphery. The ends away from the centre are designated as plus (+) ends while the ends toward the center are minus (-) ends. Microtubules have constant flux of continual polymerization and depolymerization, and hence they have a very short half-life ranging from 20 seconds to 10 minutes.
Microfilaments are also known as Actin filaments. As the name implies, these filaments are made of globular actin protein subunits. Each filament is composed of 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. Microfilaments possess polarity so they have plus (+) and minus (-) ends and they represent the growth & direction of microfilaments. Some microfilaments are involved in the contraction of cells while the rest are interconnected to form the cytoskeleton.
The filaments involved in contractile function usually exist as bundles and are concentrated below the plasma membrane. These microtubules are referred to as ‘stress fibers’ in a cell.
What is the difference between Microtubules and Microfilaments?
• Microtubules are composed of tubulin protein while microfilaments are composed of actin protein.
• Microtubule is the largest element of the cytoskeleton and microfilaments are comparatively smaller than microtubules.
• Two actin strands are twisted to form microfilaments whereas 13 protofilaments are arrayed side by side around a central core to form microtubules.
• Usually microfilaments exist as fiber bundles (stress fibers) while microtubules exist as their characteristic hollow tube shape.
• Microtubules are comparatively more stiff elements than microfilaments.
• Unlike microfilaments, many structures with which microtubules are associated are able to grow and move.
• Two sets of microtubules are arranged to form centriole which is located in the centrosome. Microfilaments have no such arrangement.
• Microfilaments have contractile function whereas, microtubules have functions including the intracellular transport and the separation of chromosomes during the mitosis.
• The diameter of microfilament is about 7 nm while that of microtubule is 25 nm.
• Microtubules have a very short half-life compared to that of microfilaments.