Cells are not merely fluid-filled sacs; they possess an intricate internal framework known as the cytoskeleton. This network of protein filaments provides cells with their characteristic shape and mechanical stability. It also enables various dynamic processes, allowing cells to move and organize their internal components. This internal scaffolding is fundamental for the proper functioning and survival of all living cells.
What Are Microfilaments?
Microfilaments, also called actin filaments, are the thinnest components of the cell’s cytoskeleton. They are primarily composed of a globular protein called actin. Individual actin units, known as G-actin monomers, link together to form long, flexible fibers. The resulting filamentous structure, or F-actin, consists of two strands of actin monomers twisted around each other in a double helix. Microfilaments typically have a diameter ranging from 5 to 9 nanometers.
Where Microfilaments Reside
Microfilaments are found throughout the cytoplasm of eukaryotic cells. They are particularly concentrated just beneath the cell membrane, forming a dense network known as the cell cortex. This cortical network provides mechanical support to the plasma membrane. Beyond this cortical layer, microfilaments also extend into the deeper regions of the cytoplasm.
Key Roles of Microfilaments
Microfilaments play diverse and important roles in maintaining cell integrity and facilitating cellular activities.
Cell Shape and Movement
They contribute significantly to a cell’s shape and structure by forming a supportive network, enabling cells to resist external forces and maintain their morphology. These filaments are also instrumental in various forms of cell movement, including amoeboid movement and cell crawling. They facilitate the formation of structures like lamellipodia and filopodia, which help cells navigate their environment.
Muscle Contraction and Cell Division
Furthermore, microfilaments are fundamental to muscle contraction, where they interact with another protein called myosin to generate contractile forces. During cell division, microfilaments form a contractile ring that pinches the cell in two, a process known as cytokinesis. They also serve as tracks for the movement of organelles and vesicles within the cell.
How Microfilaments Work
The functions of microfilaments are enabled by their dynamic nature, involving rapid assembly and disassembly. Actin monomers can quickly polymerize to extend the microfilament, primarily at a faster-growing end known as the barbed or plus end. Conversely, depolymerization occurs more readily at the slower-growing pointed or minus end. This constant adding and removing of actin units, often driven by ATP hydrolysis, allows microfilaments to change length and reorganize swiftly. Motor proteins, particularly myosin, interact with these actin filaments to produce force and movement, with myosin molecules “walk” along the actin tracks, using energy from ATP to drive processes like muscle contraction and the transport of cellular cargo.