Microfilaments- Definition, Structure, Functions and Diagram

Microfilaments are one of the three main types of protein filaments that make up the cytoskeleton, the network of protein structures that gives shape, support and movement to eukaryotic cells. The other two types are microtubules and intermediate filaments. Microfilaments are the thinnest and most flexible of the cytoskeletal filaments, with a diameter of about 6 to 7 nanometers.

Microfilaments are composed of two intertwined strands of a globular protein called actin. Each strand consists of many actin subunits, also known as globular actin (G-actin), that polymerize to form a long chain called filamentous actin (F-actin). The two strands of F-actin are twisted around each other in a helical orientation, creating a microfilament .

Microfilaments are polar structures, meaning they have a plus end and a minus end that differ in their rates of growth and shrinkage. The plus end is also called the barbed end, and the minus end is also called the pointed end. The plus end grows faster than the minus end because it has a higher affinity for G-actin subunits. The process of adding or removing subunits from the ends of microfilaments is called polymerization or depolymerization, respectively .

Microfilaments have various functions in cells, depending on their location, orientation and interactions with other proteins. Some of their main functions are:

• Muscle contraction: Microfilaments work together with another protein called myosin to generate force and movement in muscle cells. Myosin is a motor protein that can bind to actin and slide along it, causing the microfilaments to shorten and pull on each other. This process is called actomyosin contraction and it is responsible for muscle contraction and relaxation .
• Cell movement: Microfilaments also enable non-muscle cells to move by changing their shape and extending protrusions called pseudopodia (false feet), lamellipodia (flat extensions) or filopodia (thin extensions). These protrusions are driven by the polymerization and depolymerization of actin at the leading edge of the cell, creating pushing and pulling forces that propel the cell forward. This type of movement is called amoeboid movement and it is important for many biological processes such as wound healing, immune response and development .
• Cell division: Microfilaments play a crucial role in cytokinesis, the final stage of cell division when the cytoplasm of a parent cell splits into two daughter cells. Microfilaments form a ring-like structure called the contractile ring at the equator of the cell, where they contract with the help of myosin and pinch off the cell membrane, creating a cleavage furrow that separates the two cells .
• Cell shape and stability: Microfilaments provide mechanical support and rigidity to cells by resisting compression and bending forces. They also help maintain the shape and structure of cell surface projections such as microvilli, which are finger-like extensions that increase the surface area for absorption in some cells. Microfilaments are often found just beneath the plasma membrane, where they form a meshwork called the cell cortex that regulates the shape and movement of the cell surface .
• Cytoplasmic streaming: Microfilaments can also facilitate the flow of cytoplasm within cells, which is called cytoplasmic streaming. This process allows for the transport and distribution of nutrients, organelles and other molecules within the cell. Cytoplasmic streaming is especially important for plant cells, which have large vacuoles that occupy most of their volume .

Microfilaments are dynamic structures that can rapidly assemble and disassemble in response to various signals and stimuli. They are regulated by many proteins that influence their nucleation, polymerization, depolymerization, branching, cross-linking, capping, severing and binding to other molecules. Some examples of these proteins are profilin, cofilin, Arp2/3 complex, formin, fimbrin, villin, gelsolin and tropomyosin . By interacting with these proteins, microfilaments can adapt to different cellular needs and functions.

In summary, microfilaments are thin protein filaments composed of actin subunits that form part of the cytoskeleton. They have various roles in cell structure, movement and division. They are highly dynamic and regulated by many proteins that modulate their behavior.

Microfilaments are thin protein filaments that are part of the cytoskeleton of eukaryotic cells. The cytoskeleton is a network of protein structures that provides shape, support, and movement to the cell and its organelles. Microfilaments are the smallest type of cytoskeletal filaments, with a diameter of about 6 to 8 nanometers .

Microfilaments are mainly composed of actin, a globular protein that can polymerize into long chains called filamentous actin (F-actin). Two strands of F-actin twist around each other to form a helical microfilament . Microfilaments are polar, meaning they have a plus end and a minus end that differ in their growth rates and interactions with other proteins. The plus end is also called the barbed end, and the minus end is also called the pointed end.

Microfilaments have various functions in the cell, such as:

• Contracting muscles in cooperation with myosin, another protein that forms thick filaments.
• Enabling cell movement and shape changes by forming structures like filopodia, lamellipodia, and pseudopodia .
• Assisting in cell division by forming a contractile ring that pinches the cell into two daughter cells .
• Anchoring and organizing the plasma membrane and its associated proteins.
• Participating in endocytosis and exocytosis, processes that involve the uptake and release of materials by the cell.

Microfilaments are dynamic structures that can assemble and disassemble rapidly depending on the needs of the cell. They are regulated by various proteins that bind to them and affect their stability, length, branching, and interactions with other molecules. Microfilaments are usually concentrated near the cell periphery, where they form part of the cell cortex.

Microfilaments as part of the cytoskeleton

The cytoskeleton is a network of protein filaments that extends throughout the cell, giving the cell structure and keeping organelles in place. The cytoskeleton consists of three types of protein fibers: microtubules, intermediate filaments, and microfilaments. Microfilaments are the smallest and thinnest filaments of the cytoskeleton, with a diameter of about 6 to 7 nanometers .

Microfilaments are composed of two intertwined strands of a globular protein called actin, which is why they are also known as actin filaments . Each strand is made up of subunits of actin called globular actin (G-actin), which join together to form filamentous actin (F-actin). Microfilaments are polar, meaning they have a plus end and a minus end that differ in their growth rates and interactions with other proteins .

Microfilaments are dynamic structures that can assemble and disassemble rapidly in response to cellular needs . They are regulated by a number of proteins that bind to them and influence their polymerization, depolymerization, branching, capping, severing, cross-linking, and sliding. Microfilaments are usually nucleated at the plasma membrane, where they form a dense network called the cell cortex that supports the shape and movement of the cell surface.

Microfilaments have various functions in the cell, such as:

• Muscle contraction: Microfilaments work with another protein called myosin to generate forces for muscle movement. In muscle cells, microfilaments form bundles called myofibrils that are organized into sarcomeres, the basic units of muscle contraction. Myosin molecules act as motors that slide along the microfilaments and pull them closer together, causing the sarcomeres to shorten and the muscle to contract .
• Cell movement: Microfilaments also enable non-muscle cells to move by forming protrusions at the cell front, such as filopodia (thin extensions) and lamellipodia (broad sheets). These protrusions are driven by the polymerization of actin at the plus end of the microfilaments, which pushes the plasma membrane outward. The protrusions then adhere to the substrate and pull the cell forward. Microfilaments also help in cell movement by generating contractile forces at the cell rear, where they interact with myosin and cause the cell to retract .
• Cell division: Microfilaments play a crucial role in cytokinesis, which is the final stage of cell division when the cytoplasm splits into two daughter cells. During cytokinesis, microfilaments form a ring around the equator of the cell, where they cooperate with myosin to constrict the cell membrane and create a cleavage furrow. The furrow deepens until it separates the two daughter cells completely .
• Organelle placement: Microfilaments also help in maintaining the position and shape of organelles within the cell. For example, microfilaments anchor the nucleus to the cytoplasm and prevent it from moving around. They also support the structure and function of organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes by forming scaffolds or tracks for their movement.
• Cell shape and movement: Microfilaments can also change the shape and movement of cells by altering their cytoskeleton. For example, microfilaments can form stress fibers that span across the cell and provide mechanical stability. They can also form contractile bundles that bend or twist the cell. They can also form gel-like networks that allow the cell to flow or crawl.

Microfilaments are essential components of the cytoskeleton that provide structural support and enable various cellular processes. They are highly dynamic and adaptable structures that respond to internal and external signals. They interact with many other proteins that modulate their behavior and function. They are involved in muscle contraction, cell movement, cell division, organelle placement, and cell shape and movement.

To visualize the structure and function of microfilaments, it is helpful to look at some diagrams that show how they are arranged and interact with other proteins in the cell. Here are some examples of microfilament diagrams:

• This diagram shows the basic structure of a microfilament, composed of two intertwined strands of actin subunits (G-actin) that form a helical filament (F-actin). The filament has a polarity, with a barbed (+) end and a pointed (-) end. The barbed end grows faster than the pointed end by adding more actin subunits.

• This diagram shows how microfilaments are involved in muscle contraction. In muscle cells, microfilaments form myofibrils, which are bundles of actin and myosin filaments. Actin and myosin interact to form actomyosin complexes, which slide past each other and shorten the sarcomere, the basic unit of muscle tissue. This process requires ATP and calcium ions.

• This diagram shows how microfilaments are involved in cell movement. In some cells, such as amoebae and white blood cells, microfilaments form projections called pseudopodia, which extend and retract to propel the cell forward. Microfilaments also enable cytoplasmic streaming, which is the circular flow of cytoplasm within the cell. This helps distribute nutrients and organelles throughout the cell.

These diagrams illustrate some of the roles that microfilaments play in maintaining the cell`s shape, movement, and function. Microfilaments are dynamic structures that can assemble and disassemble quickly in response to the cell`s needs. They are essential for many cellular processes and interactions.

Microfilaments are primarily composed of polymers of actin, but in cells are modified by and interact with numerous other proteins. When actin is first produced by the cell, it appears in a globular form. But in microfilaments, however, they appear as long polymerized chains of the molecules are intertwined in a helix, creating a filamentous form of the protein ie. F-actin.

They are thus composed of two strands of subunits of the protein actin wound in a spiral. Specifically, the actin subunits that come together to form a microfilament are called globular actin (G-actin), and once they are joined together they are called filamentous actin (F-actin) . They are usually about 7 nm in diameter making them the thinnest filaments of the cytoskeleton.

The polymers of these linear filaments are flexible but still strong, resisting crushing and buckling while providing support to the cell. Like microtubules, microfilaments are polar. Their positively charged, or plus end, is barbed and their negatively charged minus end is pointed. Polarization occurs due to the molecular binding pattern of the molecules that make up the microfilament. Also like microtubules, the plus end grows faster than the minus end.

A microfilament begins to form when three G-actin proteins come together by themselves to form a trimer. Then, more actin binds to the barbed end. The process of self-assembly is aided by autoclampin proteins, which act as motors to help assemble the long strands that makeup microfilaments. Two long strands of actin arrange in a spiral in order to form a microfilament.

Microfilaments are typically nucleated at the plasma membrane. Therefore, the periphery (edges) of a cell generally contains the highest concentration of microfilaments. A number of external factors and a group of special proteins influence microfilament characteristics, however, and enable them to make rapid changes if needed, even if the filaments must be completely disassembled in one region of the cell and reassembled somewhere else.

When finding directly beneath the plasma membrane, microfilaments are considered part of the cell cortex, which regulates the shape and movement of the cell’s surface.

Microfilaments are primarily composed of polymers of actin, but in cells they are modified by and interact with numerous other proteins. Some of these proteins regulate the assembly and disassembly of microfilaments, while others link microfilaments to other cytoskeletal elements or to the plasma membrane. Some of the proteins that interact with microfilaments are:

• Profilin: This protein binds to actin monomers (G-actin) and promotes their addition to the plus end of microfilaments (F-actin). Profilin also stimulates the exchange of ADP for ATP on G-actin, which increases the rate of polymerization.
• Cofilin: This protein binds to ADP-actin filaments and accelerates their depolymerization by severing them. Cofilin also prevents the reannealing of severed filaments, thus increasing the pool of free actin monomers.
• Arp2/3 complex: This protein complex nucleates the formation of branched microfilaments by binding to the side of an existing filament and initiating a new filament at a 70-degree angle. The Arp2/3 complex is activated by various signaling proteins that respond to extracellular cues, such as growth factors or cell adhesion molecules.
• Formins: These proteins nucleate the formation of unbranched microfilaments by associating with the plus end and facilitating the addition of actin monomers. Formins also bind to profilin and recruit it to the growing end, thus enhancing the polymerization process. Formins are involved in the formation of stress fibers and contractile rings in animal cells.
• Tropomyosin: This protein binds along the length of microfilaments and stabilizes them by preventing their depolymerization. Tropomyosin also regulates the interaction between actin and myosin in muscle cells by blocking the myosin-binding sites on actin until calcium ions are released.
• Troponin: This protein complex binds to tropomyosin and actin in muscle cells and mediates the calcium-dependent regulation of muscle contraction. When calcium ions bind to troponin, it undergoes a conformational change that moves tropomyosin away from the myosin-binding sites on actin, allowing myosin to interact with actin and generate force.
• Myosin: This protein is a motor protein that uses ATP hydrolysis to move along microfilaments and generate force. Myosin has a head domain that binds to actin and a tail domain that interacts with other myosins or other molecules. There are many types of myosins that have different functions in different cell types. For example, myosin II forms bipolar filaments that interact with actin filaments to cause muscle contraction, while myosin V transports vesicles and organelles along actin filaments.
• Fimbrin: This protein crosslinks parallel microfilaments into tight bundles by binding to two actin filaments at a 65-degree angle. Fimbrin is found in microvilli and filopodia, where it helps maintain their shape and rigidity.
• Alpha-actinin: This protein crosslinks parallel microfilaments into loose bundles by binding to two actin filaments at a 166-degree angle. Alpha-actinin is found in stress fibers and adherens junctions, where it links actin filaments to transmembrane proteins that connect cells to each other or to the extracellular matrix.

• Spectrin: This protein forms a network of short actin filaments that are attached to the cytoplasmic face of the plasma membrane by linking them to transmembrane proteins such as ankyrin and band 3. Spectrin provides mechanical support and elasticity to the membrane and helps maintain its shape and integrity.

  Formation or Self-assembly of Microfilaments

Microfilaments are formed by the polymerization of actin monomers, also known as globular actin (G-actin). The process of microfilament formation involves three main steps: nucleation, elongation, and steady-state.

• Nucleation is the rate-limiting step that initiates the formation of a microfilament. It occurs when three G-actin molecules spontaneously associate to form a trimer, which serves as a nucleus for further polymerization. The nucleation step is slow and unfavorable because it requires overcoming the entropy loss of bringing together three molecules in a specific orientation. However, once a nucleus is formed, it can rapidly recruit more actin monomers to grow the filament.
• Elongation is the step that extends the length of the microfilament by adding more actin monomers to both ends. The two ends of a microfilament have different affinities for G-actin: the plus end (or barbed end) has a higher affinity and thus a faster growth rate than the minus end (or pointed end). This polarity is due to the asymmetric structure of actin, which has four subdomains that interact differently with neighboring subunits. The plus end has an exposed subdomain that can bind G-actin more tightly than the minus end, which has a buried subdomain. The elongation step is fast and favorable because it releases energy from the hydrolysis of ATP bound to G-actin.
• Steady-state is the step that reaches a dynamic equilibrium between the addition and loss of actin monomers at both ends of the microfilament. The steady-state is achieved when the concentration of free G-actin in the cytoplasm equals the critical concentration of each end, which is the minimum concentration required for polymerization to occur. At this point, the net growth rate of the microfilament is zero, but there is still constant turnover of actin subunits at both ends. This dynamic instability allows microfilaments to respond quickly to changes in cellular conditions or signals.

The formation of microfilaments is regulated by various actin-binding proteins that modulate the nucleation, elongation, and steady-state steps. Some examples are:

• Nucleation factors, such as Arp2/3 complex and formins, that facilitate the formation of new microfilament branches or linear filaments by lowering the activation energy for nucleation.
• Capping proteins, such as CapZ and gelsolin, that bind to either end of the microfilament and prevent further addition or loss of actin monomers, thus stabilizing or terminating its growth.
• Severing proteins, such as cofilin and gelsolin, that cut existing microfilaments into shorter fragments by breaking the bonds between actin subunits, thus increasing the number of ends available for polymerization or depolymerization.
• Cross-linking proteins, such as fimbrin and alpha-actinin, that bind to two or more microfilaments and organize them into parallel or antiparallel bundles or networks, thus enhancing their mechanical strength and structural diversity.

The formation of microfilaments is essential for many cellular functions, such as cell shape, movement, division, and signaling. By interacting with other cytoskeletal elements and membrane proteins, microfilaments provide structural support and dynamic flexibility to the cell.

Functions of Microfilaments

Microfilaments are involved in various cellular functions that require movement, shape change, or structural support. Some of the main functions of microfilaments are:

• Muscle contraction: Microfilaments work together with another protein called myosin to enable the contraction and relaxation of muscle cells. The actin and myosin filaments slide past each other in a coordinated manner, generating force and movement. This process is essential for voluntary movements, such as walking or lifting, as well as involuntary movements, such as heartbeat or digestion .
• Cell motility: Microfilaments also enable the movement of cells, especially those that have a single cell body, such as amoebae or white blood cells. Microfilaments can extend and retract parts of the cell membrane, creating protrusions called filopodia or lamellipodia that help the cell crawl or change direction. Microfilaments also power the movement of cilia and flagella, which are hair-like structures that project from some cells and beat rhythmically to propel the cell or move fluids around it .
• Cytokinesis: Microfilaments play a key role in the division of cells into two daughter cells. During cytokinesis, a ring of actin and myosin filaments forms around the middle of the cell, constricting it until it splits into two. This process is also known as the cleavage furrow. Microfilaments also help to transport organelles and other materials to the daughter cells along actin cables .
• Cell shape and stability: Microfilaments provide structural support to the cell by forming a network under the plasma membrane, called the cell cortex. The cell cortex helps to maintain the cell shape and resist external forces. Microfilaments can also interact with other cytoskeletal elements, such as intermediate filaments and microtubules, to form a complex scaffold that anchors organelles and reinforces the cell`s mechanical strength .
• Endocytosis and exocytosis: Microfilaments are involved in the processes of endocytosis and exocytosis, which are the uptake and release of materials by the cell membrane. In endocytosis, microfilaments help to form vesicles that pinch off from the membrane and carry substances into the cell. In exocytosis, microfilaments help to move vesicles to the membrane and fuse them with it, releasing their contents outside the cell. These processes are important for communication, digestion, immunity, and secretion .

Role in cellular contraction and movement

Microfilaments are involved in various cellular processes that require contraction and movement, such as muscle contraction, cell division, cell migration, and cytoplasmic streaming. These processes depend on the interaction of microfilaments with another protein called myosin, which acts as a motor protein that can bind to and slide along actin filaments.

Muscle contraction

One of the most important roles of microfilaments is to contract muscles. There is a high concentration of microfilaments in muscle cells, where they form myofibrils, the basic unit of the muscle cell. Actin is an indispensable protein for muscle movement, and microfilaments are often called actin filaments because actin is so prominent in the muscular system of the body.

In muscle cells, actin works together with myosin to allow the muscles to contract and relax. Here, neither actin nor myosin can work properly without the other, and they form a complex called actomyosin. Groups of actomyosin are found in sarcomeres, the basic unit of muscle tissue.

Sarcomeres consist of overlapping thick filaments (composed of myosin) and thin filaments (composed of actin). When a muscle cell receives a signal to contract, the myosin heads bind to the actin filaments and pull them towards the center of the sarcomere. This causes the sarcomere to shorten and the muscle to contract. The process is reversed when the muscle cell relaxes.

Cell division

Another important function of microfilaments is to help divide the cell during mitosis (cell division). Microfilaments aid the process of cytokinesis, which is when the cell “pinches off” and physically separates into two daughter cells.

During cytokinesis, a ring of actin and myosin filaments forms beneath the plasma membrane at the equator of the cell. The ring contracts and constricts the cytoplasm, forming a cleavage furrow. The furrow deepens until it reaches the midline of the cell, where it meets a band of microtubules called the midbody. The midbody acts as a bridge between the two daughter cells and helps to complete their separation.

Cell migration

Microfilaments play a role in causing cells to move. This occurs throughout the body and it is also very important for organisms whose entire body consists of one cell, such as amoebae; without microfilaments, they would not be motile.

Actomyosin plays a role here just as it does in muscle cells. In order for cells to move, one end of a microfilament must elongate while the other end must shorten, and myosin acts as a motor to make this happen.

Microfilaments also have a role in forming various cell surface projections that enable cell migration, such as filopodia, lamellipodia, and pseudopodia. Filopodia are thin extensions of the plasma membrane that contain bundles of actin filaments. Lamellipodia are broad extensions of the plasma membrane that contain networks of actin filaments. Pseudopodia are large extensions of the plasma membrane that contain both actin and myosin filaments.

These projections are dynamic structures that can extend and retract by polymerizing and depolymerizing actin filaments at their tips. They also interact with extracellular molecules and receptors that guide the direction and speed of cell movement.

Cytoplasmic streaming

Microfilaments also have a role in cytoplasmic streaming. Cytoplasmic streaming is the flow of cytoplasm (the contents of the cell, including the fluid part called cytosol and cell organelles) throughout the cell.

Cytoplasmic streaming helps to distribute nutrients, metabolites, and organelles within the cell. It also helps to maintain cell polarity and shape. Cytoplasmic streaming is especially prominent in plant cells, where it facilitates photosynthesis by moving chloroplasts around.

Cytoplasmic streaming is driven by actomyosin complexes that are attached to the plasma membrane or to organelles. The complexes generate force by sliding along actin filaments that are arranged along the cortex (the outer layer) or throughout the cytoplasm.

Role in cell division and organelle placement

Microfilaments play a crucial role in cell division, especially during cytokinesis, which is the final stage of mitosis and meiosis where the cytoplasm of the parent cell splits into two daughter cells. Microfilaments form a contractile ring at the equator of the cell, which gradually constricts and pinches off the cytoplasm into two separate compartments . This process is also known as cleavage furrow formation and requires the interaction of actin and myosin proteins. The contractile ring is composed of a bundle of microfilaments that are oriented perpendicular to the long axis of the cell. The myosin molecules slide along the actin filaments, generating tension and pulling the plasma membrane inward. The contractile ring also helps to position the mitotic spindle and the chromosomes at the center of the cell before cytokinesis.

Microfilaments also have a role in maintaining the proper placement of organelles within the cell. Microfilaments are often attached to the plasma membrane and form a network called the cell cortex, which provides mechanical support and shape to the cell surface . The cell cortex can also anchor some organelles, such as mitochondria, to specific locations near the membrane. Microfilaments can also interact with motor proteins, such as myosin, to transport organelles along the cytoplasm. For example, microfilaments can move chloroplasts in plant cells to adjust their exposure to light. Microfilaments can also rearrange the distribution of organelles during cell polarization, differentiation, and migration.

Ability to change cell shape and movement

Microfilaments are dynamic structures that can rapidly assemble and disassemble in response to various stimuli. This allows them to change the shape and movement of the cell in different ways. Some of the ways microfilaments can alter cell shape and movement are:

• Cell cortex formation: Microfilaments form a network under the plasma membrane, called the cell cortex, that provides mechanical support and regulates the shape and movement of the cell surface . The cell cortex can be modulated by various proteins that bind to or sever microfilaments, creating different patterns of filaments that affect the curvature and tension of the membrane.
• Cell surface projections: Microfilaments are involved in the development of various cell surface projections, such as filopodia, lamellipodia, and stereocilia, that extend from the plasma membrane and enable the cell to interact with its environment . Filopodia are thin, finger-like extensions that contain parallel bundles of microfilaments. Lamellipodia are broad, sheet-like extensions that contain branched networks of microfilaments. Stereocilia are long, hair-like projections that contain cross-linked bundles of microfilaments. These projections are formed by the polymerization of actin at the leading edge of the membrane, driven by proteins such as Arp2/3 complex and formins.
• Cell motility: Microfilaments enable cells to move by generating forces through interactions with motor proteins, such as myosin . Myosin is a protein that can bind to actin and use ATP to slide along the filament, creating a contractile force. This force can be used to move the cell forward, as in amoeboid movement, or to pull the cell inward, as in endocytosis. Microfilaments also cooperate with other cytoskeletal elements, such as microtubules and intermediate filaments, to coordinate cell movement.
• Cell division: Microfilaments play a crucial role in cytokinesis, which is the final stage of cell division when the cytoplasm is split into two daughter cells . During cytokinesis, a ring of microfilaments and myosin forms around the equator of the cell, called the contractile ring. The contractile ring gradually constricts, pulling the plasma membrane inward and creating a cleavage furrow. The cleavage furrow deepens until it separates the two daughter cells completely.

Microfilaments are thus essential for maintaining and changing cell shape and movement. They provide structural support, flexibility, and force generation for various cellular processes. They also interact with other proteins and cytoskeletal elements to modulate their functions and dynamics. Microfilaments are therefore key components of the cytoskeleton that enable cells to adapt to their environment and perform their functions.

Microfilaments are involved in various cellular functions that require movement, shape change, or structural support. Some of the main functions of microfilaments are:

• Muscle contraction: Microfilaments work together with another protein called myosin to enable the contraction and relaxation of muscle cells. The actin and myosin filaments slide past each other in a coordinated manner, generating force and movement. This process is essential for voluntary movements, such as walking or lifting, as well as involuntary movements, such as heartbeat or digestion .
• Cell motility: Microfilaments also enable the movement of cells, especially those that have a single cell body, such as amoebae or white blood cells. Microfilaments can extend and retract parts of the cell membrane, creating protrusions called filopodia or lamellipodia that help the cell crawl or change direction. Microfilaments also power the movement of cilia and flagella, which are hair-like structures that project from some cells and beat rhythmically to propel the cell or move fluids around it .
• Cytokinesis: Microfilaments play a key role in the division of cells into two daughter cells. During cytokinesis, a ring of actin and myosin filaments forms around the middle of the cell, constricting it until it splits into two. This process is also known as the cleavage furrow. Microfilaments also help to transport organelles and other materials to the daughter cells along actin cables .
• Cell shape and stability: Microfilaments provide structural support to the cell by forming a network under the plasma membrane, called the cell cortex. The cell cortex helps to maintain the cell shape and resist external forces. Microfilaments can also interact with other cytoskeletal elements, such as intermediate filaments and microtubules, to form a complex scaffold that anchors organelles and reinforces the cell`s mechanical strength .
• Endocytosis and exocytosis: Microfilaments are involved in the processes of endocytosis and exocytosis, which are the uptake and release of materials by the cell membrane. In endocytosis, microfilaments help to form vesicles that pinch off from the membrane and carry substances into the cell. In exocytosis, microfilaments help to move vesicles to the membrane and fuse them with it, releasing their contents outside the cell. These processes are important for communication, digestion, immunity, and secretion .

Microfilaments are involved in various cellular processes that require contraction and movement, such as muscle contraction, cell division, cell migration, and cytoplasmic streaming. These processes depend on the interaction of microfilaments with another protein called myosin, which acts as a motor protein that can bind to and slide along actin filaments.

Muscle contraction

One of the most important roles of microfilaments is to contract muscles. There is a high concentration of microfilaments in muscle cells, where they form myofibrils, the basic unit of the muscle cell. Actin is an indispensable protein for muscle movement, and microfilaments are often called actin filaments because actin is so prominent in the muscular system of the body.

In muscle cells, actin works together with myosin to allow the muscles to contract and relax. Here, neither actin nor myosin can work properly without the other, and they form a complex called actomyosin. Groups of actomyosin are found in sarcomeres, the basic unit of muscle tissue.

Sarcomeres consist of overlapping thick filaments (composed of myosin) and thin filaments (composed of actin). When a muscle cell receives a signal to contract, the myosin heads bind to the actin filaments and pull them towards the center of the sarcomere. This causes the sarcomere to shorten and the muscle to contract. The process is reversed when the muscle cell relaxes.

Cell division

Another important function of microfilaments is to help divide the cell during mitosis (cell division). Microfilaments aid the process of cytokinesis, which is when the cell “pinches off” and physically separates into two daughter cells.

During cytokinesis, a ring of actin and myosin filaments forms beneath the plasma membrane at the equator of the cell. The ring contracts and constricts the cytoplasm, forming a cleavage furrow. The furrow deepens until it reaches the midline of the cell, where it meets a band of microtubules called the midbody. The midbody acts as a bridge between the two daughter cells and helps to complete their separation.

Cell migration

Microfilaments play a role in causing cells to move. This occurs throughout the body and it is also very important for organisms whose entire body consists of one cell, such as amoebae; without microfilaments, they would not be motile.

Actomyosin plays a role here just as it does in muscle cells. In order for cells to move, one end of a microfilament must elongate while the other end must shorten, and myosin acts as a motor to make this happen.

Microfilaments also have a role in forming various cell surface projections that enable cell migration, such as filopodia, lamellipodia, and pseudopodia. Filopodia are thin extensions of the plasma membrane that contain bundles of actin filaments. Lamellipodia are broad extensions of the plasma membrane that contain networks of actin filaments. Pseudopodia are large extensions of the plasma membrane that contain both actin and myosin filaments.

These projections are dynamic structures that can extend and retract by polymerizing and depolymerizing actin filaments at their tips. They also interact with extracellular molecules and receptors that guide the direction and speed of cell movement.

Cytoplasmic streaming

Microfilaments also have a role in cytoplasmic streaming. Cytoplasmic streaming is the flow of cytoplasm (the contents of the cell, including the fluid part called cytosol and cell organelles) throughout the cell.

Cytoplasmic streaming helps to distribute nutrients, metabolites, and organelles within the cell. It also helps to maintain cell polarity and shape. Cytoplasmic streaming is especially prominent in plant cells, where it facilitates photosynthesis by moving chloroplasts around.

Cytoplasmic streaming is driven by actomyosin complexes that are attached to the plasma membrane or to organelles. The complexes generate force by sliding along actin filaments that are arranged along the cortex (the outer layer) or throughout the cytoplasm.

Microfilaments play a crucial role in cell division, especially during cytokinesis, which is the final stage of mitosis and meiosis where the cytoplasm of the parent cell splits into two daughter cells. Microfilaments form a contractile ring at the equator of the cell, which gradually constricts and pinches off the cytoplasm into two separate compartments . This process is also known as cleavage furrow formation and requires the interaction of actin and myosin proteins. The contractile ring is composed of a bundle of microfilaments that are oriented perpendicular to the long axis of the cell. The myosin molecules slide along the actin filaments, generating tension and pulling the plasma membrane inward. The contractile ring also helps to position the mitotic spindle and the chromosomes at the center of the cell before cytokinesis.

Microfilaments also have a role in maintaining the proper placement of organelles within the cell. Microfilaments are often attached to the plasma membrane and form a network called the cell cortex, which provides mechanical support and shape to the cell surface . The cell cortex can also anchor some organelles, such as mitochondria, to specific locations near the membrane. Microfilaments can also interact with motor proteins, such as myosin, to transport organelles along the cytoplasm. For example, microfilaments can move chloroplasts in plant cells to adjust their exposure to light. Microfilaments can also rearrange the distribution of organelles during cell polarization, differentiation, and migration.

Microfilaments are dynamic structures that can rapidly assemble and disassemble in response to various stimuli. This allows them to change the shape and movement of the cell in different ways. Some of the ways microfilaments can alter cell shape and movement are:

• Cell cortex formation: Microfilaments form a network under the plasma membrane, called the cell cortex, that provides mechanical support and regulates the shape and movement of the cell surface . The cell cortex can be modulated by various proteins that bind to or sever microfilaments, creating different patterns of filaments that affect the curvature and tension of the membrane.
• Cell surface projections: Microfilaments are involved in the development of various cell surface projections, such as filopodia, lamellipodia, and stereocilia, that extend from the plasma membrane and enable the cell to interact with its environment . Filopodia are thin, finger-like extensions that contain parallel bundles of microfilaments. Lamellipodia are broad, sheet-like extensions that contain branched networks of microfilaments. Stereocilia are long, hair-like projections that contain cross-linked bundles of microfilaments. These projections are formed by the polymerization of actin at the leading edge of the membrane, driven by proteins such as Arp2/3 complex and formins.
• Cell motility: Microfilaments enable cells to move by generating forces through interactions with motor proteins, such as myosin . Myosin is a protein that can bind to actin and use ATP to slide along the filament, creating a contractile force. This force can be used to move the cell forward, as in amoeboid movement, or to pull the cell inward, as in endocytosis. Microfilaments also cooperate with other cytoskeletal elements, such as microtubules and intermediate filaments, to coordinate cell movement.
• Cell division: Microfilaments play a crucial role in cytokinesis, which is the final stage of cell division when the cytoplasm is split into two daughter cells . During cytokinesis, a ring of microfilaments and myosin forms around the equator of the cell, called the contractile ring. The contractile ring gradually constricts, pulling the plasma membrane inward and creating a cleavage furrow. The cleavage furrow deepens until it separates the two daughter cells completely.

Microfilaments are thus essential for maintaining and changing cell shape and movement. They provide structural support, flexibility, and force generation for various cellular processes. They also interact with other proteins and cytoskeletal elements to modulate their functions and dynamics. Microfilaments are therefore key components of the cytoskeleton that enable cells to adapt to their environment and perform their functions.

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