What are Actin Filaments?

Actin is given as a globular protein, which polymerizes to form the long filaments. Because every actin subunit faces in a similar direction, the filament definition biology is polar, with different ends, termed “barbed” and “pointed.” An abundant protein in approximately all the eukaryotic cells, actin has been deeply studied in muscle cells. Actin filaments are arranged into regular arrays in muscle cells, which are complemented by a group of thicker filaments made from a second protein called myosin. The force that causes muscle contraction is generated by these two proteins.

Actin Filaments Function

Let us discuss the actin filaments function in detail. When the signal to contract is sent along the nerve to muscle, the myosin and actin are activated. Myosin acts as a hydrolyzing adenosine triphosphate (ATP) and motor to release energy in such a way that a myosin filament slides past an actin filament. The thick myosin and thin actin filaments are organized in a structure known as sarcomere that shortens as the filaments slide over one another. Skeletal muscles are composed of bundles of several long muscle cells; when the sarcomeres contract, every of these giant muscle cells shortens, and the entire effect is the contraction of the complete muscle. Although the stimulation pathways vary, smooth muscle (found in several blood vessels and internal organs) and heart muscle contract by the same sliding filament mechanism.

Actin Filament Structure

Let us understand the actin filament structure in detail.

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The structure of skeletal, or striated, muscle. Striated muscle tissue, such as human biceps muscle tissue, has long fine fibres, each of which is essentially a bundle of finer myofibrils. Within every myofibril are filaments of the proteins actin and myosin; these filaments slide past one another as the muscle expands and contracts. On every myofibril, regularly taking place, dark bands, known as Z lines, may be seen where myosin and actin filaments overlap. A sarcomere is an area between the two Z lines; sarcomeres can be thought of as the main functional and structural unit of muscle tissue.

Also, actin is present in the non-muscle cells, where it produces a meshwork of filaments responsible for several cellular movement types. The meshwork consists of actin filaments, which are attached to the cell membrane and every other. The filaments’ length and the architecture of their attachments define the cell’s consistency and shape. Actin filament length, number, location, and attachments are all regulated by a large number of accessory proteins that bind to it.

Various tissues and cells contain multiple accessory proteins, which account for the different movements and shapes of multiple cells. For example, in a few cells, actin filaments are bundled by the accessory proteins, and that bundle is attached to the cell membrane to produce microvilli, stable protrusions, which resemble tiny bristles. Microvilli on the epithelial cell’s surface, such as those lining the intestine, increase the surface area of the cell and hence facilitate the absorption of ingested water and food molecules. Other microvilli are involved in sound perception in the ear, where sound waves induce their movement, which sends electrical signals to the brain.

Actin Filaments in Non-Muscle Cells

Several actin filaments found in non-muscle cells are only present for a short time, polymerizing and depolymerizing in regulated ways to generate movement. For example, several cells continually send out and retract small “filopodia,” long needlelike projections of the cell membrane, which are thought to enable the cells to probe their environment and decide the particular direction to go.

Similar to microvilli, filopodia are formed when the actin filaments push out the membrane, but due to these actin filaments being less stable, filopodia contain only a brief existence. The contractile ring is the other actin structure only transiently associated with the cell membrane that is composed of actin filaments running around the cell’s circumference during the cell division. This ring, as its name implies, pinches the cell in half by pulling in the cell membrane through a myosin-dependent mechanism.


Microtubules are the long filaments formed from 13-15 protofilament strands of a globular subunit known as tubulin, with the strands arranged in the hollow cylinder form. Similar to actin filaments, microtubules are polar, with the “plus” and “minus” ends. Most of the microtubule plus ends are growing and constantly shrinking by respectively adding and losing the subunits at their ends.

Stable microtubules can be found in flagella and cilia. Cilia are hair-like structures located on the surface of certain epithelial cell types that beat in unison to transport fluid and particles across the cell surface. Cilia are closely related to the flagella’s structure. Flagella, such as those found on the sperm cells, form a helical wavelike motion, which enables a cell to propel itself through fluids rapidly. A microtubule collection is bound in a regular array in flagella and cilia by a number of accessory proteins that serve as links and spokes in the assembly.

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FAQs on Actin Filaments

1. Explain the Movement of the Cilia?

Answer: Movement of the flagella or cilia takes place when the adjacent microtubules slide past one another by bending the structures. This particular motion is caused by the motor protein dynein that uses the ATP hydrolysis energy to move along the microtubules in a way resembling the movement of myosin along the actin filaments.

2. Give the Growth of Microtubules?

Answer: In most of the cells, microtubules grow outward, from the cell center to the cell membrane, from a special cytoplasm region near the nuclear envelope known as the centrosome. The minus ends of these particular microtubules are embedded in the centrosome, while the plus ends terminate around the cell membrane. The plus ends shrink and grow rapidly; a process called dynamic instability.

3. What are Intermediate Filaments?

Answer: Intermediate filaments are so named because they are thicker compared to actin filaments and thinner compared to microtubules or the muscle myosin filaments. The subunits of intermediate filaments are not globular, are elongated, and are antipolarly related. As a result, there is no polarity in the overall filament, and no motor proteins can migrate along the intermediate filaments.

4. What is an Extracellular Matrix?

Answer: A substantial part of the tissues is given as space outside of the cells, known as the extracellular space. This is filled with a composite material, called the extracellular matrix, which is composed of a gel, where a number of fibrous proteins are suspended.

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