Microfilaments and special-purpose organelles

Molecular organization of microfilaments

Microfilaments are ultramicroscopic filaments that permeate the cell cytoplasm in a dense network. Their basis is formed by the protein actin.

• Basic structure: The molecular structure of a microfilament resembles a double helix. This helix is formed by individual actin globules that are strictly ordered and arranged into a long chain. The diameter of such a filament is 6 nm.
• Associated proteins: In addition to actin itself, functional complexes necessarily include additional protein molecules, the most important of which are myosin and troponin.
• Interaction mechanism: To perform motor functions, polymeric actin filaments form stable complexes with polymeric myosin molecules. A key condition for myosin polymerization is the presence of calcium ions in the medium. Once the actin-myosin complex is formed, the process of their mutual longitudinal sliding begins—the filaments shift relative to each other, which underlies contraction.

Functions of microfilaments in a living cell

Due to their ability to assemble, disassemble, and slide, microfilaments perform a wide range of vital tasks:

1. Structural role: They act as the main organizer of the cytoplasm, forming the supporting cytoskeleton of the cell.
2. Motor function (cyclosis): They take direct part both in the movement of the cell itself in space and in the internal cytoplasmic streaming.
3. Contractile activity: They are the main component of the contractile apparatus of muscular elements (working in an inseparable complex with myosin and other associated proteins).
4. Ensuring division: In animal cells, microfilaments are responsible for forming the contractile ring during cytokinesis (the physical division of the mother cell's cytoplasm into two daughter cells).
5. Supporting function: At the tissue level, for example, in intestinal cells, they reliably maintain the correct spatial structure of microvilli, preventing them from collapsing.

Special-purpose organelles

This category of cellular structures traditionally includes cilia and flagella. Unlike microfilaments, which are located deep within the cytoplasm, these organelles protrude beyond the main bulk of the cell, although they are tightly covered by the plasma membrane on the outside.

Comparative structural characteristics:

• Cilia are numerous, very thin, and short cylindrical cytoplasmic outgrowths.
• Flagella, by contrast, are single and significantly longer outgrowths.

Despite external differences, their internal structure (known as the axoneme) is absolutely identical. The axoneme is formed by microtubules strictly organized according to a universal pattern: 9 pairs of microtubules are located at the periphery, and 1 pair is situated precisely in the center.

Main functions:

• Active locomotion: They provide locomotion for many unicellular organisms (such as ciliates and flagellates), as well as specialized gametes of multicellular organisms (spermatozoa).
• Absorption processes: They participate in supporting the absorptive function in specialized epithelia lining the respiratory tract, intestinal lumen, and certain other organs.

Class and exam questions

What else to study on this topic

• Molecular structure and properties of the actin protein
• The role of troponin in regulating muscle contraction
• The process of cytokinesis and the formation of the contractile ring
• Structural organization of intestinal microvilli
• Features of movement in ciliates and flagellates

Other topics in the section "The Cell — The Elementary Unit of Life"

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