Cell Structures: Cytoplasm, Cytoskeleton, Centrioles, Cilia, Flagella

Cellular Components: Cytoplasm and Cytosol

The cytoplasm is the material or protoplasm within a living cell, excluding the nucleus. It comprises the *cytosol* (also known as the hyaloplasm or cytoplasmic matrix), a gelatinous substance composed of ions and dissolved inorganic and organic molecules in water, and various organelles such as centrioles, ribosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, and chloroplasts (in plant cells). These structures play specific roles in cellular function.

The cytosol is the site of various chemical reactions of metabolism, including the first stage of cellular respiration (glycolysis) and the synthesis of several substances.

The Cytoskeleton

In eukaryotic cells, the cytoskeleton is a complex network of protein filaments extending throughout the cytoplasm. It is composed primarily of:

• *Microtubules*: Hollow tubes made of spherical protein subunits called tubulin.
• *Microfilaments*: Solid rods made of a contractile protein called actin.

These two structures (tubulin and actin) form a network spread across the cytoplasm, the cytoskeleton, which helps to give the cell its shape and serves as a support structure. The cytoskeleton also participates in several cellular movements, such as amoeboid movement (characteristic of amoebas, which rely on temporary extensions of cytoplasm called pseudopods for locomotion and food capture via phagocytosis).

Cytoskeleton Functions

• Provides shape and organizes internal cell structure.
• Allows cell adhesion to the extracellular matrix and neighboring cells.
• Enables the movement or displacement of material inside the cell.
• Is responsible for the movement of cells with cilia or flagella.

Centrioles

Centrioles are small, cylindrical structures present in almost all eukaryotic cells, with some exceptions like certain unicellular organisms and higher plant cells. They are located within the *centrosome*, a denser region of cytoplasm near the nucleus.

Each centriole is composed of microtubules arranged in a characteristic pattern: nine groups of three microtubules forming the cylinder wall.

They are involved in the formation of cilia and flagella and the organization of the achromatic spindle (spindle fibers) during cell division in animal cells. Centrioles can self-replicate; new centrioles form from existing ones within the cytoplasm.

Cilia and Flagella

Some cells possess *cilia* (short and numerous) and *flagella* (long and few), whose movement creates water currents or propels the cell. Both are composed of microtubules and involve a projection of the plasma membrane.

Cilia and flagella serve for locomotion or to facilitate the capture of food, among other functions.

The flagellum of a prokaryotic cell is different from a eukaryotic one, as it does not have centrioles and is not formed by microtubules, but by strands of the protein flagellin.