Which structures are included in the cytoskeleton?

The cytoskeleton is a dynamic network of fibers that provides structural support, shape, and organization to the cell. It plays a critical role in various cellular functions, including movement, division, and intracellular transport. The three main components of the cytoskeleton are microtubules, intermediate filaments, and actin filaments.

Microtubules are hollow tubes made of tubulin proteins that help maintain cell shape, facilitate the movement of organelles, and are essential during cell division as they form the mitotic spindle. Intermediate filaments provide mechanical support to the cell and help maintain its integrity under stress. Actin filaments, also known as microfilaments, are essential for cell motility and shape changes; they are involved in muscle contraction and cell division as well.

In contrast, options regarding the cell membrane and ribosomes, chloroplasts and mitochondria, as well as the endoplasmic reticulum and Golgi apparatus, do not belong to the cytoskeleton. These structures have distinct functions in cells but are not components of the cytoskeletal framework. Therefore, the correct answer identifies the structures that are integral to the cytoskeleton and its various essential functions.