Final answer:
The large amount of G-actin is not freely available in the cell because it is bound by microfilaments. Microfilaments consist of polymerized actin monomers that are involved in muscle contraction, cell support, and cell division. They are distinct cytoskeletal structures from intermediate filaments and microtubules. The correct answer to the question is option 1: Microfilaments.
Step-by-step explanation:
In the cell, a large amount of free G-actin is not available because it is bound by microfilaments. Microfilaments are composed of actin monomers that arrange into a filamentous structure, crucial for various cell functions. These structures are also referred to as F-actin when they form the filamentous polymers of actin.
They are involved in a range of cellular processes including muscle contraction, cell support, and the pinching off of daughter cells after mitosis. Actin exists in two forms within the cell: the polymerized gel form and the unpolymerized soluble form. In ameboid organisms, for instance, actin provides stability in the ectoplasm, aiding in amoeboid movement.
It is important to understand the distinction between different elements of the cytoskeleton. Intermediate filaments are different from microfilaments; they are made up of proteins such as keratin and are crucial for maintaining cell shape and structure, resisting tensile forces that pull apart cells. On the other hand, microtubules are made up of tubulin monomers and play a role in maintaining cell shape, as well as facilitating intracellular movement and cell motility. However, neither intermediate filaments nor microtubules are responsible for sequestering the actin monomers; this function is performed by microfilaments.
Moreover, the dynamic reorganization of microfilaments, such as during cytokinesis, is a testament to their essential role in cellular processes. Electron microscopy has helped reveal these structures and their interactions with proteins like myosin to affect muscular contractions and cellular movements.
In conclusion, the answer to the question is option 1: Microfilaments.