Final answer:
Actin filaments can form a variety of structures but do not form axonemes with doublet microfilaments; those are related to microtubules. Structures like 3D networks, contractile bundles, and parallel filament bundles are indeed formed by actin filaments.
Step-by-step explanation:
Actin filaments can form various types of structures within the cell, supporting a range of functions such as providing rigidity, enabling cellular movements, and facilitating muscle contractions.
Actin filaments may form each of the following except axonemes containing doublet microfilaments. Axonemes are actually associated with microtubules, not microfilaments, and form the core of cilia and flagella. Actin filaments, in contrast, are known to form 3D filament networks (gels), contractile bundles, branched networks, and bundles of parallel filaments. These actin-based structures are integral to various cellular mechanisms like muscle contraction and cell motility. Moreover, associated with motor proteins like myosin, they facilitate the generation of forces for cellular movements, and in muscle cells, the interaction between actin filaments and myosin results in contraction.
Actin filaments are integral to the cytoskeleton, supporting the cell's structure and involved in processes like cytokinesis and maintaining cell shape. They are the thinnest component of the cytoskeleton, and with the aid of actomyosin contractility, they provide shape and enable various cellular activities.