Final answer:
The thick filament in muscle cells is comprised of the protein myosin, which interacts with actin from the thin filament during muscle contraction. Myosin heads form cross-bridges with actin, allowing for the sliding filament model of muscle action. Microfilaments, also made of actin, play a role in cellular movement and structure.
Step-by-step explanation:
The thick filament in muscle cells is composed of the protein myosin, which has several heads, also known as cross-bridges, projecting out towards the thin filaments. The myosin heads form cross-bridges by attaching to actin, which is the primary component of the thin filament.
The thin filament also contains other proteins like tropomyosin and troponin, which regulate muscular contractions. Actin has specific binding sites for the myosin heads, and conformational changes in these proteins triggered by ATP hydrolysis lead to muscle contraction. This is a part of the sliding filament model, which describes how muscles contract and relax through the interaction of myosin and actin filaments.
Additionally, in the cytoskeleton of cells, microfilaments are composed of actin and are involved in various cellular movement and structural integrity tasks. They are differentiated from the muscle contraction system but also rely on actin and interactions with motor proteins like myosin.