Final answer:
The thin filament is made up of actin, tropomyosin, and troponin, which are proteins that work together to regulate muscle contraction by controlling the interaction between actin and myosin.
Step-by-step explanation:
The thin filament in muscle cells is primarily formed of actin, which is a protein that forms chains and has binding sites for myosin attachment. The other two molecules that constitute the thin filament are tropomyosin and troponin. Tropomyosin is a regulatory protein that blocks the binding sites on actin to prevent interaction with myosin when the muscle is at rest. Troponin is composed of three globular subunits, with one subunit binding to tropomyosin, one binding to actin, and one binding calcium ions (Ca²⁺), playing a crucial role in muscle contraction by regulating the interaction between actin and myosin.