Muscles enable nearly every movement, from external motion to essential internal organ functions. To understand muscle force and movement, we must examine their basic building blocks: biomolecules.
Proteins The Core of Muscle
Proteins are the primary biomolecules responsible for both the structure and function of muscle tissue. Within muscle, certain proteins are particularly abundant and play direct roles in contraction. For instance, approximately 20% of muscle mass is composed of protein, with about 5 to 6 kilograms of muscle protein in a healthy adult.
The two most abundant proteins in muscle are actin and myosin. Myosin makes up as much as 35% of the total protein volume in skeletal muscles, while actin is highly prevalent in most eukaryotic cells. Actin and myosin are fundamental building blocks enabling muscle contraction and relaxation.
Assembling Muscle Structures
Actin and myosin proteins are organized into precise, repeating structures within muscle cells. Muscle fibers, which are long cells, contain numerous cylindrical bundles called myofibrils. Each myofibril is composed of repeating contractile units known as sarcomeres, the basic functional units of muscle contraction.
Sarcomeres feature distinct regions of thick and thin filaments. Thick filaments are primarily made of myosin, while thin filaments are composed mainly of actin, along with other regulatory proteins like tropomyosin and troponin. This organized arrangement gives skeletal muscle its characteristic striped, or striated, appearance under a microscope. The myosin molecules within the thick filaments have tails that intertwine, forming a continuous fiber, while the actin filaments are anchored at the Z-discs at either end of the sarcomere.
The Mechanism of Muscle Contraction
Muscle contraction occurs through the sliding filament theory, which explains how actin and myosin filaments interact and move past each other. During contraction, myosin heads, part of the thick filaments, bind to specific sites on the actin thin filaments, forming cross-bridges. The myosin then pulls the actin filaments inward toward the center of the sarcomere, causing the sarcomere to shorten.
This pulling action, known as the “power stroke,” shortens the muscle fiber. The filaments themselves do not shorten; instead, they slide past one another. This dynamic interaction requires energy, supplied by adenosine triphosphate (ATP), a molecule that powers the detachment and re-cocking of the myosin heads, allowing the cycle to repeat.
Supporting Molecules for Muscle Function
While proteins form the structural and contractile elements of muscle, other biomolecules are essential for muscle function and overall health. Carbohydrates, primarily stored as glycogen in the muscles and liver, serve as a readily available energy source. Muscle glycogen is particularly important as a fuel during exercise, providing quick ATP synthesis for muscle cells.
Fats, or lipids, also provide energy, especially during lower-intensity or prolonged physical activities. They are a more energy-efficient form of fuel compared to carbohydrates, storing more than twice the energy per gram. Fats contribute to hormone production and the absorption of fat-soluble vitamins, which indirectly support muscle growth and repair. Water, a significant component of muscle tissue, transports nutrients, maintains electrolyte balance, and facilitates chemical reactions.