Problem 3 - Campbell Biology

Question

During vertebrate skeletal muscle contraction, calcium ions (A) break cross-bridges as a cofactor in hydrolysis of ATP. (B) bind to troponin, exposing myosin-binding sites. (C) transmit action potentials to the muscle fiber. (D) spread action potentials through the T tubules.

Step-by-Step Solution

Answer

Option (B) is correct because calcium ions bind to troponin, exposing myosin-binding sites.

Step 1: Understand Muscle Contraction

During skeletal muscle contraction, specific interactions occur between actin and myosin filaments. The correct answer involves knowing the role of calcium ions in this process.

Step 2: Analyze Each Option

Consider each option provided:(A) Break cross-bridges as a cofactor in hydrolysis of ATP - This statement is incorrect. ATP hydrolysis is not the role of calcium ions.(B) Bind to troponin, exposing myosin-binding sites - This is a true statement. Calcium ions bind to troponin, which causes a conformational change exposing the binding sites on actin for myosin.(C) Transmit action potentials to the muscle fiber - This statement is incorrect. Action potentials are transmitted to the muscle fiber by means of motor neurons.(D) Spread action potentials through the T tubules - This statement is somewhat correct but T tubules primarily help in transmitting the action potentials within the muscle fiber.

Step 3: Identify the Correct Answer

Based on the analysis, the correct answer is option (B). Calcium ions bind to troponin and expose myosin-binding sites, facilitating the interaction between actin and myosin necessary for muscle contraction.

Key Concepts

Calcium ions

Calcium ions play a critical role in muscle contraction. When a muscle cell is stimulated by a nerve impulse, calcium ions are released from the sarcoplasmic reticulum into the cytoplasm of the muscle cell. The release of calcium ions is what initiates muscle contraction. Without calcium ions, the process of muscle contraction would not be possible. This is because calcium ions bind to specific proteins involved in the contraction process, facilitating the interaction between actin and myosin filaments.

Troponin

Troponin is a protein complex that plays an essential role in muscle contraction. It is found on the thin filaments of muscle tissue, particularly actin filaments. Troponin has three subunits—Troponin T, Troponin I, and Troponin C. When calcium ions are released into the muscle cell, they bind to Troponin C. This binding induces a conformational change in the troponin complex. As a result, the position of tropomyosin (another protein associated with actin filaments) is altered, exposing the myosin-binding sites on the actin filament. This exposure is crucial for the interaction between actin and myosin, ultimately leading to muscle contraction.

Myosin-Binding Sites

Myosin-binding sites are specific regions on the actin filaments where the myosin heads attach during muscle contraction. Under resting conditions, these binding sites are covered by tropomyosin, preventing the interaction between actin and myosin. When calcium ions bind to troponin, it causes a shift in tropomyosin, uncovering the myosin-binding sites on actin. This exposure is essential as it allows the myosin heads to attach to the actin filaments and form cross-bridges. The formation and breaking of these cross-bridges during the actin-myosin interaction generate the force necessary for muscle contraction.

Actin-Myosin Interaction

The interaction between actin and myosin is at the core of muscle contraction. This process is often referred to as the sliding filament theory. Once the myosin-binding sites on actin are exposed, the myosin heads attach to these sites forming cross-bridges. The energy for this attachment and the subsequent movement comes from the hydrolysis of ATP. After attaching, the myosin heads pivot, pulling the actin filaments toward the center of the sarcomere, thereby shortening the muscle fiber. This movement is called a power stroke. After the power stroke, a new ATP molecule binds to the myosin head, causing the myosin to detach from the actin filament. This cycle of attachment, pivoting, and detachment repeats as long as calcium ions and ATP are available, resulting in sustained muscle contraction.