Problem 3 - Campbell Biology

Question

\begin{equation} \begin{array}{l}{\text { During the contraction of a vertebrate skeletal muscle fiber, }} \\ {\text { calcium ions }} \\ {\text { (A) break cross- bridges as a cofactor in hydrolysis of ATP. }} \\ {\text { (B) bind with troponin, changing its shape so that the myosin- }}\end{array} \end{equation}\begin{equation} \begin{array}{l}{\text { binding sites on actin are exposed. }} \\ {\text { (C) transmit action potentials from the motor neuron to the }} \\ {\text { muscle fiber. }} \\ {\text { (D) spread action potentials through the T tubules. }}\end{array} \end{equation}

Step-by-Step Solution

Answer

Option B

Step 1 - Understand the question

The question is about the role of calcium ions during the contraction of vertebrate skeletal muscle fibers. It's a multiple-choice question with four options, A through D.

Step 2 - Review the functions of calcium ions

Remember that calcium ions are crucial for muscle contraction. They are released from the sarcoplasmic reticulum and play a role in the interaction between actin and myosin.

Step 3 - Evaluate Option A

Option A states that calcium ions break cross-bridges as a cofactor in the hydrolysis of ATP. This is incorrect as ATP hydrolysis and cross-bridge cycling do not primarily involve calcium ions in this manner.

Step 4 - Evaluate Option B

Option B states that calcium ions bind with troponin, changing its shape to expose myosin-binding sites on actin. This is correct—calcium ions bind to troponin causing the tropomyosin strands to shift and expose the active sites on actin.

Step 5 - Evaluate Option C

Option C states that calcium ions transmit action potentials from the motor neuron to the muscle fiber. This is not correct as the transmission of action potentials involves neurotransmitters like acetylcholine at the neuromuscular junction.

Step 6 - Evaluate Option D

Option D states that calcium ions spread action potentials through the T tubules. This is incorrect since action potentials are transmitted through T tubules by voltage changes, not directly by calcium ions.

Step 7 - Select the correct option

Having reviewed all the options, the correct choice is Option B: calcium ions bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed.

Key Concepts

skeletal muscle contraction

Skeletal muscle contraction is the process that allows muscles to produce movement. This process is based on the sliding filament theory.
The main components involved are actin, myosin, and various regulatory proteins. When a signal from the nervous system reaches the muscle, it triggers a series of events that lead to muscle contraction.
The myosin heads bind to actin and pull the filaments toward the center of the sarcomere, the basic unit of muscle fiber. This pulling action is often called the power stroke, and it results in the shortening of the muscle, which is evident as muscle contraction.

calcium ions

Calcium ions (Ca²⁺) play a critical role in the process of muscle contraction. They are stored in the sarcoplasmic reticulum within the muscle cells and are released in response to an action potential.
Once released, calcium ions bind to troponin, a regulatory protein associated with the thin filaments of the muscle fibers.
This binding event is crucial for the contraction process as it leads to subsequent events that make the myosin-binding sites on actin available for interaction.

troponin

Troponin is a complex of three proteins (Troponin C, Troponin I, and Troponin T) that is essential for the regulation of skeletal muscle contraction. Troponin C is the component that binds calcium ions.
When calcium ions bind to Troponin C, a conformational change occurs, which shifts the position of tropomyosin.
This movement uncovers the myosin-binding sites on the actin filament, thereby allowing the myosin heads to attach and perform the power stroke.

myosin-binding sites

Myosin-binding sites are specific locations on the actin filaments where the myosin heads can attach. These sites are initially blocked by tropomyosin, a protein filament that lies along the grooves of the actin filaments.
When calcium ions bind to troponin, this causes a shift in the position of the tropomyosin, revealing the binding sites on actin.
This exposure allows myosin heads to bind to these sites, initiating the power stroke and sliding the actin filaments past the myosin, resulting in muscle contraction.