Problem 9 - Biology: Science for Life with Physiology

Question

During muscle contraction, ______ A. the actin heads pull the sarcomere closed; B. myosin attaches to actin, pulls it toward the center of the sarcomere, releases it, and reattaches further along; C. myofibrils shrink due to the actions of testosterone; D. muscle fibers contract under the actions of testosterone

Step-by-Step Solution

Answer

The correct answer is B: myosin attaches to actin, pulls it toward the center of the sarcomere, releases it, and reattaches further along.

Step 1: Understand the Question

The question is asking what happens during muscle contraction. There are four options provided that describe different processes or mechanisms. We must identify the correct option that accurately describes the process occurring during muscle contraction in biological terms.

Step 2: Analyze Each Answer Option

We need to review each of the four options given: - Option A suggests actin heads pull the sarcomere closed. - Option B suggests myosin attaches to actin, pulls it, releases it, and reattaches further along. - Option C suggests testosterone causes myofibrils to shrink. - Option D suggests testosterone causes muscle fibers to contract.

Step 3: Identify the Process of Muscle Contraction

Muscle contraction occurs through the sliding filament theory, where actin and myosin filaments slide past each other to shorten the sarcomere. Myosin heads attach to actin, form cross-bridges, pull actin toward the center (power stroke), then release and attach further along to repeat the process. This matches the description in Option B.

Step 4: Confirm and Explain the Correct Answer

Option B correctly describes the mechanism of muscle contraction involving myosin and actin. In the contraction cycle, myosin heads bind to actin, pulling the thin filaments towards the center of the sarcomere. This is key to muscle shortening during contraction, and there is no involvement of testosterone in these steps.

Key Concepts

Sliding Filament Theory

Muscle contraction is beautifully explained by the sliding filament theory. Imagine it like a complex dance, where proteins glide past one another to achieve movement.
In this theory:

• Muscle fibers are composed of thick and thin filaments.
• The thick filaments are made mainly of myosin.
• Thin filaments consist mostly of actin.

These filaments slide over each other, bringing about contraction. The sarcomere, the functional unit of a muscle, shortens as actin and myosin interact. Picture it like two hands with interlaced fingers that pull together, making the whole structure more compact. This elegant interaction gives muscles the ability to shorten, generating the force we recognize as contraction.

Actin and Myosin Interaction

The interaction between actin and myosin heads is central to muscle contraction. Consider it like a latch and hook system, where these proteins bind strongly and pull.
Here’s how it works:

• Myosin heads, energized by ATP, attach to specific sites on the actin filament.
• Once attached, the myosin pulls the actin filament toward the center of the sarcomere.
• After the move, the myosin head releases, resets using another ATP molecule, and reattaches to a new actin spot, continuing the process.

This cyclic process, known as cross-bridge cycling, is repeated numerous times during a muscle contraction. Such interactions are critical for producing the muscular force needed for various physical activities.

Sarcomere Contraction

The sarcomere is the smallest contractile unit of a muscle fiber, and its contraction is a pivotal part of muscle movement. Picture it as a segment of a rope, which shortens during contraction.
In this process:

• The Z-lines, which define the boundaries of a sarcomere, move closer together.
• Actin filaments are pulled by myosin towards the center, shortening the overall length of the sarcomere.
• Thousands of sarcomeres contracting simultaneously lead to the shortening of the entire muscle.

This organized shortening results in the entire muscle fiber contracting, allowing for movement and strength. Sarcomere contraction is crucial for translating microscopic interactions into visible muscular actions.

Power Stroke

The power stroke is the key force-generating step within the muscle contraction process. Think of it as the climax of a piston’s stroke in an engine.
Here’s the breakdown:

• Once a myosin head forms a cross-bridge with actin, it pivots toward the center of the sarcomere.
• This pivoting action pulls the actin filament along, causing a shift known as the power stroke.
• ATP is crucial here, as its hydrolysis powers the myosin head, allowing it to release, re-cock, and prepare for the next cycle.

The power stroke is essential, converting chemical energy (from ATP) into mechanical work, driving the fundamental action of muscle contraction.