Question

Which of the following statements is true? a. A muscle fiber is a collection of cells embedded in connective tissue. b. A sarcomere contains both actin and myosin molecules arranged in a parallel fashion. c. The function of Ca2+ in contraction is to bind to tropomysin. d. Myofibrils are individual muscle cells. e. The I band of a sarcomere is the region where thin and thick filaments overlap.

Short answer

The true statement is B: A sarcomere contains both actin and myosin molecules arranged in a parallel fashion.

Step-by-step solution

Analyzing each statement

Start by considering each statement individually and assess if it is true concerning known scientific knowledge about muscle cells and their mechanisms.

Investigate Statement A

Statement A suggests that a muscle fiber is a collection of cells embedded in connective tissue. This is not true. In fact, a muscle fiber is a single muscle cell, not a collection of several cells.

Investigate Statement B

Statement B states that a sarcomere contains both actin and myosin molecules arranged in a parallel fashion. This is true. Sarcomeres, which are the basic contractile units of muscle fiber, do contain actin (thin filaments) and myosin (thick filaments) arranged in a parallel manner.

Investigate Statement C

Statement C alleges that the function of Ca2+ in contraction is to bind to tropomysin. This is not entirely correct. In the process of muscle contraction, Ca2+ binds to troponin, not tropomysin. Troponin then moves tropomyosin away from the binding sites on actin, allowing myosin to attach to actin and cause contraction.

Investigate Statement D

Statement D posits that myofibrils are individual muscle cells. This is not correct. Myofibrils are actually subunits of muscle cells (muscle fibers), consisting of a bundle of myofilaments (actin or myosin).

Investigate Statement E

Statement E affirms that the I band of a sarcomere is the region where thin and thick filaments overlap. This is not accurate. The I band is a region containing only thin (actin) filaments, whereas the A band contains regions where thin and thick filaments overlap.

Key concepts

Sarcomere Structure

Understanding how muscles contract begins with the sarcomere, the fundamental unit of a muscle's contraction mechanism. Imagine the sarcomere as a tiny component within a muscle fiber, akin to the individual bricks in a long and sturdy wall.

Within this structure, two main types of protein filaments are arranged in a specific pattern: actin, known as the thin filaments, and myosin, known as the thick filaments. They are organized in an end-to-end fashion to form what's called a myofibril. Now, visualize actin as a set of strings, and myosin as thicker ropes interspersed with those strings.

The sarcomere is delineated by Z-lines, which serve as boundary markers. Between these lines, actin and myosin do not just lie parallel but also exhibit areas of overlap. When a muscle contracts, these filaments slide past one another, shortening the overall length of the sarcomere, thus leading to contraction.

Muscle Fiber Anatomy

Zooming out from the microscopic sarcomere, let's understand the larger structure it resides within - the muscle fiber. Muscle fiber anatomy is intricate but essential for appreciating how our muscles function.

A single muscle fiber is, in essence, a muscle cell and is often engulfed by a specialized plasma membrane named the sarcolemma. Within this cell, an array of myofibrils exist, which are packed with repeating units of sarcomeres. It's much like a rope consisting of multiple strands, each contributing to its overall strength and function. The muscle fiber itself is encapsulated within connective tissue, which aids in transferring the force generated within the fibers to the bones they are attached to, thus facilitating movement.

Each fiber contains numerous nuclei necessary to manage the cell's substantial protein synthesis needs required for muscle function and repair. Furthermore, the fibers are categorized based on their properties and functions into slow-twitch and fast-twitch fibers, each with unique characteristics fitting for different types of physical activities.

Calcium's Role in Muscle Contraction

Every movement we make hinges on a molecular ballet involving calcium ions (Ca²⁺). Their role in muscle contraction is pivotal and refined.

When a nerve impulse triggers a muscle contraction, it leads to the release of Ca²⁺ within the muscle fiber. Calcium does not approach tropomyosin, rather it binds to another protein named troponin. This binding instigates a conformational change in troponin that, in turn, propels tropomyosin away from actin's myosin-binding sites.

With these sites exposed, myosin heads eagerly bind to actin, initiating the contraction process through a ratcheting motion that pulls the actin filaments closer together within the sarcomere. The role of calcium here is tantamount to turning a key that unlocks the door, allowing contraction to proceed. Once the contraction is complete, Ca²⁺ ions are pumped back into the sarcoplasmic reticulum, a specialized endoplasmic reticulum in the muscle cell, ensuring relaxation of the muscle fiber until the next nerve impulse arrives.