Question

3\. Fill up the blanks in the following sentence by selecting the correct option. The thin filaments of myofibril contain (A) actin and two filaments of \(\underline{(B)}\) protein along with \(\underline{\text { CC }}\) protein for masking binding site for myosin. A \(\quad\) B \(C\) (a) \(1 \mathrm{~F}\) troponin tropomyosin (b) IF tropomyosin troponin (c) \(2 \mathrm{~F}\) troponin tropomyosin (d) \(2 \mathrm{~F}\) tropomyosin troponin

Short answer

The correct option is (b) 1F tropomyosin troponin.

Step-by-step solution

Understand myofibril structure

Myofibrils are long filaments found within muscle cells, and they consist of repeating units called sarcomeres. The sarcomeres contain thick filaments (mostly myosin) and thin filaments. The thin filaments are primarily made up of the protein actin, but they also contain regulatory proteins.

Identify the regulatory proteins of thin filaments

The regulatory proteins associated with the actin filaments of the myofibril are troponin and tropomyosin. Tropomyosin covers the myosin binding sites on the actin molecules, preventing contraction until the muscle receives a signal to contract. Troponin is bound to tropomyosin and helps to control its position on the actin filament.

Select the correct option

Given the structure and components of a thin filament, 'A' should be actin which is correct in all the options. The protein that associates with actin to block myosin binding sites is tropomyosin, (B) and the protein associated with tropomyosin is troponin (CC). Therefore, the correct option is the one with tropomyosin as (B) and troponin as (CC).

Key concepts

Actin Filaments

Actin filaments are fundamental to the muscle contraction process and play a pivotal role within the myofibril structure of muscle cells. Composed of globular actin molecules, these filaments form a helical structure crucial in muscle contraction.

Actin's Role in Muscle Contraction

During muscle contraction, actin filaments slide over myosin filaments, causing the muscle to shorten. This sliding occurs due to the myosin heads binding to the actin, creating cross-bridges and pulling the actin towards the center of the sarcomere. The process repeats, drawing the actin filaments closer together, which leads to muscle contraction.

Muscle Contraction Regulatory Proteins

Regulatory proteins are essential in controlling muscle contractions and ensuring that these contractions occur at the appropriate times. Troponin and tropomyosin, the primary regulatory proteins, work in tandem to either allow or prevent the myosin heads from binding to actin filaments.

How Troponin and Tropomyosin Regulate Contraction

Under relaxed conditions, tropomyosin blocks the myosin-binding sites on actin. When a muscle receives a contraction signal, calcium ions bind to troponin, causing a conformational change. This shift moves the tropomyosin away from the binding sites, permitting the myosin heads to engage the actin filaments, and thus, contraction occurs. In this intricate dance, each protein's role is indispensable for the finely tuned regulation of muscle contractions.

Sarcomere Anatomy

The sarcomere is the basic functional unit of the muscle fiber responsible for muscle contraction. Its highly ordered structure is necessary for the complex mechanism of muscle shortening.

Structural Components of the Sarcomere

The sarcomere is delineated by Z-discs, with the M-line running down its center. Actin filaments extend from the Z-discs towards the middle of the sarcomere, while myosin filaments are located centrally, overlapping with the actin. A sarcomere spans from one Z-disc to the next, and during contraction, these Z-discs move closer together as the actin and myosin filaments slide past each other. It is this precise organization and movement of filaments within the sarcomere that leads to muscle contraction.