Structure and Mechanism of Contraction
Skeletal muscles are composed of muscle bundles or fascicles held together by fascia. Each muscle bundle contains muscle fibers surrounded by the sarcolemma and sarcoplasm. The sarcoplasmic reticulum stores calcium ions, and myofibrils, composed of myofilaments, give the myofiber a striated appearance. Actin is predominantly found in the I-band, while myosin is primarily located in the A-band. Actin and myosin filaments are parallel to each other and arranged along the myofibril’s longitudinal axis. The Z-line divides the I-band, and the M-line holds the thick filaments together in the A-band. Sarcomeres, located between Z-lines, are the functional units responsible for muscle contraction. In a resting state, thin filaments overlap partially with thick filaments, leaving the H-zone unoverlapped.
Structure of Contractile Proteins
The structure of an actin (thin) filament is composed of two helically wound “F” (filamentous) actins. Each “F” actin is a polymer formed by monomeric “G” (globular) actins. Alongside the “F” actins, two filaments of the protein tropomyosin run parallel to them. Tropomyosin is a complex protein that is evenly distributed along the length of the actin filaments. In the resting state, a subunit of troponin covers or masks the active binding sites on the actin filaments, preventing myosin from binding to them.
On the other hand, the myosin (thick) filament is also a polymerized protein. It is composed of multiple monomeric proteins called meromyosins. A thick filament consists of two main components: heavy meromyosin (HMM) and light meromyosin (LMM).
The HMM component of the myosin filament includes the globular head, which possesses an active ATPase enzyme. It also has binding sites for ATP, as well as active sites for actin. The globular heads with short arms project outward from the surface of the polymerized myosin filament at regular intervals and angles, forming cross-arms.
In short, the actin filament is made up of polymerized “F” actins, accompanied by tropomyosin and troponin, while the myosin filament consists of polymerized meromyosins, with the globular heads containing ATPase and actin-binding sites.
NCERT Notes for Class 11 Biology Chapter 17: Locomotion and Movement
NCERT and CBSE Notes for Class 11 Biology Chapter 17: Locomotion and Movement: The given article is a detailed introduction to the class 11 chapter “Locomotion and Movement”. The article discusses all the important topics and their related sub-topics which include muscles, types of muscles, muscle contraction, skeletal system, and a brief illustration of different types of bones present in the human body.
Notes on Locomotion and Movement of NCERT Class 11 The article also looks into the structure of contractile protein and its functioning. A bunch of frequently asked questions are also included in this article for better understanding.
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