How did data on sarcomere structure inspire the sliding-filament model of muscle contraction? Explain why the observation that muscle cells contain many mitochondria and extensive smooth endoplasmic reticulum turned out to be logical once the molecular mechanism of muscular contraction was understood.

Verified step by step guidance

1. 1. The sliding filament model of muscle contraction was inspired by the structure of the sarcomere, the basic unit of a muscle. Sarcomeres are composed of thick and thin filaments, known as myosin and actin respectively. The observation that these filaments do not change length during muscle contraction, but rather slide past each other, led to the development of the sliding filament model. This model proposes that muscle contraction occurs as the myosin heads bind to actin, forming cross-bridges, and then bend, pulling the actin filaments towards the center of the sarcomere.
2. 2. The sliding filament model was further supported by the discovery of the proteins troponin and tropomyosin, which regulate the interaction between actin and myosin. In a relaxed muscle, these proteins block the myosin-binding sites on actin. When a nerve impulse reaches the muscle, calcium ions are released, binding to troponin and causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin. This allows the myosin heads to bind to actin and begin the process of muscle contraction.
3. 3. The presence of many mitochondria in muscle cells is logical given the energy requirements of muscle contraction. The process of muscle contraction requires ATP, which is produced by mitochondria through cellular respiration. Therefore, muscle cells need a large number of mitochondria to meet their high energy demands.
4. 4. The extensive smooth endoplasmic reticulum (SER) in muscle cells, also known as the sarcoplasmic reticulum, plays a crucial role in muscle contraction. The SER stores and releases calcium ions, which are necessary for the regulation of muscle contraction. When a nerve impulse reaches the muscle, the SER releases calcium ions, which bind to troponin and initiate the process of muscle contraction.
5. 5. In conclusion, the structure of the sarcomere and the presence of many mitochondria and extensive SER in muscle cells are all integral to the process of muscle contraction. These observations have led to a deeper understanding of the molecular mechanisms underlying muscle contraction.