Question

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.

Short answer

The sliding-filament model was inspired by the observation of sarcomere structure, which showed the interaction between actin and myosin filaments, resulting in muscle contraction. The presence of many mitochondria in muscle cells provides the necessary ATP for muscle contraction, while the extensive smooth endoplasmic reticulum (sarcoplasmic reticulum) stores and releases calcium ions (Ca2+) required for muscle contraction and relaxation, supporting the sliding-filament model.

Step-by-step solution

1. Understanding the Sliding-Filament Model

The sliding-filament model of muscle contraction is a biological mechanism that explains how muscle contraction occurs at the molecular level. The muscle is made up of small structural units called sarcomeres, which consist of two main protein filaments: actin (thin filament) and myosin (thick filament). In the sliding-filament model, the binding of myosin to actin and subsequent movement of the myosin head cause the thin actin filament to slide over the thick myosin filament, resulting in muscle contraction.

2. Data on Sarcomere Structure that Inspired the Model

The observation of sarcomere structure using electron microscopy provided insights into the arrangement and organization of actin and myosin filaments within the sarcomere. Researchers observed that the overlapping regions of actin and myosin filaments exhibited a striated pattern, suggesting a mechanism of interaction between the filaments. Additionally, during muscle contraction, the length of the sarcomere shortens, but the length of the individual actin and myosin filaments remains constant. These observations inspired the sliding-filament model, which suggests that muscle contraction occurs due to the movement of actin filaments over myosin filaments.

3. Role of Mitochondria in Muscle Contraction

Mitochondria are the cellular organelles responsible for energy production through aerobic respiration. Muscle contraction requires energy in the form of adenosine triphosphate (ATP). During muscle contraction, the myosin head utilizes ATP to bind to actin and change its conformation, enabling it to pull actin filament towards the center of the sarcomere. The presence of many mitochondria in muscle cells provides the necessary ATP to drive the muscle contraction process.

4. Role of Smooth Endoplasmic Reticulum in Muscle Contraction

The smooth endoplasmic reticulum (SER) in muscle cells, also known as the sarcoplasmic reticulum (SR), plays a crucial role in regulating muscle contraction through calcium ion (Ca2+) storage and release. When a muscle cell is stimulated by a nerve impulse, Ca2+ is released from the sarcoplasmic reticulum, which in turn activates the binding of myosin to actin, initiating the contraction process. After muscle contraction, calcium ions are actively pumped back into the sarcoplasmic reticulum, allowing the muscle to relax. The extensive smooth endoplasmic reticulum in muscle cells serves to store and release calcium ions required for muscle contraction and relaxation. In conclusion, the sliding-filament model was inspired by the data on sarcomere structure, which highlighted the interaction mechanisms between actin and myosin filaments, resulting in muscle contraction. The presence of many mitochondria and an extensive smooth endoplasmic reticulum in muscle cells logically supports this model as they supply the necessary energy (ATP) and calcium ions (Ca2+) required for muscle contraction and relaxation.

Key concepts

Sarcomere Structure

The fascinating structure of sarcomeres forms the foundation for understanding the sliding-filament model of muscle contraction. Sarcomeres are the fundamental units of striated muscle tissue, characterized by their distinct arrangement of actin and myosin filaments. Electron microscopy has revealed that these filaments are organized in a striated pattern, due to their overlapping regions.
This striation is a crucial insight, suggesting a potential mechanism of interaction between actin and myosin, where the arrangement allows for movement and sliding.
The sarcomere's unique organization ensures that during contraction, the unit shortens in length while the individual filaments maintain their size. This shortening without change in filament length is a pivotal observation that inspired the sliding-filament model, illustrating how the movement of actin over myosin leads to muscle contraction.

Muscle Contraction

Muscle contraction is an intricate process driven by the interaction between the proteins actin and myosin within the sarcomere. The sliding-filament model explains this at the molecular level by describing how myosin heads bind to actin filaments, utilizing ATP. Each myosin head pivots, pulling the actin towards the sarcomere's center, a process that repeats cyclically.

• The energy for this action is derived from ATP, which is hydrolyzed on the myosin head, providing the necessary force for movement.
• This continual cycle of attachment, movement, and detachment of myosin heads pulling actin filaments past them, is what causes the muscle to contract.

As each sarcomere shortens, the entire muscle fiber contracts, facilitating movement in response to neural signals. The sliding-filament model provides a comprehensive framework to understand this sophisticated biological mechanism.

Mitochondria in Muscle Cells

Mitochondria, often referred to as the powerhouse of the cell, play a critical role in muscle contraction by supplying the much-needed energy in the form of ATP. Muscle cells are packed with numerous mitochondria to meet these high energy demands.
During muscle contraction, ATP is essential for myosin heads to bind and rebind to actin filaments continuously. This cycle consumes a significant amount of ATP
Multiple functions of mitochondria in muscle cells include:

• Converting nutrients into ATP through aerobic respiration.
• Ensuring ample ATP is available for sustained muscle activities.

Mitochondria ensure that muscle cells have a steady supply of ATP, aligning perfectly with the demands of the sliding-filament model, where constant ATP is essential for ongoing contraction and relaxation cycles.

Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum, or sarcoplasmic reticulum (SR) in muscle cells, is vital for regulating calcium ions, which are crucial for muscle contraction. It acts as a storage depot for calcium ions, releasing them swiftly when a muscle cell receives a neural signal.

• Upon nerve stimulation, calcium ions flood the sarcoplasm from the SR, triggering myosin to bind to actin.
• This surge of calcium and subsequent binding initiate the contraction process outlined by the sliding-filament model.

When contraction ends, calcium ions are actively pumped back into the SR, allowing the muscle to relax.
This efficient storage and release process showcases how integral the smooth endoplasmic reticulum is to muscle function, closely supporting the roles of mitochondria and the structure of sarcomeres in effective muscle contraction and relaxation.