Problem 10 - Biology Today and Tomorrow with Physiology

Question

A sarcomere shortens when _____ a. actin filaments shorten b. myosin filaments shorten c. both actin and myosin filaments shorten d. actin filaments slide past myosin filaments

Step-by-Step Solution

Answer

A sarcomere shortens when actin filaments slide past myosin filaments (option d).

Step 1: Identifying Key Terms

First, understand the key components involved in the contraction of a sarcomere: actin filaments and myosin filaments. These structures are part of the muscle fiber's contractile machinery.

Step 2: Understanding Sarcomere Function

A sarcomere is the basic contractile unit of muscle fiber. During contraction, it is not the filaments that shorten, but rather they slide over each other, resulting in the shortening of the entire sarcomere.

Step 3: Analyzing Available Options

Evaluate the given options: a) actin filaments shorten, b) myosin filaments shorten, c) both actin and myosin filaments shorten, d) actin filaments slide past myosin filaments. Options a, b, and c suggest filament shortening, which is incorrect.

Step 4: Selecting the Correct Option

Choose the option that correctly describes the mechanism of sarcomere shortening. The correct understanding is that the actin filaments slide past the myosin filaments, which matches option d.

Key Concepts

Actin Filaments

Actin filaments are thin strands of protein that play a critical role in muscle contraction. They are part of the sarcomere, the smallest contractile unit within a muscle fiber. These filaments are composed mainly of the protein actin, which interacts with the thicker myosin filaments during muscle contractions.

• Actin filaments are anchored to the Z-discs at the ends of each sarcomere.
• During muscle contraction, they remain of constant length but change their position.
• This movement allows them to slide past the static myosin filaments.

The sliding mechanism is powered by the energy from ATP, and is known as the sliding filament theory. Essentially, the heads of myosin filaments pull on the actin filaments, drawing them toward the center of the sarcomere. This action results in the shortening of the sarcomere, and thus the entire muscle contracts.

Myosin Filaments

Myosin filaments are thick, rope-like structures composed mainly of the protein myosin. These filaments are central to the contraction process within a sarcomere. They interact with the actin filaments to produce the force needed for muscle contraction.

• Myosin molecules have heads that bind to actin filaments using cross-bridge links.
• These heads perform a pulling motion powered by ATP, causing actin to slide past.
• The myosin filaments themselves do not move, only their heads are actively involved in the motion.

This pulling action creates tension within the muscle, leading to contraction. Myosin's ability to bind and "walk" along actin filaments positions it as the motor behind muscle movement. The interaction between myosin and actin is what drives the entire process of muscle contraction.

Muscle Fiber

Muscle fibers are the building blocks of muscles, comprising many sarcomeres aligned in series. Each muscle fiber consists of long cylindrical cells filled with myofibrils, which in turn contain sarcomeres.

• Muscle fibers are covered by a cell membrane known as the sarcolemma.
• Within each muscle fiber, sarcomeres create the pattern needed for contraction.
• The synchronized contraction of all sarcomeres results in the muscle contraction we observe.

Muscle fibers are responsible for converting chemical energy from ATP into mechanical energy, enabling movement. Depending on their arrangement and size, different types of muscle fibers are specialized for endurance, strength, or speed. They work cooperatively to ensure efficient body movements and maintain posture.