Biological Systems MCAT Practice Exam 2025 - Free Biological Systems Practice Questions and Study Guide

The primary structural unit of a skeletal muscle fiber is the sarcomere. Sarcomeres are the fundamental repeating units found within myofibrils, which are long strands of muscle tissue that give muscle fibers their striated appearance. Each sarcomere is composed of two types of protein filaments: thick filaments made primarily of myosin, and thin filaments made primarily of actin, along with regulatory proteins like troponin and tropomyosin.

When muscle contraction occurs, the interaction of these filaments results in the shortening of the sarcomere, which leads to overall contraction of the muscle fiber and, consequently, the entire muscle. Therefore, understanding the role of sarcomeres is crucial for comprehending muscle function and physiology, as they are the key components responsible for muscle contraction.

The other terms have specific meanings in the context of muscle structure. Myofibrils consist of many connected sarcomeres, while myofiber refers to an entire skeletal muscle cell, and sarcoplasm is the cytoplasm of muscle fibers, which contains organelles and other proteins needed for muscle function. However, none of these alternatives serve as the primary structural unit in the same way that the sarcomere does.