Biological Systems MCAT Practice Exam

What functional unit of muscle cells contracts in response to an influx of calcium ions?

1. Myofibril

2. Actin-myosin fibers

3. Sarcoplasm

4. Endoplasmic reticulum

The correct answer is: Actin-myosin fibers

The correct response highlights the role of actin-myosin fibers in muscle contraction. In muscle cells, the fundamental process of contraction is driven by the interaction between actin and myosin, which are the primary proteins that make up the myofibrils. When an action potential reaches the muscle cell, it triggers the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm (the cytoplasm of a muscle cell). The influx of calcium ions binds to troponin, a regulatory protein associated with actin filaments. This binding causes a conformational change in troponin, which in turn moves tropomyosin away from the myosin-binding sites on actin. As a result, the myosin heads can attach to the exposed binding sites on the actin fibers. The power stroke initiated when myosin heads pivot pulls the actin filaments inward, leading to the shortening of the muscle fiber and thus contraction. Therefore, the actin-myosin interaction, facilitated by an influx of calcium ions, is crucial for the contraction of muscle cells. Understanding this process is essential as it forms the basis of how muscles produce movement and generate force.