Myofibrils are __________.

• A. the plasma membrane of a muscle cell
• B. organelles responsible for producing ATP in muscle cells
• C. connective tissue layers surrounding muscles
• D. rod-like structures composed of repeating sarcomeres within muscle fibers

Answer: (D)

Verified step by step guidance

1. Understand the context: Myofibrils are structural components of muscle cells, so the question is asking about their specific role and structure within the muscle fiber.
2. Recall the anatomy of a muscle cell: Muscle fibers (muscle cells) contain myofibrils, which are long, rod-like structures that run parallel to the length of the cell.
3. Recognize the composition of myofibrils: Myofibrils are made up of repeating units called sarcomeres, which are the functional contractile units of muscle. Sarcomeres are composed of actin (thin filaments) and myosin (thick filaments).
4. Eliminate incorrect options: The plasma membrane of a muscle cell is called the sarcolemma, not myofibrils. Organelles responsible for producing ATP are mitochondria, and connective tissue layers surrounding muscles include the epimysium, perimysium, and endomysium.
5. Conclude the correct answer: Myofibrils are rod-like structures composed of repeating sarcomeres within muscle fibers, which aligns with the provided correct answer.