The ______ belongs to the thoracic cage, whereas the ______ belongs to the pectoral girdle.

Choice A reason: The sarcomere is the smallest contractile unit of a skeletal muscle. It is composed of actin and myosin filaments arranged in a precise pattern that allows for muscle contraction. Sarcomeres are repeated along the length of a myofibril and are responsible for the striated appearance of skeletal muscle.

Choice B reason: The sarcoplasmic reticulum is a specialized form of endoplasmic reticulum in muscle cells that stores and releases calcium ions, which are essential for muscle contraction. However, it is not the contractile unit itself.

Choice C reason: The sarcolemma is the plasma membrane of a muscle fiber. It plays a role in conducting electrical signals but is not the structural unit responsible for contraction.

Choice D reason: A myofibril is a long, cylindrical organelle found within muscle cells that contains many sarcomeres. While it contributes to muscle contraction, it is composed of multiple sarcomeres and is not the fundamental unit.

Choice A reason: Myoglobin is a red pigment found in muscle cells that binds oxygen molecules. It serves as an oxygen reservoir, especially important during periods of intense muscular activity when oxygen demand exceeds supply. Its high affinity for oxygen allows it to release oxygen when intracellular levels drop, supporting aerobic metabolism.

Choice B reason: The protein involved in the direct phosphorylation of ADP is creatine kinase, which facilitates the transfer of a phosphate group from creatine phosphate to ADP. Myoglobin is not involved in this process.

Choice C reason: Glycogen breakdown is catalyzed by enzymes such as glycogen phosphorylase. Myoglobin does not participate in carbohydrate metabolism or glycogenolysis.

Choice D reason: Oxygen transport throughout the body is primarily the function of hemoglobin in red blood cells. Myoglobin stores oxygen locally within muscle cells rather than transporting it across tissues.

Choice E reason: The end plate potential is generated at the neuromuscular junction due to the influx of sodium ions following acetylcholine binding. This is an electrical event and does not involve myoglobin.