What is the structural unit of compact bone?

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.

Choice A reason: Pronation refers to the rotation of the forearm so that the palm faces downward. It does not describe movement away from the midline.

Choice B reason: Protraction refers to the anterior movement of a body part, such as pushing the shoulders forward. It does not involve lateral movement of the arm.

Choice C reason: This is correct. Abduction is the movement of a limb away from the midline of the body. Raising the arm sideways away from the torso is a classic example of abduction.

Choice D reason: Adduction is the opposite of abduction. It involves moving a limb toward the midline of the body.

Choice E reason: Retraction refers to the posterior movement of a body part, such as pulling the shoulders backward. It does not describe lateral arm movement.