Which component does an O² molecule attach to in the blood?

The O₂ molecule primarily attaches to heme within hemoglobin located in red blood cells. Heme is an iron-containing component of hemoglobin, the protein responsible for transporting oxygen from the lungs to the tissues throughout the body. Each heme group can bind one oxygen molecule, allowing for efficient oxygen transport and release based on the body's needs.

The other components listed do not play a role in oxygen binding. Plasma is the liquid component of blood but does not directly bind oxygen. Leukocytes, or white blood cells, are involved in the immune response and do not carry oxygen. Platelets are cell fragments important for blood clotting but also do not participate in oxygen transport. Therefore, heme is the critical component for oxygen attachment in the blood’s transport system.