Final answer:
Hemoglobin is the protein in red blood cells that transports oxygen from the lungs to tissues and carbon dioxide back to the lungs, made up of four subunits and containing iron for binding oxygen. Option a.
Step-by-step explanation:
The protein in red blood cells (RBCs) that carries oxygen from the lungs to tissues is hemoglobin. Hemoglobin is composed of four symmetrical subunits and four heme groups, where iron in the heme binds oxygen. It is this iron in hemoglobin that imparts blood its characteristic red color.
Not only does hemoglobin transport oxygen, but it also carries carbon dioxide back to the lungs. Each hemoglobin molecule can carry up to four oxygen molecules, enabling the efficient transfer of oxygen to body tissues.
Alternative proteins, such as myoglobin, are responsible for storing oxygen in muscle tissues, while oxyhemoglobin is a form of hemoglobin that has oxygen bound to it. Hemoglobin's ability to carry oxygen and carbon dioxide is due to the structure-function relationship in proteins, highlighting hemoglobin's vital role in the respiratory process.
So Option a is correct.