Final answer:
The biconcave shape of red blood cells (RBCs) is maintained by hemoglobin. Hemoglobin is made up of proteins and iron, consisting of four folded chains of a protein called globin. Hemoglobin's shape optimizes gas exchange and allows RBCs to fold up as they move through narrow blood vessels.
Step-by-step explanation:
The biconcave shape of red blood cells (RBCs) is maintained by a protein known as hemoglobin. Hemoglobin is a large molecule made up of proteins and iron, consisting of four folded chains of a protein called globin. Each of these globin molecules is bound to a red pigment molecule called heme, which contains an ion of iron. Hemoglobin's biconcave shape optimizes the ratio of surface area to volume, facilitating gas exchange and allowing RBCs to fold up as they move through narrow blood vessels.