Final answer:
Hemoglobin is the protein that makes up 95% of the dry weight of red blood cells and is responsible for their red pigment. This iron-containing protein carries oxygen and carbon dioxide. Red blood cells are packed with hemoglobin to transport oxygen efficiently throughout the body.
Step-by-step explanation:
The molecule that constitutes approximately 95% of the dry weight of each red blood cell and endows it with its characteristic red color is hemoglobin. This iron-containing protein is critical for the transportation of oxygen and carbon dioxide within the body. A single red blood cell contains about 250 million molecules of hemoglobin, and each molecule can bind up to four oxygen molecules, thus each cell can transport about one billion molecules of oxygen. Furthermore, the structure of hemoglobin comprises four subunits, each with a heme group that contains iron, which is responsible for the red coloration as it binds oxygen. This allows arterial blood, which has hemoglobin carrying four oxygen molecules, to appear bright red, while deoxygenated venous blood appears darker.
Notably, erythrocytes, or red blood cells, lack organelles and a nucleus in mammals, which provides more room for hemoglobin and ensures that oxygen is not used for metabolic respiration within these cells. Instead, these cells metabolize anaerobically, utilizing a primitive metabolic pathway to generate ATP and optimize oxygen transport efficiency.