The iron reversibly associates with oxygen, and in so doing is oxidized from Fe2+ to Fe3+. Hemoglobin is composed of four protein subunits, two alpha chains and two beta chains, and a heme group that has iron associated with it. In most vertebrates, (a) hemoglobin delivers oxygen to the body and removes some carbon dioxide. Despite the name, hemerythrin does not contain a heme group and its oxygen-carrying capacity is poor compared to hemoglobin. Hemerythrin, a red, iron-containing protein is found in some polychaete worms and annelids and is illustrated in Figure 21.6 c. Chlorocruorin, a green-colored, iron-containing pigment is found in four families of polychaete tubeworms. Hemocyanin, a blue-green, copper-containing protein, illustrated in Figure 21.6 b is found in mollusks, crustaceans, and some of the arthropods. Invertebrates have a variety of other respiratory pigments. These pigments use copper or iron to the oxygen. Invertebrates that utilize hemolymph rather than blood use different pigments to bind to the oxygen. Not all organisms use hemoglobin as the method of oxygen transport. Anucleated red blood cells metabolize anaerobically (without oxygen), making use of a primitive metabolic pathway to produce ATP and increase the efficiency of oxygen transport. The advantage of nucleated red blood cells is that these cells can undergo mitosis. Only mammals have anucleated red blood cells, and some mammals (camels, for instance) even have nucleated red blood cells. In mammals, the lack of organelles in erythrocytes leaves more room for the hemoglobin molecules, and the lack of mitochondria also prevents use of the oxygen for metabolic respiration. There are approximately 25 trillion red blood cells in the five liters of blood in the human body, which could carry up to 25 sextillion (25 × 10 21) molecules of oxygen in the body at any time. Each hemoglobin molecule binds four oxygen molecules so that each red blood cell carries one billion molecules of oxygen. Hemoglobin is packed into red blood cells at a rate of about 250 million molecules of hemoglobin per cell. The principal job of this protein is to carry oxygen, but it also transports carbon dioxide as well. The red coloring of blood comes from the iron-containing protein hemoglobin, illustrated in Figure 21.6 a. In birds and non-avian reptiles, a nucleus is still maintained in red blood cells. In mammals, red blood cells are small biconcave cells that at maturity do not contain a nucleus or mitochondria and are only 7–8 µm in size. Red blood cells, or erythrocytes (erythro- = “red” -cyte = “cell”), are specialized cells that circulate through the body delivering oxygen to cells they are formed from stem cells in the bone marrow. Blood is 20 percent of a person’s extracellular fluid and eight percent of weight. In humans, cellular components make up approximately 45 percent of the blood and the liquid plasma 55 percent. Interstitial fluid that surrounds cells is separate from the blood, but in hemolymph, they are combined. The platelets are responsible for blood clotting. The cells are responsible for carrying the gases (red cells) and immune the response (white). Blood plasma is actually the dominant component of blood and contains the water, proteins, electrolytes, lipids, and glucose. Blood is actually a term used to describe the liquid that moves through the vessels and includes plasma (the liquid portion, which contains water, proteins, salts, lipids, and glucose) and the cells (red and white cells) and cell fragments called platelets. The blood is more than the proteins, though. Hemoglobin is responsible for distributing oxygen, and to a lesser extent, carbon dioxide, throughout the circulatory systems of humans, vertebrates, and many invertebrates.
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