|MadSci Network: Anatomy|
The color of blood comes from the red blood cells (RBC's, also called erythrocytes), which make up about 40% of blood, by volume. Each red blood cell is filled with hemoglobin, the protein which carries oxygen to tissues and carbon dioxide away from tissues. Hemoglobin carries oxygen by using heme, a large ring-like molecule which has at its center a single atom of iron, which is what actually binds to the oxygen.
Heme is not found only in hemoglobin, and it does not bind only iron: heme is also found in cytochromes and vitamins, and its relative, chlorophyll, is found in plants; it can bind magnesium, zinc, and other metal ions beside iron. In most cases, heme is used to move electrons from one place to another, but in hemoglobin, this activity has been modified to also carry oxygen from place to place. The nature of heme that gives it these abilities is in the many double covalent bonds that form the ring. These double bonds can be shifted into many different configurations, so that any electrons added to the molecule can move freely around the ring, dispersing the negative charge across the whole molecule.
The movement of double bonds like this is called resonance, and it is an important part of many other molecules which are used as pigments to give things color. All of these molecules are called chromogens, and their color depends on the resonant frequency of the molecule: heme makes blood red, beta-carotene makes carrots orange, chlorophyll makes leaves green, and a variety of chromogens, including xanthophylls and caro- tenoids, give flowers their many colors. Even most food colorings and clothing dyes are resonant organic chromogens.
When oxygen binds to the iron atom in heme, the iron changes its shape slightly, which alters the resonance of the heme molecule. This new resonance gives off a different frequency of light, so the perceived color of the heme goes from dark red to bright red. When the oxygen is released into the tissues, the iron goes back to its original shape, and the heme returns to its normal resonance, so the color goes back from scarlet to maroon.
Sorry it took so long getting to the last part; I hope this helps.
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