Chapter 7: Capturing Solar Energy: PhotosynthesisIssues in Biology |
Chlorophyll is responsible for the beautiful green color of the plants around us, but did you realize that a molecule very similar to chlorophyll is also responsible for the red color of your blood? The chlorophyll molecule has a complex ring structure that binds a magnesium ion in its center. This basic structure is known as a porphyrin ring. A porphyrin ring is also at the heart of a molecule called heme, which carries oxygen from our lungs and delivers it to cells throughout our body. However, instead of magnesium, heme binds to an iron ion in its center. Heme also lacks the long “tail” that is present in chlorophyll. In our red blood cells, each heme molecule is bound to a protein called globin, forming hemoglobin. Heme absorbs yellow and green light, so it appears red, giving our blood its characteristic deep crimson color.
Red blood cells live only about 120 days. Each day, about a billion red blood cells finish out their life span and are destroyed in the spleen, liver, and bone marrow. The globin in these cells is broken down to amino acids, to be used for making other proteins. The iron from the heme is rescued and conserved in the body’s iron stores, so it is available to make more heme. Interestingly, the heme itself is broken down and excreted out of the body! A major intermediate in heme degradation, bilirubin, is produced by the liver. Instead of the red color characteristic of heme, bilirubin absorbs different wavelengths of light so that it appears yellowish. The excess production of bilirubin produces noticeable yellowing of the skin and whites of the eye in individuals with jaundice. You can also see the first stages of heme degradation when a bruise changes color from purple to yellow as the heme is broken down to bilirubin in the wounded tissue. If you have had a bruise that is greenish in color, the bilirubin has undergone alterations for biliverdin, which has a greenish color.
Bilirubin is normally secreted in the bile that the liver produces and dumps into the intestine to aid in digestion. Bacteria in the intestine further break down or modify the bilirubin, producing waste products that exit in feces and urine. In fact, the yellow or brown color of our wastes is produced by these products of heme digestion.
So what happens to the chlorophyll that we eat when we consume green vegetables? You can browse the Internet and come across dozens of claims for chlorophyll as a health food. For example, it is recommended that you consume chlorophyll in the juice or extracts from the young blades of green grasses such as wheat-grass or barley. Because of the similarity of chlorophyll and heme structure, the claims of the benefits of chlorophyll may seem plausible. Perhaps chlorophyll in our diet can save our cells some time and effort in making all those heme molecules to fill up the billions of red blood cells that our body produces every day. Among several assumptions, this idea presumes that increased heme production will result in production of more red blood cells, thus helping to treat conditions such as anemia. However, even the heme made in our own bodies is not reused but instead is broken down and excreted. Consequently, it is very unlikely that the chlorophyll we eat with our veggies does much more than pass through our system. In fact, consistent with this possibility, one warning about a potential side effect of chlorophyll supplements is that they can turn your feces green! This example illustrates how difficult it can be to separate fact from fiction, particularly when the fiction is mixed with truths. So when you need to make important decisions about your health and well-being, a distinct advantage can be a healthy dose of skepticism.