Isotopes

Carbon is a very important atom for biological organisms, and we will talk about the chemical reasons for that. If you have ever watched Star Trek or other science fiction shows, you'll remember that they talk about carbon-based life forms. Life on Earth is considered to be carbon-based because the macromolecules that are the building blocks of life have a carbon "skeleton" to which other atoms attach. All carbon atoms have six protons. But different types of carbon have different numbers of neutrons. The element is defined by the number of protons but if there are different numbers of neutrons in the nucleus of that atom then you have a different isotope of that particular atom or element.

Carbon 12 is the most common form of carbon. It has six protons and six neutrons so it has an atomic weight of 12. Almost all of the carbon atoms in your body are Carbon 12. There is another form of carbon, Carbon 13, which is very rare. It is a stable form with six protons and seven neutrons. There also is a Carbon 14, which has six protons and eight neutrons. Carbon 14 occurs in very small quantities in the environment, with about one Carbon 14 atom for every trillion Carbon 12 atoms. All organisms have a little bit of Carbon 14 in their bodies. The carbon atoms in your body are constantly changing; you bring in food that has carbon atoms[,] which are then used to replace other carbon atoms in your body. Carbon 14 is incorporated into your body at about the same percent that it exists in nature. Chemically, these isotopes are identical in our body and they function the same way. The nucleus of Carbon 14 is not stable, however, and over time it breaks down or decays. When the nucleus is not stable we say that an isotope is radioactive. The rate of decay of a particular isotope is constant as are the particles that it decays into. We can use this information.

When an organism dies, incorporation of new carbon atoms stops. Death means that all of the carbon atoms that an organism has at that point are fixed, with the ratio of Carbon 12/Carbon 14 that exists in nature. As Carbon 14 decays over time, it is converted to an isotope of nitrogen, N14. This decay is the basis of Carbon 14 dating of fossils (Figure 3.7 in your text). We can measure the ratio of C12/C14 in the fossil and, knowing the rate of decay for C14, calculate its age; the smaller the ratio of C14, the older the specimen. We use radioactive decay of isotopes in a lot of different ways. Radioactivity has a bad public image because people associate radioactivity with mutation, but radioactive decay is a natural process.

There are many elements that have isotopes that are not stable. The decay of Carbon 14 does not release energy that can damage tissue, but there are some other radioactive decay processes that can cause tissue damage. Those isotopes release gamma rays or x rays, but even these can be used to benefit humans. Some are used in medicine. For example, if you have problems with your thyroid, the doctor may want to test thyroid function. Your thyroid uses the element iodine. The physician will give you a radioisotope of the element iodine. The half-life (the time that it takes for half of this radioactive form of iodine to decay) is very short. So physicians can measure the concentration of the element to see how well your thyroid is functioning. Radioisotopes are commonly used in scientific research. You can label DNA with a radioisotope of phosphorous and then visualize the DNA on a piece of film. You expose it to x-ray film because it releases energy when it decays. This allows you to determine the base composition of DNA in a process called DNA sequencing.