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How Radioactive Isotopes Track Biological Molecules

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  • 0:42 What Are Radioactive Isotopes
  • 2:21 Tracking Atoms in…
  • 4:27 Using Radioisotopes in…
  • 6:25 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Radioactive isotopes can be used to track atoms and label biological molecules. This lesson explores how this can be applied to microbiology to learn more about the way a cell works.

Visualizing Biological Processes

Have you ever heard the phrase, 'I'll believe it when I see it'? While this phrase might make sense in our daily lives, there are so many things in biology that we just can't see with our own eyes. Organisms like bacteria are constantly taking up chemical compounds and rearranging the atoms, using some for building cell components, digesting some, and respiring others. How can we track or observe a process that is happening at this almost incomprehensibly tiny scale? One answer is radioactive isotopes.

What Are Radioactive Isotopes?

Let me refresh your memory on isotopes. Each element in the periodic table has a set number of protons but can have a variable number of neutrons. Atoms of the same element that have a different number of neutrons are referred to as isotopes of the element. For example, carbon has three major, stable isotope forms: C-12, which has six protons and six neutrons; C-13, which has six protons and seven neutrons; and C-14, which has six protons and eight neutrons. You get the pattern?

So, all elements can have multiple isotope forms. But some of these isotope forms are even more special and are referred to as radioactive isotopes. These isotopes are unstable and will convert to a more stable isotope form spontaneously, releasing energetic particles in the process. Some radioactive isotopes convert incredibly quickly, with lifetimes measured in seconds or less, while others are moderately fast, with lifetimes measured in minutes to hours to days, while some are much slower, with lifetimes measured in decades, centuries, or millennia.

In biology, we can exploit the radioactive isotopes that have moderately long lifetimes to track atoms and molecules. We can do this by detecting or measuring the energy or particles that are released when the isotope converts to a more stable form. Some useful radioactive isotopes used in microbiology experiments include hydrogen-3, carbon-14, phosphorus-32, and sulfur-35.

Tracking Atoms in Major Macromolecues

Let's use radioactive isotopes to see how this works and to learn something about the major macromolecules in the cell. Remember that the major macromolecules are things like lipids, carbohydrates, proteins, and nucleic acids like DNA and RNA. The macromolecules are made up of building blocks that contain certain elements, as this table illustrates.

For bacteria to grow and reproduce, they must have a source of all these elements. In nature, bacteria have to scavenge compounds containing these elements from the environment. But in the lab, we can provide them with sources of these elements and track their incorporation into the cell.

Let's say we are interested in determining which elements can be incorporated into nucleic acids and proteins. In order to answer this question, we will do a set of experiments. First, we will grow some bacteria in a flask with a phosphorus source. The trick is that the phosphorous source is made up of the radioactive isotope phosphorus-32. Now we let the cells grow for a while. Then we collect the cells, break them open and separate and collect the nucleic acids and proteins.

So we have a tube containing the cellular nucleic acids and a tube containing the cellular proteins. Now we measure the tubes to detect radiation, or, more specifically, the emission of radioactive particles. Which tube do you think will emit radiation? If you said the nucleic acid tube, you would be right. We can see that nucleic acids contain phosphorus while proteins do not.

Now, what if we grew our cells with a source of radioactive sulfur-35? Would we detect radiation in the nucleic acid tube or the protein tube? If you said the protein tube, you would be right again. This may have seemed like a pointless set of experiments since we already know what major elements are in the cell's major macromolecules, but this basic kind of experiment can be used to learn a lot about bacterial growth and metabolism.

Using Radioisotopes in Microbiology

Bacteria live in complex communities consisting of many different species. These communities inhabit a diverse range of habitats, from soil to the human gut to the atmosphere. In every case, multiple organisms can work together to carry out various chemical and metabolic processes. These microbes play an important role in controlling many of the global element cycles. This means that microbes can play a role in things like climate change or the breakdown of pollutants in the environment. But scientists have to find a way to measure and detect the microbial activities so they can accurately account for their contribution.

But there is a problem. When we take bacteria into the lab and begin to grow them separately in culture, they become tame and domesticated and we lose the complex interactions that take place in nature. Some scientists use radioisotopes to try to capture some of the chemical reactions that take place in complex bacterial communities. Let me give you an example.

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