Elemental building blocks of biological molecules | Chemistry of life | AP Biology | Khan Academy

What we have here is just a small sample of the types of molecules that you will see in a biological system. At the top left, right over here, you have an example of an amino acid. Amino acids are the building blocks of proteins. If we were to take a look at what an amino acid is made up of, in this dark gray color, those are carbon atoms. In the white, you see hydrogen atoms. In the red, you see oxygen atoms, and this blue right over here, that is a nitrogen atom.

As you can see, a lot of these elements keep showing up in these various molecules, especially carbon and hydrogen; but also, you see a lot of oxygen and nitrogen. As we're about to see, phosphorus also pops up a lot. Now, this isn't a comprehensive list. You'll also see other elements, but these tend to show up fairly frequently.

For example, this is a model of ATP, adenosine triphosphate. As we study biology, you'll see that it's often viewed as the currency of energy, the molecular currency of energy in biological systems. Once again, we see a lot of carbons in the dark gray. We see the hydrogens in this off-white color, or the light gray, I guess you could say. You see your oxygens again here; the nitrogen is in this light blue color. Then you see the phosphoruses right over there in that yellow color.

This is a model of a triglyceride, often known as a fat molecule. Fat molecules are used for energy storage, and once again, you see many carbons in the dark gray. Then you see these hydrogens and then a few oxygens. This is a model of DNA, a small segment of DNA, and this is a much more complex molecule than the other ones we've seen. In fact, this could extend far beyond our screen in either direction. But once again, you see these same familiar elements. You see the carbon in the dark gray, the hydrogen in that white color, you see the oxygens in the red, the nitrogens in the blue, and the phosphorus in the yellow.

So the big takeaway here is that biological molecules tend to be made up of the same set of elemental building blocks. In fact, it isn't just at the elemental level; it can even be at the molecular level. For example, in ATP, you have what's known as a nitrogenous base right over here, you have a five-carbon sugar right over here, and you have three phosphate groups or a triphosphate group.

In DNA, you have something very similar; the nitrogenous bases are hard to see. They are kind of the rungs of the ladder here. You have your five-carbon sugars, also hard to see, and then you have these phosphates as well. In fact, the backbone of DNA is made up of these five-carbon sugars and these phosphates.

Now, why do these elements keep showing up? Well, these are elements that you will see a lot on Earth. For example, nitrogen makes up most of our atmosphere. We have a lot of water on the surface of our planet, which is made up of oxygen and hydrogen. Carbon actually makes up a surprisingly small percentage of our atmosphere, about 0.04% of our atmosphere. But photosynthetic organisms, like plants, are good at fixing carbon and storing energy in carbon bonds.

When we eat those, those become part of our bodies. Just to get an appreciation of what we are made up of in terms of elements, we can look at this chart right over here, where we see that we are primarily made up of oxygen, which is a high percentage in the body. That's because we're primarily made up of water, and water is primarily oxygen. It also has hydrogen.

Now, second to oxygen is carbon, and then you see nitrogen, phosphorus. We, of course, have a lot of calcium. Calcium, of course, is used in bones, but it's also used for things like muscle contractions. I could keep on going down this list, and you will see these other elements in your study of biology.

But the big picture is that even though biological systems can get fairly complex, they're made up of similar building blocks, and these elemental building blocks come from the environment in which these biological systems exist and evolved.