Molecular variation | Cellular energetics | AP Biology | Khan Academy

We are now going to discuss molecular variation in cells. You're probably familiar with the idea that you have a variation of genetic makeups in a population. But even within an organism, you have variation in the types of molecules that an organism can produce and when they produce them.

So, for example, we know that we all have DNA. All organisms, living organisms that we know about, they have DNA. I'll just do this as a quick drawing of DNA. We know that we have genes in our DNA that code. Eventually, they go from DNA to messenger RNA and then they go to the ribosomes to be translated into proteins. These proteins are a major way of expressing what is encoded in our DNA.

Now, it turns out that our DNA will encode for not only multiple proteins but multiple types of the same protein. It can encode for some of these proteins more under certain circumstances and other proteins more in other circumstances based on environmental factors. Those environmental factors might influence what part of the DNA is being transcribed into mRNA, which then is translated into proteins at different times.

And there are several very interesting examples of this. It turns out that hemoglobin, which you might recognize as the protein complex that binds to oxygen in our red blood cells, the type of predominant hemoglobin changes from when we are inside our mother's wombs to when we become independent beings.

So this right over here is a picture of a hemoglobin molecule. You see your four heme groups that each bind to oxygen. When you're a fetus, the primary type of hemoglobin is hemoglobin F. Then, once we come out of our mother's womb, the hemoglobin F stops getting produced and we go to hemoglobin A.

Now you might say, "Well, why do we have this variation in the type of hemoglobin?" And the answer is that those are two different environments. When a fetus is in the mother's womb, it's not directly breathing. It's getting its oxygen from the mother's blood. The mother's blood does not mix directly with the baby's blood, but there's a boundary where you have the mother's blood here and I'll say this is the baby's blood right over here. You have the gas exchange of the oxygen going through that boundary.

And then, of course, the release of the carbon dioxide going the other way. This environment, where the baby's red blood cells have to bind to the oxygen, is a relatively low oxygen environment compared to, say, our lungs. This is because it has oxygenated and deoxygenated blood mixing in that same place, and it does not have direct access to, say, the lungs.

So, in this low oxygen environment, the fetal hemoglobin molecules have to be really, really, really good at binding to oxygen. We can see that from this diagram right over here, where the horizontal axis is the partial pressure of oxygen and the vertical axis is how saturated with oxygen these different hemoglobin molecules can become. You can see that the fetal hemoglobin, which is depicted by this blue curve, gets 50% saturated at a lower partial pressure of oxygen than the adult hemoglobin.

So, one way to think about it is that it is stickier. It binds with that oxygen; it can pull that oxygen out of the blood far better. This makes sense for the environment that the fetus is in, but once it comes out of the mother's womb, it doesn't need that stickiness. There are some drawbacks to that stickiness as well because it makes it hard for that oxygen to go into as many of the body's tissues. So that's why you have this transition from hemoglobin F to hemoglobin A.

And it's not just hemoglobin where we see this molecular variation. Plants and other organisms that conduct photosynthesis contain multiple types of chlorophyll. Remember, chlorophyll is a very important molecule in capturing light energy, which can then be used to help synthesize carbohydrates in things like plants.

Here we see how two different chlorophyll molecules, both that would be found in plants, absorb light of different frequencies. You can see chlorophyll A is really good at absorbing the violet bordering on blue light, while chlorophyll B is better at the blue-green type of light. Then you have another peak here where chlorophyll B is better at absorbing an orangish-red, while chlorophyll A is better at absorbing a, I guess you could say, a red bordering on infrared wavelength.

The reason why this is valuable is that the light that the plant gets, especially at different times of day and at different times of year, is going to have different wavelengths. So, this just lets the plant capture more energy that it can use in photosynthesis.

These are just two examples of molecular variation. In our cellular membranes, there are multiple types of phospholipids that are forming the phospholipid bilayer, and those multiple types are to have different levels of how fluid they are at different temperatures. There are animal studies that show that the variations change depending on the conditions.

For example, a cold-blooded animal might have more of the fluid phospholipids when it is very cold so that the membranes don't become overly rigid. But I will leave you there. This is just to appreciate this idea that we have all sorts of molecular variation inside organisms' cells, and it allows those organisms to better adapt to their environment or different stages of their development.