ATP synthase | Cellular energetics | AP Biology | Khan Academy

In this video, we're going to talk about what is arguably my favorite enzyme, and that is ATP synthase. You might be able to predict from its name what it does: it synthesizes ATP.

Now, you've probably seen it before. We saw it when we looked at respiration, or you will see it when you look at respiration, which is going on in most of the cells of your body. You also see it when you study photosynthesis. The general thing that it does is it sits across a phospholipid membrane, and through other processes, you will have hydrogen ion concentration increase on one side of the membrane.

You have a higher hydrogen ion concentration on one side than on the other side. You still might have a few over here, and a hydrogen ion is essentially a proton. On this side of the membrane, it'll be more positive, so there will be an electromotive force to go to the other side. Additionally, you just have a higher concentration, so there's a chemical gradient, a concentration gradient, where if there's some way for these protons to get to this side, they would want to get there.

So, there's an electrochemical gradient that they would want to go down, and ATP synthase provides a channel for those protons. As those protons travel through the ATP synthase, they turn this part of it, which drives this axle; this axle nudges these parts of the protein so that they jam together an ADP with a phosphate group to produce ATP.

So down here, going into this part of the complex, you'll have an ADP and a phosphate group. Then that rotation force, that's provided by that electrochemical gradient, produces our ATP. That's going to be the case both in respiration, which occurs in the mitochondria, and in photosynthesis, which occurs in chloroplasts.

Now, there are a few differences. In mitochondria, the hydrogen ions, these protons, the concentration builds up in the intermembrane space right over here because of the electron transport chain, and we study that in other videos. Then, the protons travel through the ATP synthase. You can see a little mini version right over here; you can imagine that what we see really big is a blown-up version of this part of the mitochondria. Of course, this is not to scale.

In the case of a mitochondrion, this would be the inner membrane. Right over here would be the intermembrane space between the inner and the outer membrane. The intermembrane space, and right over here would be the matrix of the mitochondria. As the protons go through, they're able to produce ATP in the matrix.

Now, in chloroplasts, the hydrogen protons build up inside the thylakoids, which are these parts of the chloroplast. That space inside the thylakoid is often called the thylakoid space, sometimes called the lumen. That proton buildup inside the thylakoids happens because of the light reactions, the first phase of photosynthesis. But then, those protons will travel through the thylakoid membrane to this area, which is known as the stroma in chloroplasts, and they produce the ATP in the stroma.

But then the ATP is used in the second phase of photosynthesis to synthesize carbohydrates, which you could use as one of the end products of photosynthesis.

So, the big takeaway of this video is, one: ATP synthase is incredibly cool. If you look up on the internet, you can find some simulations that show ATP synthase and how it acts like a motor to jam the phosphate group to the ADP to produce ATP.

ATP synthase in mitochondria and chloroplasts are remarkably similar, although they sit in different parts of these organelles. The ATP in mitochondria can be viewed as the end product of respiration, while the ATP produced in chloroplasts is an intermediary store of energy, which is then used to synthesize carbohydrates.