Light dependent reactions actors

In a previous video, we gave an overview of the light-dependent reactions, which are essentially occurring across the thylakoid membranes. Right, and we zoomed in on one, and we saw, okay, we have some energy from light exciting the electrons within the chlorophyll pair, that P680 chlorophyll a pair. That energized electron will then be transferred from one molecule to another, and as it does so, it'll go to lower and lower energy states. The released energy, some of it will be used to transfer hydrogen protons across the membrane. Eventually, that electron will make its way to photosystem I, where it can get excited again.

If we think of it as the same electron, it doesn't necessarily have to be the exact same electron, but we can think of that same electron as being excited again by light energy. Then it can, once again, go to lower and lower energy states. This time, it's going to be used to reduce NADP+ to NADPH. Now, NADPH itself is an input into the Calvin cycle, but ATP is another input we need for the Calvin cycle. The way that we produce ATP is that hydrogen ion concentration increases on the inside due to it being essentially pumped across the membrane, as well as the leftover hydrogen ions from the water after it's stripped of electrons to replace that originally excited electron in that P680 chlorophyll pair.

Well, that increased hydrogen ion concentration can be used to drive ATP synthase, which creates ATP from phosphate and ADP. We saw it. We saw that over here without seeing the different components. You get light exciting the electron; the electron goes to lower and lower energy states as it does so. As it's going from photosystem II to photosystem I, some of that energy is being used to pump hydrogen ions into the thylakoid lumen. Then the electron gets excited again, and as it gets transferred and goes to lower and lower energy states, it can be used to produce NADPH, where once again its electrons are still at a fairly high energy state.

So it's a strong reducing agent, and that's why it's valuable in the Calvin cycle. That energy from acting as a strong reducing agent can be used to help in the creation or the eventual creation of the sugar. Once again, where is that electron? Once it gives it away, how does it get replaced? Well, it snags it from the water.

What I have here is a more detailed diagram that labels some of the actors. The important thing is really what we just covered and what we covered in more detail in the previous video: the conceptual idea of what's happening in the light-dependent reactions. A lot of times in your biology class or in your biology book, you'll see discussions of things like a cytochrome complex and plastoquinone and things like that. I want you to look at that right now so that you're not intimidated when you see it and that you see that these are just the actors that we talked about.

Right over here, this is photosystem II. I give credit for where this image comes from—it's modified from the light-dependent reactions of photosynthesis, Figure 8 by Open Stacks College. But this right over here, we see the light interacting. The way it's depicted here is not directly with the chlorophyll pair within photosystem II, that P680 chlorophyll a pair. We see it acting on some of these neighboring molecules as their electrons get excited and then go to lower energy levels. That energy can be used to excite neighboring electrons. This kind of keeps happening; that energy gets transferred eventually to excite the electron in that P680 pair.

That electron is the first electron acceptor. You'll see this sometimes spoken of in your biology textbooks as “ferredoxin.” Then it can transfer the electron to plastoquinone, and that plastoquinone is interacting in this cytochrome complex, which transfers the electron from plastoquinone to plastocyanin. As it's doing it, you see the hydrogen ions being transferred from the outside of the thylakoid to the inside of the thylakoid, which is exactly what we've been talking about.

Then as we go to photosystem I, that electron can be transferred from plastocyanin to the chlorophyll pair, the P700 chlorophyll. That can get excited again. Once again, it doesn't have to be the light directly exciting it; it can be exciting other molecules within photosystem I. But that energy eventually gets transferred to that chlorophyll, excites its electron, and then it goes from one molecule to another.

Eventually, it goes to ferredoxin, which is being used in conjunction—it’s one of the actors along that the enzyme NADP+ reductase needs along with NADP+. So it's essentially just reducing NADP+ along with this electron that's on the ferredoxin to produce NADPH. Once again, what's going on here? Well, this is the ATP synthase that is using all this increased hydrogen ion concentration on the inside of the thylakoid to power the—you could say the motor. The ATP synthase is the motor that is powered as these hydrogen ions go down their concentration gradient. That energy is used to jam the phosphate onto the ADP to produce ATP.

So, I've essentially said the same thing two or three times already in the last two or three videos, but I'm doing it because when you first see this, it seems very, very intimidating and very, very complex. And it is complex. Frankly, it's amazing that things like this are happening on the plant that I'm looking at outside of my window right now. It kind of boggles my mind that this kind of thing is happening in nature. There are bits and pieces of it that aren't fully understood yet and still need to be discovered. But at the same time, the general idea is not as intimidating as these diagrams appear.

Hopefully, you find this as inspiring as I do and not as intimidating as what some of these words might make you feel initially.