Conceptual overview of light dependent reactions

We've seen in previous videos that photosynthesis can be broken down into the light dependent reactions and the Calvin cycle. The light dependent reactions is where we take light as an input along with water, and we'll see the water is actually a source of electrons. We can use that to store energy in the form of ATP and NADPH, and as a byproduct, we produce molecular oxygen, which is very important for us to breathe.

Then, that ATP and that NADPH can be used in the Calvin cycle, along with carbon dioxide, to actually synthesize sugar. What we're going to focus on in this video are the light dependent reactions. How does this process right here work? To help us think about this, we are going to zoom in onto a thylakoid membrane. This is a thylakoid right over here sitting inside of the chloroplast, and if we zoom in on its membrane, we see it's a phospholipid bilayer, like many membranes that we see in biology.

At first glance, this might seem like a very complex diagram, and that's because it is a complex diagram. You will often see things like this in your biology textbooks, and it can be very intimidating. These proteins and molecules and complexes have very complicated sounding names. But the general idea of what's going on is you'll hopefully find a pretty straightforward explanation: you have the energy from light photons from light that are going to either directly or indirectly excite electrons.

Those excited electrons are in a high energy state, and they're going to be transferred from one molecule to another, going to lower energy states. That's what allows those transfers to be spontaneous for them to actually occur. They are going from a higher energy state to a lower energy state, and the electrons are getting more and more and more comfortable. Some of that energy that's released as the electron goes from a higher energy state to a lower energy state is used to pump hydrogen ions across the membrane from the outside of the membrane in the stroma to the inside of the membrane within the thylakoid lumen.

So you are building a hydrogen ion concentration gradient, where you have a higher concentration inside than you have outside. This, by itself, this concentration gradient, as we'll see, can be used to fuel the production of ATP by ATP synthase. Those hydrogen ions want to get back out; they want to go down their concentration gradient. As they go back out through the ATP synthase, it essentially turns that motor that can jam the phosphate group onto ADP to produce ATP.

One way to think about it is that this process is producing a hydrogen ion gradient, which is then being used to produce the actual ATP. Now, the electrons going from a high energy state to a lower energy state in this part of the light dependent reactions, that by itself isn't the only thing that is contributing to the hydrogen ion concentration gradient. Once that electron gets donated, you might ask, "Well, how does it get replaced?"

Well, the thing that's doing the donating, the thing that eventually gets excited and donates that electron, it's a chlorophyll a variation called P680. P680 is referring to the "P" which stands for pigment, and 680 stands for 680 nanometers, the wavelength of light that it absorbs best. When it gets excited, you'll see the notation often of P680*. That's when it has an excited electron.

After it gives away its electron, it becomes P680+, which we could call P680 with a positive charge or a P680 ion. This is actually a very strong oxidizing agent, one of the strongest, if not the strongest we know in biological systems. It really likes to grab electrons from other things, and the thing that is around that it can grab electrons from is actually water.

This is such a strong oxidizing agent that it can essentially oxidize the oxygen in water. Oxygen, as itself, I mean oxidizing is named after oxygen because oxygen is such a strong electronegative element; it's the thing that's normally doing the oxidizing. So anyway, it grabs its electrons. Once P680+ grabs an electron from water, the water essentially falls apart, so you're left just with the oxygen and then the hydrogen ions.

Those hydrogen ions also contribute to the increased hydrogen ion concentration on the inside, and this is where we get the oxygen byproduct. Here, we have one-half of O2. If you do this twice, you're going to have a molecular oxygen. So, so far, I've talked about how the oxygen gets produced and how the ATP gets produced. What about the NADPH?

Well, we started our process in Photosystem II. You might ask, "Why is it called Photosystem II if that's where we start?" Well, that's because it's the second photosystem to be discovered. You might ask, "What is a photosystem?" Well, these photosystems and complexes are combinations of proteins and molecules. A photosystem in particular has chlorophyll and variations of chlorophyll and pigment molecules that are responsive to light, that have electrons that can get excited by light.

They can transfer that energy back down to the P680 chlorophyll a pair, which then can have its electron excited and then it can give that to an acceptor molecule, which can go to lower energy states and pump those hydrogen ions out. But that's not the entire light dependent reactions. That electron can eventually make its way over to Photosystem I.

Why is it called Photosystem I? Well, because it's the first one that was discovered. In Photosystem I, there's another chlorophyll a pair called P700, and that's because it optimally absorbs light of a wavelength of 700 nanometers. You have something similar that happens: light can either directly or indirectly excite its electron. That electron, as it goes to a lower energy level, goes from one molecule to another, can be used to reduce NAD+ into NADPH.

So that's where the NADPH comes from. Then, once again, once P700 has given its electron, it wants an electron. It can get that from the electron that has been going from lower to lower and lower energy states, essentially the electron that has been making its way from Photosystem II. That's why you'll often see these diagrams: light comes in, the electron gets energized, it gets excited, and goes to lower energy states.

As it's doing that, it's being transferred from one molecule to another, facilitated by enzymes. Part of that energy is being used to transfer hydrogen ions into the thylakoid lumen. In Photosystem I, you have another excitation event. The thing that got excited can grab that electron that went to lower energy states, and its excited electron can once again be transferred from one molecule to another in order to fuel or provide the energy for NADP+ being converted into NADPH.

Once again, the whole idea of the hydrogen ion concentration increasing can fuel ATP synthase, which allows us to jam a phosphate onto ADP to produce ATP. So that is where we actually get all of these things, and the by-product, of course, is our oxygen. If you wanted to see that same idea but think from an energetic point of view without all the complexity of seeing the physical components involved, you see it right over here: light energy comes, excites the electrons.

Once P680 has given that electron away, it wants an electron really badly and gets it from the water. Then, as that electron goes to lower and lower energy states, it can eventually be grabbed by P700, which has given away its electron. That electron, excited at P700 by more light energy, can be transferred from one molecule to another to fuel the creation of NADPH. This part, this phase, as the energy goes from a high energy state to as the electron goes from a higher energy state to a lower energy state, fuels the pumping of hydrogen protons into the actual thylakoid.