Photophosphorylation 2: Study with Video Lessons, Practice Problems & Examples

Photosynthesis occurs in chloroplasts, where light-dependent reactions produce ATP and NADPH. The stroma contains thylakoids, which house photosystems that capture light energy through resonance energy transfer. This energy excites electrons in the reaction center, leading to their ejection and transport through an electron transport chain, creating a proton gradient. Electrons can reduce NADP⁺ to NADPH or contribute to ATP synthesis. The overall process is essential for autotrophs, facilitating the Calvin cycle for carbohydrate production.

$2^H(2HO)+NADP+\to 2^{NADPH}+H++O₂$

Photophosphorylation 2

Photosynthesis occurs in chloroplasts, and these are organelles that have electron transport chains and ATP synthesis and convoluted inner structures very similar to mitochondria. This is also the organelle in which the Calvin cycle occurs, where carbon dioxide is fixated into carbohydrates, but we're not going to get into any of that. Now, we should talk a little bit about chloroplast anatomy. The goopy fluid that fills chloroplasts is called the stroma, and these structures inside the chloroplast, these infoldings, are called thylakoids, and thylakoids, as you can see over here, have a lumen inside. So there is an internal space we're going to cover more on this subject momentarily.

The reactions that we're going to be covering are called the light-dependent reactions, and that's because they require light energy to proceed with the Calvin cycle, sometimes called the light-independent reactions because they don't actually need sunlight energy to go. Sometimes they're also called the dark reactions, but that's kind of misleading because they do occur in the light as well. But anyways, we are going to focus on the light reactions, and in light reactions, what's going to happen is we're going to produce ATP. That is one of the main things. We're also going to be producing NADPH. And remember, NADPH is very similar to NADH. It's an electron carrier. And NADPH is actually what's used in the Calvin Cycle, to generate carbohydrates, but we're not going to get into that. So that gets used up in the Calvin cycle and comes back as NADP⁺ to the light reactions. The light reactions will actually see split water and take electrons from it and oxidize the oxygens to molecular oxygen, which will be released from these reactions, and that is the oxygen that is released from plants that we breathe. Pretty cool, right? You can see the equation for the light reactions right here. You have 2 H2O, 2 NADP⁺ and you form as should be 2 NADPH, 2H⁺ O₂.

Now, the photosystems that we're going to be talking about are basically complexes of proteins and photopigments and other organic molecules embedded in the thylakoid membrane. Photosystems can basically be broken down into 2 parts. You have your light harvesting complex, which is this outer portion. And then you have the reaction center, which is this interior portion. So this whole thing here is a photosystem.

The light harvesting complex is basically an antenna. It's chock full of tons of chlorophyll, carotenoids, and other photopigments. Its job is to pick up light energy, get it, and as soon as it gets it, it wants to transfer it and move it through the photosystem or through the light harvesting complex to the reaction center. The way this occurs is a very interesting thing. It's basically an energy transfer, a very special type of energy transfer called resonance energy transfer. And essentially what happens is, an electron in one photopigment gets excited by light, a photon hits it, it bounces up into this excited state, and when it relaxes, it actually excites its neighbor. So its neighbor gets excited, and then that electron relaxes and excites its neighbor, and that's how this energy is relayed around the light harvesting complex until it is transferred to the reaction center, to these special pigments in the reaction center.

Now the reaction center contains chlorophylls, cytochromes, quinones, and these things called pheophytin, which are basically chlorophylls without magnesium. They're all electron carriers, as I'm sure you know. And essentially, once the energy from the light harvesting complex hits the reaction center, it will excite an electron in the reaction center, excite an electron from chlorophyll in the reaction center so much that it actually gets ejected. So I almost like to think of the photosystem like, you know, a magnifying glass. If you've ever seen a magnifying glass focus sunlight energy, right? Focus sunlight energy and then you can burn something with it. That's kind of what's going on here. The light harvesting complex focuses the energy onto the reaction center, and that allows the reaction center to excite its electron enough to actually eject it. Not just excite it but eject it so that it gets picked up by the electron carriers and taken around in a manner that's very similar to the electron transport chain. It is an electron transport chain, just a different one than the one we were just looking at in the mitochondria.

Now, the electrons can actually come back to the reaction center. The energy will get passed around, you'll excite that electron, and it will move through an electron transport chain into this cytochrome complex, and we'll talk all about this in more detail momentarily. Moving through the cytochrome complex is actually what causes that protein complex to pump protons and create a proton gradient. This process will, as we'll see, play out in a fashion where the electrons can be delivered to the reaction center of another photosystem, right? So once a photosystem loses its electrons, it needs them replaced. They can be delivered to the reaction center of another photosystem and ultimately be given to NADP⁺ reductase to turn NADP⁺ into NADPH. Right? And then that's going to go off to the Calvin Cycle. So that's one thing that can happen. But what can also happen is instead of delivering them to NADP reductase, this molecule, or this protein rather, called ferredoxin, can actually shuttle it in a cyclic fashion. So essentially, electrons can be used just to create the proton gradient that's going to ultimately power ATP synthase, or the electrons can be used to reduce NADP⁺ to NADPH. And I have the P in parentheses here because in some circumstances, it will be NAD⁺, and in others NADP⁺ for various photosynthetic organisms. For plants, it's NADP, NADP⁺.

Let's flip the page and take a look at this process in more detail.

Here’s what students ask on this topic:

What is the role of chloroplasts in photosynthesis?

Chloroplasts are organelles in plant cells where photosynthesis occurs. They contain thylakoids, which house photosystems that capture light energy. This energy is used in light-dependent reactions to produce ATP and NADPH. The stroma, the fluid inside chloroplasts, is where the Calvin cycle takes place, converting carbon dioxide into carbohydrates. Chloroplasts are essential for converting light energy into chemical energy, which is then used to fuel various cellular processes.

How do light-dependent reactions produce ATP and NADPH?

Light-dependent reactions occur in the thylakoid membranes of chloroplasts. Photosystems capture light energy through resonance energy transfer, exciting electrons in the reaction center. These electrons are ejected and transported through an electron transport chain, creating a proton gradient. This gradient powers ATP synthase to produce ATP. Electrons can also reduce NADP⁺ to NADPH, which is used in the Calvin cycle for carbohydrate production.

What is the function of the thylakoid membrane in photosynthesis?

The thylakoid membrane in chloroplasts is crucial for photosynthesis. It houses photosystems that capture light energy and initiate the light-dependent reactions. These reactions involve the transport of excited electrons through an electron transport chain, creating a proton gradient that drives ATP synthesis. The thylakoid membrane also facilitates the reduction of NADP⁺ to NADPH, which is essential for the Calvin cycle.

What is resonance energy transfer in photosynthesis?

Resonance energy transfer is a process in photosynthesis where energy is transferred between photopigments in the light-harvesting complex of a photosystem. When a photon excites an electron in one pigment, the energy is transferred to a neighboring pigment, exciting its electron. This process continues until the energy reaches the reaction center, where it excites an electron enough to be ejected and used in the electron transport chain.

How is the proton gradient used to produce ATP in photosynthesis?

In photosynthesis, the proton gradient is created by the movement of electrons through the electron transport chain in the thylakoid membrane. As electrons move, protons are pumped from the stroma into the thylakoid lumen, creating a high concentration of protons inside the lumen. This gradient drives protons back into the stroma through ATP synthase, a protein complex that synthesizes ATP from ADP and inorganic phosphate.