Photophosphorylation 3 - Online Tutor, Practice Problems & Exam Prep

Photosystem II initiates the light reactions of photosynthesis by absorbing light at 680 nm, splitting water to replace lost electrons, and generating oxygen and protons. Electrons flow through the cytochrome B6F complex, creating a proton motive force that powers ATP synthase. Photosystem I, absorbing light at 700 nm, receives electrons from plastocyanin, leading to NADPH production. Non-cyclic electron flow produces both ATP and NADPH, while cyclic flow generates only ATP. This process, known as photophosphorylation, parallels oxidative phosphorylation, utilizing sunlight as the energy source.

Photophosphorylation 3

Our story here actually begins with photosystem 2, which might seem odd, but the naming convention has to do with the fact that photosystem 1 was discovered before photosystem 2. So, photosystem 2 has a chlorophyll in its reaction center called P680, and that basically means this chlorophyll absorbs light at 680 nanometers really well. What's going to happen in photosystem 2 is when it loses electrons to the electron acceptors from the reaction center, it's going to split water to replace those lost electrons. So it's not going to have them returned by the electron carriers like we mentioned before. And that's because photosystem 2's main job is to provide the electrons for what's called non-cyclic or linear electron flow. So it's going to split water into electrons. Let me do like this. We're breaking up water and we're turning it into electrons, and we are giving off oxygen and protons as a result of this. And just to be clear, the membrane is not drawn here, but there is a membrane, and on this side is the stroma. What's important to remember is that the cytochrome complex, its job is to pump protons through to this side. So, the fact that the splitting of water also generates protons means that the proton gradient.

Now, splitting of water actually uses manganese in the protein, and it actually uses all 5 redox states of manganese. So pretty cool. Just one of those rare instances where manganese is used in biology. And after the electrons leave photosystem 2, they go to this cytochrome complex, which is cytochrome B6f. It contains hemes, iron-sulfur proteins, beta-carotene, and it acts as a proton pump that receives electrons from this electron carrier right here called plastoquinone, abbreviated PQ as you see, and it's going to act as a proton pump. Let me just clear up this space and bring through protons. Now, the electrons will move through this cytochrome complex, first to the cytochrome b6 portion, then to the cytochrome f portion, and finally to a copper in plastocyanin. Plastocyanin is going to deliver its electrons to photosystem 1. So here is photosystem 2, and here we have photosystem 1.

The reaction center of photosystem 1 contains chlorophyll P700, right? So I'm sure you can guess what that name's significance is. It absorbs light best at 700 nanometers. And what's important to realize about all of this is that the original wavelength we started at was 680. Remember, the shorter the wavelength, the higher energy the photon, right? So 680, and now we're at 700. So, slightly lower wavelength, slightly less energy from these electrons, so to speak. That is, in essence, what this figure is trying to show: the energy changes along this. If you imagine like a Y-axis along here, right, this is showing the energy changes of the electrons, roughly as they progress through this system. So when they get to the reaction center of photosystem 1, remember that photosystem 1 is also absorbing sunlight. It is needing to replace the electrons it loses to the electron acceptors. So that is what photosystem 2's electrons do. They come in. They replace the electrons that photosystem 1 loses and travel from the reaction center to the electron acceptors to this electron carrier ferredoxin, which will either bring them to NADP+ reductase, which is going to ultimately form NADPH by reducing NADP+ or ferredoxin can also bring these electrons back to the cytochrome complex and drop them off there, where they will move through the complex again, pumping more protons through and they will be picked up by plastocyanin and delivered back to photosystem 1. So that is the difference between cyclic and non-cyclic electron flow essentially. In cyclic electron flow, we basically have this cycle from photosystem 1 to ferredoxin to the cytochrome complex to plastocyanin, and then back to photosystem 1 again. This cannot generate any NADP+, right? It only produces, and this is important, a proton motive force, so it only leads to the generation of ATP. I'm going to write that in:

Only produces ATP.

Whereas non-cyclic electron flow produces ATP and NADPH.

Alright. So the ultimate purpose of creating this proton gradient, just like in oxidative phosphorylation, is to power ATP synthase. So here's kind of an overview of what's going on in this whole process. We have photosystem 2, photosystem 1, and electron flow from photosystem 2 goes this way. And we can also have cyclic electron flow, which will cycle like that. And this builds a proton gradient, which, you can see here, we have a proton motive force right there. That is going to power ATP synthase. Just to be clear, this is the lumen, this is the stroma, and this is the stroma, right. So, you know, we're just seeing a cross-section, but this will, you know, this membrane will have edges to it, so to speak. This is a contained area in the lumen with the stroma surrounding it. And this whole process is called photophosphorylation, and you can see that it's very similar to oxidative phosphorylation. The difference being that sunlight energy rather than the oxidation of nutrients is providing the energy source ultimately for ATP synthase.

So, this is sort of the other big ATP generation process in addition to oxidative. Now that's all I have in terms of concept videos for this review. So let's turn over to practice problems.

Here’s what students ask on this topic:

What is the role of Photosystem II in photophosphorylation?

Photosystem II (PSII) plays a crucial role in the light reactions of photosynthesis. It absorbs light at 680 nm, which excites electrons in its reaction center chlorophyll, P680. These excited electrons are transferred to electron acceptors, initiating the electron transport chain. To replace the lost electrons, PSII splits water molecules, a process known as photolysis, producing oxygen, protons, and electrons. The protons contribute to the proton gradient across the thylakoid membrane, which is essential for ATP synthesis. Thus, PSII is vital for generating the initial electron flow and maintaining the proton motive force required for ATP production.

How does the cytochrome B6F complex contribute to ATP synthesis in photophosphorylation?

The cytochrome B6F complex is a key component in the electron transport chain of photophosphorylation. It receives electrons from plastoquinone (PQ) and facilitates their transfer to plastocyanin (PC). During this process, the complex acts as a proton pump, moving protons from the stroma into the thylakoid lumen. This proton translocation contributes to the creation of a proton gradient across the thylakoid membrane. The resulting proton motive force drives ATP synthase, an enzyme that synthesizes ATP from ADP and inorganic phosphate. Therefore, the cytochrome B6F complex is essential for coupling electron transport to ATP production.

What is the difference between cyclic and non-cyclic electron flow in photophosphorylation?

In photophosphorylation, cyclic and non-cyclic electron flows are two pathways for electron transport. Non-cyclic electron flow involves both Photosystem II (PSII) and Photosystem I (PSI). Electrons move from PSII to PSI, ultimately reducing NADP⁺ to NADPH and generating ATP. This pathway produces both ATP and NADPH. In contrast, cyclic electron flow involves only PSI. Electrons are cycled back from ferredoxin to the cytochrome B6F complex and then to plastocyanin, returning to PSI. This process generates a proton gradient used solely for ATP production, without producing NADPH. Thus, cyclic flow only produces ATP, while non-cyclic flow produces both ATP and NADPH.

How does Photosystem I contribute to NADPH production?

Photosystem I (PSI) absorbs light at 700 nm, exciting electrons in its reaction center chlorophyll, P700. These high-energy electrons are transferred to electron acceptors and then to ferredoxin, a mobile electron carrier. Ferredoxin transfers the electrons to NADP⁺ reductase, an enzyme that catalyzes the reduction of NADP⁺ to NADPH. This process is crucial for the Calvin cycle, where NADPH is used as a reducing agent to convert carbon dioxide into glucose. Therefore, PSI is essential for the production of NADPH, which is necessary for the synthesis of carbohydrates in photosynthesis.

What is the significance of the proton gradient in photophosphorylation?

The proton gradient across the thylakoid membrane is a critical component of photophosphorylation. It is established by the movement of protons into the thylakoid lumen during the electron transport chain, particularly by the cytochrome B6F complex and the splitting of water by Photosystem II. This gradient creates a proton motive force, which drives ATP synthase, an enzyme that synthesizes ATP from ADP and inorganic phosphate. The energy stored in the proton gradient is thus converted into chemical energy in the form of ATP, which is essential for various cellular processes, including the Calvin cycle in photosynthesis.