Cyclic vs. Non-Cyclic Photophosphorylation: Study with Video Lessons, Practice Problems & Examples

Photophosphorylation occurs during light reactions in photosynthesis, with two pathways: noncyclic and cyclic. Noncyclic photophosphorylation produces both ATP and NADPH through a linear electron flow involving photosystems II and I, essential for the Calvin cycle. In contrast, cyclic photophosphorylation generates only ATP, cycling electrons back to the electron transport chain via photosystem I when NADPH is not needed. The choice between these pathways depends on the cell's requirements for energy and reducing power, highlighting the dynamic nature of photosynthetic processes.

Cyclic vs. Non-Cyclic Photophosphorylation

In this video, we're going to begin our lesson on the cyclic versus non-cyclic photophosphorylation pathways that are possible during the light reactions. Now the term photophosphorylation, as its name implies, is going to be the phosphorylation of ADP into ATP, creating energy for the cell, and it does this phosphorylation by using solar energy or the energy of sunlight, which is where the photo root originates from since photo means light. Now there are actually 2 types of photophosphorylation pathways that are possible during the light reactions. And so the first possibility for a photophosphorylation pathway is the non-cyclic photophosphorylation pathway. And the second possible pathway is the cyclic photophosphorylation pathway. Now as we move forward in our course we're first going to talk about the non-cyclic photophosphorylation pathway because it's more consistent with what we've already talked about in some of our previous lesson videos. But then after we talk about the non-cyclic photophosphorylation pathway, then we'll move on to talk about the cyclic photophosphorylation pathway in its own separate video. Now what ends up determining which pathway the cell ends up using, is going to be the cell's specific requirement of reducing power in the form of NADPH as well as the cell's specific requirement for ATP. And so, really, we'll be able to talk more about this specific idea right here as we move forward in our course and talk about each of these pathways. And so that concludes our brief introduction to the cyclic and noncyclic photophosphorylation pathways, and I'll see you in our next lesson video to talk about the noncyclic photophosphorylation pathway. So I'll see you all there.

Non-Cyclic Photophosphorylation

This video, we're going to talk more about the noncyclic photophosphorylation pathway that is a possibility during the light reactions. And so, really this noncyclic photophosphorylation pathway is the normal light reactions that we've already covered in our previous lesson videos. And so when the cell requires the production of both ATP and NADPH, then the cell is going to use the noncyclic photophosphorylation pathway. And so, the noncyclic photophosphorylation pathway will produce both ATP and NADPH. And this is going to be different when we talk about the cyclic photophosphorylation pathway. And so this noncyclic photophosphorylation pathway, as its name implies with the noncyclic part, is going to have a linear pathway or a linear path of electrons where the electrons are going to be passing through photosystem 2 or PS2 and photosystem 1 in a linear fashion in order to make, once again, both ATP and NADPH. And so, the electrons will be taking a linear path. And so once again, as I already stated, the noncyclic photophosphorylation pathway is pretty much the normal pathway of the light reactions as we already covered it in our previous lesson video. And so it is going to produce ATP and NADPH, and that ATP and NADPH are going to be used in the Calvin cycle. And we'll talk more about the Calvin Cycle later in our course.

Now if we take a look at our image down below, notice we're showing you an image of the noncyclic photophosphorylation pathway. And so notice that the electrons, represented in blue here, are going to be stripped away from the oxygen, and the electrons will go to photosystem 2. Then the electrons will go from photosystem 2 through the electron transport chain back to photosystem 1, and then they will continue forward to NADPH here. And so, notice that the electrons are taking somewhat of a linear path. They start here, and then the electrons are going in this direction as you see, and they end up on NADPH. And so, it is a noncyclic path. It does not come back and cycle into a cyclic pathway. These electrons are taking a linear path and so this is why it's called the noncyclic photophosphorylation pathway. And so, we know from our previous lesson videos that as the electrons are making their way through the electron transport chain, there is a large hydrogen ion gradient that's being built here within the thylakoid space, and this hydrogen ion gradient can be used to create ATP.

And so, the noncyclic photophosphorylation pathway, notice, as we mentioned above in our text, generates both NADPH and it also generates ATP. So it generates both of them. And so, the NADPH and ATP that are generated are going to be used in the Calvin cycle to power the Calvin cycle. And so this here concludes our brief introduction to the noncyclic photophosphorylation pathway, and we'll be able to compare this to the cyclic photophosphorylation pathway as we move forward in our course. So, I'll see you all in our next video.

The main sources of energy in photophosphorylation are sunlight and _________.

• A. Proton motive force.
• B. Inorganic phosphate (PO₄^3-)
• C. High-energy phosphate bond.
• D. CO₂ and H₂O.
• E. Chlorophylls.

Non-cyclic photophosphorylation is used to synthesize:

• A. ADP and NADP⁺
• B. ATP only.
• C. ADP and ATP.
• D. NADPH only.
• E. ATP and NADPH.

Cyclic Photophosphorylation

In this video, we're going to introduce the cyclic photophosphorylation pathway that is a possibility during the light reactions. And so when the cell only requires ATP production, but it does not require NADPH production, the cell will actually use the cyclic photophosphorylation pathway rather than using the noncyclic photophosphorylation pathway, which recall from our previous lesson videos will produce both ATP and NADPH. And this is because the cyclic photophosphorylation pathway, as its name implies, is going to include a cyclic path of electrons, and it's only going to be using photosystem 1 to make ATP. However, no NADPH will be made during the cyclic photophosphorylation pathway.

And so, what happens during the cyclic photophosphorylation pathway is that high energy electrons from photosystem 1 are actually going to be cycled back to the prior electron transport chain or ETC in order to continue generating a proton motive force. And, of course, the proton motive force is going to be used to produce more ATP. And so, ultimately, what we're saying is that the cyclic photophosphorylation pathway is going to have a cyclic path of electrons. It's only gonna be using photosystem 1, and it will only be making ATP. It will not be making NADPH.

And so, if we take a look at our image down below, notice we're showing you an image of the cyclic photophosphorylation pathway. And so notice that the electrons, you know, they will go from photosystem 2 over to the normal photosystem 1 here. However, once they reach photosystem 1, notice that the electrons are taking a cyclic pathway back to the previous electron transport chain. Okay? So, not that they are transported through this exact path that you see here. However, the electrons in photosystem 1 are eventually shipped back to a previous protein in the electron transport chain. And then the electron makes its way back to photosystem 1, and then it just continuously goes in this cyclic pathway.

And so when the electrons are taking this cyclic pathway here, that cycles from photosystem 1 back to the electron transport chain, back to photosystem 1, and so on, the electrons never are going to go to NADPH as long as they continue in this cyclic photophosphorylation pathway. And so it is possible for the cell to switch from cyclic photophosphorylation to the noncyclic photophosphorylation. But during this cyclic photophosphorylation, what happens is there is going to be a continuous generation of a proton motive force, a continuous generation of a hydrogen ion gradient, and that hydrogen ion gradient can continue to make ATP. And so only ATP is going to be made during the cyclic photophosphorylation pathway, and NADPH will not be made.

And so notice that we have this region and this region over here grayed out because they are not really used during the cyclic photophosphorylation pathway. And the regions that are colorful, including this region right here and this region right here, are colorful because these regions are being used by the cyclic photophosphorylation pathway. And so once again, what determines whether a cell will use the noncyclic pathway or the cyclic pathway is going to be the need for reducing power, how much NADPH does it need. If it does not need NADPH and it only needs ATP, then it's only gonna perform the cyclic photophosphorylation pathway. However, if it does need NADPH and ATP, it will use the noncyclic photophosphorylation pathway.

And so this here concludes our brief lesson on the cyclic photophosphorylation pathway, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

Photophosphorylation is:

• A. The phosphorylation of ADP to ATP using light energy of photosynthesis.
• B. The reduction of NADP+ to NADPH using light energy of photosynthesis.
• C. The phosphorylation of glucose to glusocse-6-phosphate during glycolysis.
• D. The oxidation of water during the light reactions of photosynthesis.

What is the important difference between cyclic & non-cyclic photosynthesis?

• A. Cyclic photosynthesis generates NADPH but not ATP.
• B. Cyclic photosynthesis generates ATP but not NADPH.
• C. Cyclic photosynthesis generates ADP but not NADPH.
• D. Cyclic photosynthesis generates ATP but not NAD⁺.
• E. Cyclic photosynthesis generates NADP⁺ but not ATP.

Here’s what students ask on this topic:

What is the difference between cyclic and non-cyclic photophosphorylation?

The main difference between cyclic and non-cyclic photophosphorylation lies in the electron flow and the products generated. Non-cyclic photophosphorylation involves a linear electron flow through photosystems II and I, producing both ATP and NADPH. This pathway is essential for the Calvin cycle. In contrast, cyclic photophosphorylation involves only photosystem I, where electrons cycle back to the electron transport chain, generating ATP but not NADPH. The choice between these pathways depends on the cell's need for ATP and NADPH, with cyclic photophosphorylation being used when only ATP is required.

Why does cyclic photophosphorylation only produce ATP?

Cyclic photophosphorylation only produces ATP because it involves a cyclic flow of electrons that return to photosystem I after passing through the electron transport chain. This cyclic electron flow generates a proton gradient across the thylakoid membrane, which drives ATP synthesis via ATP synthase. However, since the electrons do not move to NADP⁺ to form NADPH, no NADPH is produced. This pathway is utilized when the cell requires additional ATP but not NADPH.

What role do photosystems I and II play in non-cyclic photophosphorylation?

In non-cyclic photophosphorylation, photosystem II (PSII) and photosystem I (PSI) play crucial roles in the linear flow of electrons. PSII absorbs light energy, which excites electrons and splits water molecules, releasing oxygen. The high-energy electrons travel through the electron transport chain to PSI, creating a proton gradient that drives ATP synthesis. PSI then re-energizes the electrons with light energy, allowing them to reduce NADP⁺ to NADPH. This process produces both ATP and NADPH, essential for the Calvin cycle.

How does the cell decide between cyclic and non-cyclic photophosphorylation?

The cell decides between cyclic and non-cyclic photophosphorylation based on its specific needs for ATP and NADPH. If the cell requires both ATP and NADPH, it will use non-cyclic photophosphorylation, which involves a linear electron flow through photosystems II and I. However, if the cell only needs ATP and not NADPH, it will switch to cyclic photophosphorylation, where electrons cycle back to photosystem I, generating ATP without producing NADPH. This dynamic regulation ensures the cell meets its energy and reducing power requirements efficiently.

What is the significance of the proton gradient in photophosphorylation?

The proton gradient in photophosphorylation is crucial for ATP synthesis. During both cyclic and non-cyclic photophosphorylation, the movement of electrons through the electron transport chain pumps protons (H⁺) into the thylakoid lumen, creating a high concentration of protons inside the thylakoid. This proton gradient generates a proton motive force, which drives protons back into the stroma through ATP synthase. The energy released during this process is used to convert ADP and inorganic phosphate (P_i) into ATP, providing the cell with the energy needed for various metabolic processes.