Light Reactions of Photosynthesis - Online Tutor, Practice Problems & Exam Prep

The light reactions of photosynthesis occur in the thylakoids of chloroplasts, utilizing photons and water to produce ATP and NADPH while releasing oxygen as a byproduct. The process begins with photosystem II absorbing light, energizing electrons from water, which are then transferred through an electron transport chain to photosystem I. Here, the electrons are further energized and ultimately reduce NADP⁺ to form NADPH. The generated hydrogen ion gradient facilitates ATP production via chemiosmosis, linking the light reactions to the Calvin cycle for carbon fixation.

Light Reactions of Photosynthesis

In this video, we're going to begin our lesson on the light reactions of photosynthesis. The light reactions, as we already know from our previous lesson videos, is the first stage of photosynthesis, and it specifically occurs in the thylakoids of the chloroplast. Recall that the thylakoids are those green pancake-looking structures, and the light reactions occur in the thylakoid membrane as well as in the thylakoid space, or the space that's on the inside of the thylakoids. The light reactions synthesize chemical energy in the form of ATP and NADPH. This ATP and NADPH are going to be used to power the Calvin Cycle, which is the second stage of photosynthesis while producing oxygen gas or O₂ as a byproduct.

NADPH is really just another electron carrier that will carry or transport 2 energized electrons. NADPH functions as an electron taxicab. We covered electron carriers in our previous lesson videos when we talked about aerobic cellular respiration. In the image below, you'll notice that we've got an image showing photosynthesis. In the background, we have the chloroplast; notice that the regions we want to focus on in this image are colored and enlarged, and the regions that we don't want to focus on right now are grayed out and smaller. For instance, the Calvin cycle—we're going to talk about the Calvin cycle more in another video later in our course. For now, we want to focus on the first part of this reaction, the light reactions.

The light reactions take solar energy or sunlight in the form of photons and, using this solar energy along with water, generate chemical energy in the form of ATP and NADPH. It also creates oxygen gas as a byproduct, as we can see below. In terms of the reactants, the light reactions use solar energy or photons along with water, and in terms of the products, the light reactions create oxygen gas and chemical energy in the form of ATP and NADPH. This ATP and NADPH will be utilized in the Calvin cycle when we talk about the Calvin cycle later in our course. As you can see in this image, the NADPH are carrying 2 energized electrons.

In conclusion, this brief introduction to the light reactions and how they occur in the thylakoids within the chloroplast. In our next lesson video, we'll discuss the specific steps that occur in the light reaction. I'll see you all in our next video.

Light Reactions of Photosynthesis Example 1

Alright. So here we have an example problem that wants us to fill in the 2 blanks that we have here in each of these sentences using one of the 4 potential answer options down below. And so notice that the first sentence says the light reactions are powered by blank energy, and then the second sentence says in normal photosynthesis, the products of the light reactions are used to power blank. And so what you can see is when we look at these four answer options, options a and b both mention photorespiration, which we have not yet talked about in our course yet, but we will talk about this later in our course in a different video. But for now, because we have not yet introduced photorespiration, we should have been able to identify both option a and option b as being incorrect answer options here. Now moving on we have here option c and d, and really the difference is that option c says sunlight and option d says potential, for this first blank. And, it's true that in normal photosynthesis, the products of the light reactions are used to power the Calvin cycle. So this part here is going to be the same for the second blank. And really, again, what we're looking for is which one of these answer options is going to be best for the first blank. And of course, we know that photosynthesis is going to be driven by solar energy or sunlight, energy from sunlight. And so the correct answer here is going to be answer option c. And so we can go ahead and indicate that c here is the correct answer for this problem and that concludes this example, so I'll see you all in our next video.

Describe the primary function of the light reactions of photosynthesis.

a) Production of NADPH used in cellular respiration.

b) Use of ATP to make glucose.

c) Conversion of chemical energy to light energy.

d) Production of ATP and NADPH.

• A. Production of NADPH used in cellular respiration.
• B. Use of ATP to make glucose.
• C. Conversion of chemical energy to light energy.
• D. Production of ATP and NADPH.

Steps of the Light Reactions

In this video, we're going to talk about the steps of the light reactions. And recall from our previous lesson videos that the light reactions is the first stage of photosynthesis. And also recall from our previous lesson videos that the light reactions occur in the thylakoids within chloroplasts. And recall that the thylakoids are really just those green pancake looking structures that are within chloroplasts. And also recall from our previous lesson videos that if, within the thylakoid membranes, they contain photosystems. And recall from our previous lesson videos that photosystems are really just light harvesting complexes that are made up of pigments, proteins, and other molecules as well. And so because they are light harvesting complexes, they're gonna be needed for the light reactions.

And so if we take a look at our image down below, notice that we're zoomed into one of the thylakoids within a chloroplast because here we're showing you the thylakoid membrane. So all the way across this represents the thylakoid membrane or the membrane of one of those green pancake looking structures within the chloroplast. And then down below here what we have is the thylakoid space which represents the inside of the thylakoid or the inside of one of those green pancake looking structures. And then up above what we have is the stroma of the chloroplast, which is the space just outside of the thylakoids, the fluid filled space that fills up the innermost region of the chloroplast. And so, this is important to note as we make our way through the steps of the light reactions.

Now notice that here in our lesson, we've consolidated the steps of the light reaction down to 5 steps that are labeled a, b, c, d, and e. And notice that each of these 5 steps labeled with these letters corresponds with each of the letters that we have down below for the steps of the light reaction, so that's also important to keep in mind. Now also recall from our previous lesson videos that most plants contain 2 photosystems, and those photosystems are conveniently named photosystem 1 and photosystem 2. And it turns out that these photosystems were named just based on the order of their discovery. And so photosystem 1 was discovered first, so they called it photosystem 1. And then photosystem 2 was discovered later on, so they called it photosystem 2. But it turns out that later, they found out that photosystem 2 actually comes before photosystem 1 in the steps of the light reaction. And so it turns out that this entire process starts with photosystem 2, and then photosystem 1 comes later on. And so the very first step of the light reaction, step a, is gonna start with photosystem 2. And recall photosystems are light harvesting complexes so they can absorb photons of light and that's exactly what photosystem 2 does.

Photosystem 2 is going to absorb photons of light and those photons of light contain energy. And so the Photosystem 2, it's going to absorb the photons of light and transfer that energy to electrons in order to energize those electrons. And these electrons that are being energized they're actually coming from a water molecule. And so these electrons are being donated by a water molecule. And really this is the entire reason why, one of the biggest reasons why water is needed for the process of photosynthesis. And so it turns out that these water molecules are actually going to be split and oxidized in order to provide electrons to this process. And when the water molecules are split and oxidized in order to provide electrons, they're going to react to form oxygen gas or O₂. And really this is the oxygen gas that's associated with photosynthesis, producing oxygen.

And so let's take a look down below at our image at step a to get a better understanding of step a. And so once again, in the steps of the light reaction, we're zoomed into one of the thylakoid membrane. So notice that within the thylakoid membrane, we have these photosystems. We have one photosystem here and a second photosystem over here. But it turns out that photosystem 2 is actually the photosystem that comes first in this process. And so this is gonna be photosystem 2 right here. And photosystem 1. What we'll see is it comes later in the process in step c when we get there. But taking a look here, we can see photosystem 2 is right here. And in the first step of the light reaction step a, photosystem 2 is going to absorb photons of light.

And so you can see that these accessory pigments and the primary pigments are able to absorb photons of light and transfer the energy of those photons of light to electrons which are being represented here as these circles with these little negative symbols on the inside. And so, these electrons are being provided by water molecules and when water molecules get split in half, they are able to generate oxygen and this oxygen gas here is really the oxygen that's associated with photosynthesis and photosynthesis producing oxygen. It all comes from the splitting of this water molecule And also splitting the water molecule also provides the electrons that are going to be used in this process. And so once again, photosystem 2 is going to absorb photons of light and transfer those photons of light, the energy of those photons of light to electrons.

And so in step b, the second step, of the light reactions, what's going to happen is these energized electrons are going to move from photosystem 2 to photosystem 1, and they're going to do this via an electron transport chain which is really just going to allow for electrons to move from component to component in a series of redox reactions. And as these electrons move from photosystem 2 over to photosystem 1, these electrons are gonna be used to generate a hydrogen ion gradient or an H⁺ gradient or a proton gradient. And so we'll be able to see that down below in our image of step b. And so you can see here in step b, these electrons that are here, they're going to make their way over to photosystem 1 over here. And they do that the way that these electrons go from photosystem 2 over to photosystem 1 is via this electron transport chain that we see here.

And so, in this electron transport chain, the electrons get passed from component to component to component and those at the energy in those energized electrons is gonna be used to pump hydrogen ions into the thylakoid space. And so you can see that the thylakoid space here is building up a hydrogen ion concentration gradient, just as we indicated up above. And so ultimately this hydrogen ion concentration gradient is going to be used in a process called chemiosmosis later on to generate some ATP. But we're gonna continue to follow these electrons here before we jump over to how these hydrogen ions are gonna be used.

And so in the next step, step c, of course the electrons have now made their way over to photosystem 1 and photosystem 1 is also a light harvesting complex just like photosystem 2 is. And so photosystem 1 is also going to absorb photons of light and it's going to, take those electrons and energize them even more using the photons of light and the energy that it absorbs from those photons of light. And so photosystem 1 is going to, photosystem 1 electrons are gonna get energized even more and when they're energized even more, they're going to continue their way through the electron transport chain.

And so down below, if we take a look at step c here, notice again what we have is photosystem 1 right here and it is also a light harvesting complex similar to_photosystem 2. And so it's also going to absorb photons of light, and those photons of light are going to contain energy that can be transferred to the electrons, re energizing the electrons so that they can make their way through another, continue their way through the electron transport chain. And so this leads us into the 4th step of the light reactions here, which is step d. And in this step d, what's going to happen is something called NADP⁺ is going to serve as the final electron acceptor.

NADP⁺ is really just the oxidized form of NADPH. So it is basically an electron carrier, an electron taxicab that has 2 empty seats. And so the NADP⁺ has 2 empty seats in this electron taxicab and it's gonna serve as the final electron acceptor. So those electrons which they started on water, those water gets split to provide the electrons, the electrons get energized, make their way through an electron transport chain, they get re energized and continue through the electron transport chain, and ultimately those electrons are going to end up on NADP⁺ and, generate NADPH. And that's what we're saying here in step d is that NADP⁺ serves as the final electron acceptor and is going to be reduced it is going to be reduced to form NADPH.

NADPH is the full electron carrier that's carrying 2 electrons. And so if we take a look at our step d down below, what we can see is that we're seeing, NADP⁺, reduction. So NADP⁺ is going to be gaining electrons being reduced to NADPH and that's exactly what we see right here is NADPH is being formed, through the reduction of NADP⁺ and so NADP⁺ acts as the electron carrier, the empty, electron taxi cab if you will, and 2 electrons are going to be transferred to NADP⁺ to generate NADPH. And ultimately these 2 electrons are going to originate from the water molecule that was split over here. And so ultimately the water molecule is providing the electrons that are used in this process and end up on NADPH. And so this takes us to the 5th and final step of the light reaction, step e, and in this step, the hydrogen ion or H⁺ concentration gradient that was generated earlier is actually going to be utilized to generate some ATP.

And this ATP that's gonna be generated is generated via the process of chemiosmosis or the osmosis of ions across a semipermeable membrane. And so here in step e, notice that we have an ATP synthase protein that's embedded right here and this ATP synthase is going to allow hydrogen ions to diffuse down their concentration gradient and as these hydrogen ions diffuse down their concentration gradient it's going to energize the phosphorylation of ADP to generate ATP. And so ultimately, what we have is some ATP being generated here in the final step of chemiosmosis. And so what you can see here is that ultimately, what the light reactions is producing, or actually let's start with the reactants, with what the light reaction, starts off with the ingredients of the light reaction are photons of light and water molecules,

And ultimately, the water molecules are being split to provide electrons. The photons of light are going to be utilized to energize those electrons which allow the electrons to move through the electron transport chain, from photosystem 2 to photosystem 1, and those electrons end up on NADPH. And so in terms of the products of the light reactions it ends up forming oxygen, oxygen gas from the splitting of water molecules to provide electrons. It also ends up providing NADPH and it also ends up providing ATP. And ultimately, the oxygen gas that's produced, it can either be utilized by the plant for aerobic cellular respiration or this oxygen can actually diffuse out of the plant through the stomata of the plant, the openings in the leaves, and be released into the atmosphere. This oxygen can be released by the plant into the atmosphere. But the NADPH and the ATP that are generated are actually going to be utilized in the 2nd stage of photosynthesis which is the Calvin Cycle and that's really one of the biggest takeaways here of this lesson is that the light reactions is going to generate NADPH and ATP, chemical energy, And the NADPH and the ATP are going to go to the second stage of photos..."

Where do the electrons that are excited in photosystem II come from?

a) CO₂.

b) O₂.   

c) Glucose.   

d) Photosystem I.   

e) Water.

• A. CO_2..
• B. O_2..
• C. Glucose/
• D. Photosystem I.
• E. Water.

During the light reactions, photosystem I functions to_________, and photosystem II functions to __________.

a) Reduce CO₂; oxidize NADPH.

b) Synthesize ATP; Produce O₂.

c) Produce O₂; oxidize NADPH.

d) Reduce NADP⁺; oxidize H₂O.

• A. Reduce CO₂; oxidize NADPH
• B. Synthesize ATP; Produce O₂.
• C. Produce O₂; oxidize NADPH.
• D. Reduce NADP⁺; oxidize H₂O.

How to Memorize the Light Reactions of Photosynthesis

In this video, we're going to talk about how to memorize the steps of the light reactions of photosynthesis. And so here at Clutch Prep, we've come up with a really unique and interesting story designed specifically to help you guys remember the most important components and the most important events of the light reactions in the correct order. And so really, you'll only be able to find this story here at Clutch Prep. So you guys are pretty lucky. Alrighty. So let's get started here with this image.

And so when you're thinking about the light reactions, you want to think about Luke and Ryan. So, Luke and Ryan, when you're thinking about the light reactions. And so notice that we have a picture of Luke and Ryan down below right here. And so when you're thinking about the light reactions, you think about these two kids, Luke and Ryan, who just want to play their PlayStation 2. That's all they want to play is their PlayStation 2, and their PlayStation 2 is of course abbreviated as PS 2. And if you don't know what a PlayStation 2 is, it's a video gaming console. You can play video games on it. And so PlayStation 2, this PS 2 here is supposed to represent photo system 2. And so by remembering this PlayStation 2, you'll be able to remember that photo system 2 is actually coming first in this process of the light reactions.

But then after Luke and Ryan wanted to play their PlayStation 2, they realized that they could not find their electronic controllers to play their PlayStation 2. And so the ETC here in the electronic is going to represent the electron transport chain. And so this represents the ETC. And so after the photosystem 2, there is the electron transport chain.

And so because Luke and Ryan tried to play their PlayStation 2 but they couldn't find their electronic controllers to play their PlayStation 2, they then decided to try to play their older PlayStation 1 or PS 1 for short, and of course, the PS 1 is older. It was discovered first before PlayStation 2, but, of course, the kids want to play their PlayStation 2 first before they play their older PlayStation 1. And so here, PlayStation 1, the PS 1 here is representing photo system 1, which again was, discovered first before photo system 2, and that's why they called it photosystem 1. But then later they realized that photosystem 2 comes first in the process of the light reactions before photosystem 1.

So then Luke and Ryan, they went to play their PlayStation 2. They couldn't find the electronic controllers to play their PlayStation 2, so they decided to try to play their PlayStation 1. But then they forgot that their mom had reduced the number of games that they had for their PlayStation 1. And so what that means, what this is supposed to represent, the reduction of the number of games, is supposed to represent the reduction of NADP+ into NADPH. And so the ends here and the number can remind you of the n in NADPH and NADP+.

And so it's NADP+ that's going to get reduced here. It's going to gain electrons and become NADPH. And so because Luke and Ryan, they went to play their PlayStation 2, but they couldn't find their electronic controllers, they tried to play their PlayStation 1, but forgot that their mom had reduced the number of their games, so they couldn't even play their PlayStation 1, they then decided to just study chemistry instead. They decided to just study chemistry. And the "chem" in chemistry, I'm sorry, in chemistry, is supposed to represent the "chem" in chemiosmosis.

So the "chem" in chemistry is supposed to represent the "chem" in chemiosmosis. And so you can see here that, Luke and Ryan are now studying their chemistry here. And so you can see by just remembering this crazy unique and interesting story here, you can remember the most important components and the most important events of the light reactions in the correct order. And so this here concludes our lesson on how to memorize the light reactions of photosynthesis, 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.

What is the correct order of steps of the light reactions of Photosynthesis?

a) photosystem I, ETC, photosystem II, NADP⁺ reduction, chemiosmosis.

b) photosystem I, photosystem II, ETC, NADP⁺ reduction, chemiosmosis.

c) photosystem II, ETC, photosystem I, NADP⁺ reduction, chemiosmosis.

d) photosystem II, photosystem I, ETC, NADP⁺ reduction, chemiosmosis.

• A. photosystem I, ETC, photosystem II, NADP⁺ reduction, chemiosmosis.
• B. photosystem I, photosystem II, ETC, NADP⁺ reduction, chemiosmosis.
• C. photosystem II, ETC, photosystem I, NADP⁺ reduction, chemiosmosis.
• D. photosystem II, photosystem I, ETC, NADP⁺ reduction, chemiosmosis.

Here’s what students ask on this topic:

What are the light reactions of photosynthesis?

The light reactions of photosynthesis are the initial stage of the photosynthetic process, occurring in the thylakoid membranes of chloroplasts. These reactions utilize solar energy to convert water (H₂O) into oxygen gas (O₂), ATP, and NADPH. The process begins with photosystem II absorbing photons, which energize electrons. These electrons travel through an electron transport chain, creating a hydrogen ion gradient that drives ATP synthesis via chemiosmosis. Finally, NADP⁺ is reduced to NADPH, which, along with ATP, is used in the Calvin cycle to synthesize glucose.

Where do the light reactions of photosynthesis occur?

The light reactions of photosynthesis occur in the thylakoid membranes of chloroplasts. Thylakoids are membrane-bound compartments within chloroplasts that resemble green pancakes. These membranes house photosystems and other components essential for capturing light energy and converting it into chemical energy in the form of ATP and NADPH, while also producing oxygen gas as a byproduct.

What are the main products of the light reactions of photosynthesis?

The main products of the light reactions of photosynthesis are ATP, NADPH, and oxygen gas (O₂). ATP and NADPH are forms of chemical energy that are used in the Calvin cycle to synthesize glucose. Oxygen gas is produced as a byproduct when water molecules are split to provide electrons for the light reactions.

How do photosystems I and II function in the light reactions of photosynthesis?

Photosystems I and II are crucial components of the light reactions of photosynthesis. Photosystem II (PSII) absorbs photons and uses the energy to split water molecules, releasing oxygen and energizing electrons. These electrons travel through an electron transport chain to Photosystem I (PSI). PSI absorbs additional photons to re-energize the electrons, which are then used to reduce NADP⁺ to NADPH. This process also creates a proton gradient that drives ATP synthesis via chemiosmosis.

What role does water play in the light reactions of photosynthesis?

Water (H₂O) plays a crucial role in the light reactions of photosynthesis by providing electrons and protons. When water molecules are split by the energy absorbed by Photosystem II, they release electrons, protons (H⁺), and oxygen gas (O₂). The electrons are used to replace those lost by chlorophyll in Photosystem II, while the protons contribute to the formation of a proton gradient that drives ATP synthesis. The oxygen gas is released as a byproduct.