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

The light reactions of photosynthesis occur in the thylakoids of chloroplasts, converting solar energy into chemical energy as ATP and NADPH while producing oxygen gas as a byproduct. The process begins with photosystem II absorbing photons, energizing electrons sourced from split water molecules. These electrons travel through an electron transport chain, generating a hydrogen ion gradient used in chemiosmosis to produce ATP. Ultimately, NADP⁺ is reduced to NADPH, which, along with ATP, fuels the Calvin cycle, highlighting the interdependence of these stages in photosynthesis.

Light Reactions of Photosynthesis

In this video, we're going to begin our lesson on the light reactions of photosynthesis. And so the light reactions we already know from our previous lesson videos is the first stage of photosynthesis, and it specifically occurs in the thylakoids of the chloroplast. And so recall that the thylakoids are those green pancake-looking structures, and the light reactions are going to occur in the thylakoid membrane as well as in the thylakoid space or the space that's on the inside of the Thylakoids. Now the light reactions are going to generate chemical energy in the form of ATP and NADPH, and 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. Now NADPH is really just another electron carrier that will carry or transport 2 energized electrons. And so NADPH is really just another electron taxi cab, if you will, which you'll recall we covered electron carriers in our previous lesson videos when we talked about aerobic cellular respiration.

And so down below here in this image, what you'll notice is that we've got the image showing photosynthesis. And once again in the background what we have is the chloroplast and notice that, we have the regions that we want to focus on in this image colored, and they're also enlarged. And the regions that we don't really want to focus on right now are grayed out and they're smaller. So like the Calvin cycle for instance, 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, which is again the light reactions. And the light reactions occur specifically in the thylakoids or these green pancake-looking structures within the chloroplast.

And so really the light reactions are going to take solar energy or sunlight in the form of photons and use this solar energy of these photons along with water, to generate chemical energy in the form of ATP and NADPH, which is another electron carrier, an electron taxicab, if you will. And it also creates oxygen gas as a byproduct, which we can see down below. And so 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. And this ATP and NADPH are going to be utilized in the Calvin cycle when we talk about the Calvin cycle later in our course. But for now, what you can see is that the NADPH are going to be carrying 2 energized electrons as you can see here in this image. And of course, the Calvin cycle is going to utilize this chemical energy and convert it back into the lower energy forms, ADP and NADP⁺, which are needed, for the light reactions so that they can convert them into their higher energy forms. And so this here really concludes our brief introduction to the light reactions and how they occur in the thylakoids within the chloroplast. And in our next lesson video, we'll get to talk about the specific steps that occur in the light reaction. So 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. Recall from our previous lesson videos that the light reactions are the first stage of photosynthesis. Also recall that the light reactions occur in the thylakoids within chloroplasts. Remember that the thylakoids are really just those green pancake-looking structures inside chloroplasts. From our previous lesson videos, be reminded that within the thylakoid membranes, they contain photosystems which are light-harvesting complexes made up of pigments, proteins, and other molecules.

If we take a look at our image below, notice that we're zoomed into one of the thylakoids within a chloroplast. This shows the thylakoid membrane representing the membrane of one of those green pancake-looking structures within the chloroplast. Below what we have is the thylakoid space, which represents the inside of the thylakoid. Above what we have is the stroma of the chloroplast, the space just outside the thylakoids, the fluid-filled space that fills up the innermost region of the chloroplast. This is important to note as we make our way through the steps of the light reactions.

In our lesson, we've consolidated the steps of the light reaction down to five steps labeled a, b, c, d, and e, each corresponding with the letters for the steps of the light reaction below. Recall from our previous lesson videos that most plants contain two photosystems, named photosystem 1 and photosystem 2 based on the order of their discovery. However, photosystem 2 actually comes before photosystem 1 in the steps of the light reaction.

The first step of the light reaction, step a, starts with photosystem 2 which absorbs photons of light, transferring that energy to electrons from a water molecule. These electrons are donated by a water molecule, necessitating why water is crucial for photosynthesis. When water molecules are split and oxidized to provide electrons, they react to form oxygen gas (O₂), which is associated with photosynthesis producing oxygen. Let's better understand step a by looking at our image below, where you can see photosystem 2 absorbing photons of light and transferring the energy to electrons.

In step b, the second step of the light reactions, these energized electrons move from photosystem 2 to photosystem 1 via an electron transport chain in a series of redox reactions. This process is used to generate a hydrogen ion gradient or proton gradient, which we'll be able to see below in our image of step b. The electrons move through the electron transport chain to photosystem 1, whose role is also to absorb photons of light, re-energizing the electrons further.

In the fourth step, step d, NADP⁺ serves as the final electron acceptor. NADP⁺ is the oxidized form of NADPH and serves as an electron carrier. In this process, electrons that started from water are transferred to NADP⁺ to generate NADPH, completing its reduction. This process is illustrated below in our image depicting NADP⁺ reduction to NADPH, showing NADPH formation through the reduction of NADP⁺.

The fifth and final step of the light reaction, step e, utilizes the hydrogen ion or H⁺ concentration gradient generated earlier to produce ATP via chemiosmosis. This process is facilitated by an ATP synthase protein, allowing hydrogen ions to diffuse down their concentration gradient, energizing the phosphorylation of ADP to generate ATP. This concludes our lesson on the steps of the light reactions of photosynthesis. We'll practice applying these concepts as we move forward in our course. See you all in our next video.

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 2 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 photo system 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 photo system 1. But then later they realized that photosystem 2 comes first in the process of 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, the chemi, I'm sorry, in chemistry, is supposed to represent the chemi in chemiosmosis. So, the chemi and chemistry is supposed to represent the chemi and chemiosmosis.

So, you can see here that, 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 convert solar energy into chemical energy in the form of ATP and NADPH while producing oxygen gas (O₂) as a byproduct. The process begins with photosystem II absorbing photons, which energize electrons sourced from split water molecules. These electrons travel through an electron transport chain, generating a hydrogen ion gradient used in chemiosmosis to produce ATP. Ultimately, NADP⁺ is reduced to NADPH, which, along with ATP, fuels the Calvin cycle.

Where do the light reactions of photosynthesis occur?

The light reactions of photosynthesis occur in the thylakoids of chloroplasts. Thylakoids are membrane-bound structures that look like green pancakes and are found within the chloroplast. The thylakoid membrane contains photosystems and other components essential for capturing light energy and converting it into chemical energy. The thylakoid space, the interior of the thylakoid, also plays a crucial role in generating a hydrogen ion gradient used in ATP synthesis.

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, the second stage of photosynthesis, to synthesize glucose and other carbohydrates. Oxygen gas is produced as a byproduct when water molecules are split to provide electrons for the light reactions. This oxygen can be released into the atmosphere or used by the plant for cellular respiration.

What is the role of photosystem II in the light reactions of photosynthesis?

Photosystem II (PSII) plays a crucial role in the light reactions of photosynthesis by absorbing photons of light and using this energy to energize electrons. These electrons are sourced from water molecules, which are split in the process, releasing oxygen gas (O₂) as a byproduct. The energized electrons from PSII are then transferred to the electron transport chain, where their energy is used to pump hydrogen ions into the thylakoid space, creating a gradient that drives ATP synthesis through chemiosmosis.

How is ATP produced during the light reactions of photosynthesis?

ATP is produced during the light reactions of photosynthesis through a process called chemiosmosis. As electrons move through the electron transport chain from photosystem II to photosystem I, their energy is used to pump hydrogen ions (H⁺) into the thylakoid space, creating a concentration gradient. This gradient drives the diffusion of H⁺ ions back into the stroma through ATP synthase, a protein embedded in the thylakoid membrane. The flow of H⁺ ions through ATP synthase provides the energy needed to phosphorylate ADP, forming ATP.