Hi, welcome to another lecture video about photosynthesis. This is our second lecture video on the topic and for today we're just going to continue on our previous discussion about photosynthesis. This will be our learning targets for this lecture video.
So all the things that we had learned in the previous lecture video, we'll try to integrate that today and we'll try to see whether how photosynthesis progresses from a simple looking process to a more detailed yet very complex one. So first objective for this lecture video is we have to differentiate the two stages of photosynthesis which are light reaction and the dark reaction in terms of its process, importance, and location. Third. I mean, second, identify the reactants and products in the light and dark reaction. What are the reactants from the general equation that is being used in each stage of photosynthesis?
Third, illustrate the sequence of events in both reactions through a flowchart. So, both the light and the dark reaction has a series of steps that occurs because these are processes. the two cores and we have to sequence them through a flow chart for us to better understand it.
And finally, explain the effect to the plant when the reactants of photosynthesis are removed. So, what can happen if we remove the water from the photosynthesis equation? What will happen if there is no light in the photosynthesis equation or even carbon dioxide. But before we proceed to our lecture video, let's have a look back on the last lecture video. So last time, we defined photosynthesis.
So technically, we said that photosynthesis is the process of where plants make their own food through inorganic materials. such as water, carbon dioxide, and sunlight. And what do we call this food?
This is glucose or the carbohydrates. And also plants produce oxygen as a byproduct of photosynthesis. So in general, photosynthesis has a chemical equation of 6CO2 plus 6H2O plus sunlight. will give us C6H12O6 or glucose plus 6O2 or oxygen.
So the oxygen produced by plants in photosynthesis are waste products or byproducts. Second, last time, it was very important and meaningful for us to see the importance of photosynthesis in our ecosystem. So we said that photosynthesis plays a very vital role in the propagation of life on earth.
So kung walang photosynthesis, walang life on earth. Or it's safe to say na without photosynthesis, there will be no variation of life on earth. So plants, and not only plants, but plant-like organisms such as phytoplankton please a very important role in the succession and in the evolution of life.
Third, we also talk about the cross-section of a leaf, the parts of a cross-section of the leaf. So we were able to determine that plants or the leaves has a cuticle, has a mesophyll, has a stomata, and also the epidermis. So each layer and each part of the cross-section of the leaf plays an important role in the process of photosynthesis. And to be more specific, we also talk about the chloroplast.
So the chloroplast is the main organelle inside the cell that performs photosynthesis. So they are color green because they contain the pigments which we were also able to talk which we were able to discuss also last time so this pigments help plants absorb light and at the same time they are the leading the leading reason why plants has this kind of green color particularly the chlorophyll okay and in relation to chlorophyll and pigments we also talk about the nature of light so we said that visible light has different wavelengths and these wavelengths vary in vary in the colors that we see We have the red, the orange, the yellow, the green, the blue, the indigo, and the violet. So the ROYGB.
The ROYGB are all colors that our naked eyes can see. And these are part of the visible light. So we said that this visible light is being used by the plant to undergo photosynthesis. These are the things that we were able to discuss and these are the things that we bring today in this lecture video to better understand and appreciate the process.
So to jump start with the process, let me show to you the two stages of photosynthesis. Photosynthesis, although it's a process it might it it might look like a simple process but this process is divided into two stages. First, the light-dependent reaction or also known as the light reaction and the light-independent reaction or the Calvin cycle or the dark reaction.
So from the word itself, we can already get an idea about the key characteristics and difference of each stage. of photosynthesis. And let's dig deeper by moving on to the next slide.
Let's start with the light-dependent reaction. So the light-dependent reaction happens in the thylakoid of the chloroplast. So if we're going to go back to our discussion on the parts of the chloroplast, So the thylakoids are the... the coin-like structures, the pancake-like structures that we see here in the illustration. So this is where the light reaction happens.
And from the word itself, light is required and absorbed by the chlorophyll to be converted into energy. So what happens in the light reaction? This is the absorption of light and we turn this light into energy.
Okay. So what are those? What do we call this energy?
We'll get to know them in the succeeding slides. So the chlorophyll or the pigment in the thylakoid membrane or in the thylakoids here traps energy from the light. And using this light, okay, water is also being used as raw material.
So, in the light reaction, we have two ingredients. We have water and we also have light. Okay?
Then, light energy is needed. Okay? Why does light need photosynthesis? For it to split water into hydrogen and oxygen molecules.
Okay? H2O. Water will separate from light energy. It will be hydrogen and oxygen at the end of the light reaction.
So here, NADPH and ATP are produced from the electron transport chain. So these two molecules are called the energy molecules. It's something new to us. NADPH is something new to us.
ATP, we already know ATP. This is our energy currency. While NADPH are energy molecules, these are high energy molecules that will help power up the succeeding processes of photosynthesis.
Let's think. of NADPH as a battery. Okay? Ang mga battery, these are materials that contains energy. Okay?
Parang NADPH sila. Okay? Mga molecules ng NADPH that contains energy.
Na kapag they are used in in other processes. They power up these processes and they are able to keep these processes working. So they store energy until they are transferred to the stroma and they play an important role in the light-independent reaction or in the dark reaction.
So technically, in light-dependent reaction, This is where the production of NADPH and ATP occurs. So why do we need NADPH and ATP? Because this is what we will use in the next reaction, the dark reaction.
Okay? So the light-dependent reaction in detail looks like this one. Alright?
I'm just showing you this. We do not have to... to memorize this for the sake of our lesson.
I'm just showing you this one for the sake of visualization. Although we will be talking about this in the succeeding slides, but we will not be dwelling much on the details and the terminologies. So how does light dependent reaction occurs. So it starts with this one.
So what you can see on the screen, this area here, that's what we call electron transport chain. Later on, you'll realize why it's called electron transport chain. So, as you can see, this is also the thylakoid.
Ito yung membrane ng... thylakoid okay so the thylakoid membrane contains what we call photosystem 1 and photosystem 2 okay this one is photosystem 2 and this one is photosystem 1 so photosystem 1 and photosystem 2 solar panels because they are light harvesters okay Now, each photosystem consists of chlorophyll-absorbing molecules. Kaya sila tinawag na mga light absorbers because they contain chlorophyll.
And the main purpose of chlorophyll is to absorb light. Now, when light is absorbed, it excites the electrons present in the photosystem. So, the photosystem absorbs the light and because the light is absorbed, the light which is a form of energy, the electrons that are there get excited. Now, when the electrons get too excited, the excitation will be gathered into the reaction Center.
which will pass the electrons to a series of proteins. So, it absorbed all the energy. So, because it's so excited, it passed the electrons to the proteins beside it. Okay? When these proteins are passed on, the energized...
Sorry, when these energized electrons are passed from the reaction center... of the photosystem 2 to the electron transport chain. Okay po.
Mauubusan ng electron ang photosystem. Okay? So para ma-replenish yung electron na nawala, okay, na nawala dahil sa pagiging excited niya, papalitan yun ng water.
The lost electrons are replenished when water is split. By light through the process of photolysis. Photo meaning light, lysis meaning separation. So water is being separated.
Okay? Napaghihiwalay natin yung water gamit ang ilaw. Okay, why does water have to be separated into oxygen and hydrogen?
So that the electrons that are gone can be replenished because the electrons that are excite have already passed to the proteins. Okay, so this is what we're saying right away. that at the end of The light reaction that we produce is water. I mean, sorry, I'm sorry. We produce oxygen.
Because water is being split into hydrogen and oxygen. Sir, where is the hydrogen? I didn't see it.
The red is the oxygen. yung may mga positive, yung may mga plus-plus, yan yung hydrogen. Okay po, yan yung hydrogen. Hydrogen, naging hydrogen ions na siya. Yan.
So, how did we get the oxygen as a by-product? So, water is produced when light splits the water molecules into oxygen and hydrogen in the process of photolysis. So if this is a water molecule, because of light, we can separate it into oxygen and hydrogen molecules.
We call that photolysis. To continue the story, as we can see, the electrons are now being passed from the photosystem 1 to the electron transport chain. Originally, it's here in this part.
But because the electrons are excited, because there's a lot of light coming in, it passes the electrons to the proteins next to it. Okay. Now, as the electrons move to the electron transport chain or ETC, hydrogen ions are moved from the stroma from outside of the thylakoid okay to the inside of the thylakoid.
This movement creates a concentration gradient. So we can see here, okay, the hydrogen ions, the plus plus that we can see here, there are some that enter here to the inside okay, because the electrons are moving. So, in other words, They are the ones that pass the electrons.
Because there is a transfer of electrons, in the electron transport chain, there are those that pass hydrogen ions that will enter the thylakoid. And the concentration gradient powers a protein that we call the ATP synthase. This protein creates an adenosine triphosphate or ATP which will be used in the next stage. Dahil kanina maraming mga pumasok na hydrogen ions galing dito sa stroma papasok dito sa thylakoid, nagkaroon ng concentration gradient.
With the power of membrane transport, the hydrogen ions that enter the thylakoid are released again through this structure. But what's good about this structure is while it releases the hydrogen ions, It produces ATP. That's why we call this structure ATP synthase.
Because this structure synthesizes ATP. Okay po. So kaya in the light reaction, we are also able to produce ATP.
Next. Now, low energy electrons from the photosystem 2 are shuttled to the photosystem 1 where they will be re-energized again. So naipasa-pasa na yung mga electrons from photosystem 2 going to photosystem 1. Pagdating niya sa photosystem 1, It's low energy because it's already traveling through different molecules. But even though it's low energy, it will be re-energized again because it's in the photosystem again. Remember that the photosystem is a structure that is like a solar panel.
they absorb energy okay they absorb energy and they will be the energized again and once re-energized the the electrons pass through an electron transport chain again where they wait where they are used to produce nadph so from photosystem one okay which is here When it's re-energized, it will now pass through the electron transport chain. And from the electron transport chain, it will produce NADPH. So we can see here in this animation, the transition from photosystem. One, going to the last area wherein this is where NADPH is produced.
So in summary, this is the light-dependent reaction in a nutshell. So light-dependent reaction makes use of light and water and it happens in the thylakoid. So in this process, water is being... into hydrogen and oxygen.
The oxygen that was produced will now be emitted in the atmosphere. It will be emitted by the plant because it doesn't need a plant. A plant doesn't need a lot of oxygen.
Then, at the end of this reaction, energy is produced in the form of ATP and NADPH. So here, this is just a summary of the whole process of the light-dependent reaction. Why is it called light-dependent reaction or light reaction?
Because here, light is being used. Okay, so... Just to summarize, light strikes the thylakoids. We have the photosystems 1 and 2, wherein they act as a solar panel and absorb the light that is passing through them. In the photosystems, light excites the electrons found in these photosystems.
and electron is passed down to the ETC or the electron transport chain. So as we can see here in this particular area, the light is absorbed. And because the light is absorbed, it is excited. It is passed here in the electron transport chain.
And when we get here, it is low energy again. The light is absorbed. It will be passed again to...
proteins in the electron transport chain. Now, the electrons that are lost will be replenished by splitting water into oxygen and hydrogen. The oxygen produced after splitting the water will be released to the atmosphere for the use of different organisms, particularly animals.
We will also notice that when the electrons move or pass along the electron transport chain, hydrogen ions move along or move going inside the thylakoid. So from the stroma, from the outside environment, pupunta siya sa loob. Okay, nagsalisi sila. And when this happened, okay, mataas ang concentration ng hydrogen ions dito sa loob, dito na papasok yung role ng ATP synthase.
Okay, wherein ATP is produced because yung tipon ng hydrogen ions dito sa loob, ilalabas ulit siya. And when it's released, the ATP plants can be produced. Then finally, NADPH is produced when the electron is again transported to the ETC at the second time around.
So when electrons arrive here in photosystem 1, they are low energy. may energize ulit sila at kapag na-energize na ulit sila, pupunta na sila dito, ipapasa na naman sila. At dito sa last na pagpapasahan, makakaproduce na tayo ng NADPH. So in summary, balikan natin yung photosynthesis formula. Alin na yung mga nagamit natin.
We have water and light. And what was produced after that? after the sequence or after the process we have oxygen we're done with the light reaction now after the light reaction of course there will be other products other than oxygen that is very essential inside the plant and these are the atp and nadph like what i said earlier for what are these two energy molecules. So, technically, they are produced to energize the next stage of photosynthesis.
And this is the dark reaction or also known as the Calvin Cycle. So, contrary to the popular idea that plants do not photosynthesize at night, actually, photosynthesis still continues even though there is darkness. But it's only the dark reaction that occurs. Not to mention that there are also alternative sources of light such as street lamps, cars, and bombillia that can also help trigger the plants to undergo light-dependent reaction.
So technically, kung walang ilaw, Especially during the night, okay, walang light reaction na nangyayari. Pero hindi ibig sabihin na walang photosynthesis na nangyayari. Kasi dito, okay, at night, pwede pa naman sila mag-undergo ng dark reaction or ng Calvin cycle.
Okay? Or naman, kung available yung mga sources of light, During the night, say for example, moon, yung buwan, may flashlight tayo, may mga streetlights tayo, then pwede pa din mag-photosynthesize ang mga halaman through the light reaction. The dark reaction also occurs during daytime.
Let me clarify that one. Baka isipin natin na si dark reaction, only happens at night, pero hindi naman talaga. It's only called the dark reaction kasi it does not use or it does not need light to occur. So the dark reaction happens in the stroma of the chloroplast.
So balikan natin, saan yung stroma? If this is the thylakoid, yung pang parang piso, coins, pancake, The stroma is the fluid inside the chloroplast. So it is a cyclical series of reactions that use enzymes to assemble sugar molecules from carbon dioxide and the products of the light reaction.
So in the dark reaction, we will now be using the other reactants of photosynthesis. such as or particularly carbon dioxide. And what is produced here in the dark reaction?
It is the sugar or the glucose already. So yeah, now carbon dioxide is required in this stage. Okay? Dito na natin siya kakailanganin.
And it uses NADPH and ATP formed in the light reaction. So yung naproduce kanina ng products ng light reaction, gagamitin na natin siya in the dark reaction. So it makes glucose from...
carbon dioxide and hydrogen. And it has four stages. The carbon fixation, the reduction, and the carbohydrate formation and the regeneration. These are the four stages.
So technically, this is the process. In the dark reaction, we have carbon dioxide, hydrogen ions, plus ATP and NADPH produced in the light reaction will give us G3P or the sugar. Or, as the saying goes, glucose.
So how does that happen? so inside the inside the chloro inside the chloroplast, the ATP and NADPH that are produced here in the thylakoids will go to the stroma, to the fluid of the chloroplast. And this is where the dark reaction happens.
So this is the dark reaction in a nutshell. Again, you are not asked to memorize all these molecules. We'll just be passing through the four stages of the Calvin Cycle so we know what's going on.
So the first stage is the carbon fixation. So from the word itself, carbon fixation. Carbon dioxide is being fixed at this stage.
Okay po, it is being fixed at this page. Kasi carbon dioxide is inorganic. Okay?
So para magamit siya ng plant, aayusin muna siya ng plant. Kaya meron tayong carbon fixation. So carbon is fixed in order to be converted into usable organic form.
Kasi hindi naman talaga magigamit si carbon dioxide. Basta-basta. So you have to convert the format of carbon dioxide. It's just like our files on the internet or our laptop. Carbon dioxide is not supported in the file.
So to support that file or carbon dioxide, you have to convert this into a usable form. Then we also have the reduction stage. So here, once carbon dioxide has been fixed, NADPH and ATP will be added to produce the G3P molecules.
So we will see that the fixed carbon dioxide will react to the ATP. and NADPH. Okay po, ATP and NADPH. Okay, so yun.
Saan galing yung ATP at NADPH? Okay, galing yun sa light reaction. Okay, yung mga na-produce nun. Now at the end of stage 2 or the reduction phase, makakapag-produce na tayo ng G3P molecules.
Okay. Then, after which, there will be a carbohydrate formation. Some G3P molecules will be converted into sugar.
Before proceeding to the third stage, stage 2 will repeat itself. Stage 2 will repeat itself. and we can gather a lot of G3P molecules.
Now, not all G3P molecules convert into sugar because later on, they will have another process to go through. The sugar produced will be used by the plant as their food. So we will see here at the end of this stage that there are other G3P molecules that will come out of the cycle and there are other G3P molecules that will continue the cycle because stage 4 or regeneration will occur.
The remaining G3P molecules that were not used to form carbohydrates will be recycled to become RUBP molecules once more and continue the cycle. Because it's a cyclical process, it's a cycle, and so that the cycle won't break, the other G3P molecules will use it. to become RUBP.
Okay? Or the ribulose-1,5-bisphosphate. Okay?
Now, this ribulose-1,5-bisphosphate or RUBP, this is what will be used again to fix the carbon dioxide during stage 1 in the carbon fixation. Okay? Do I get it? There. So just to summarize the Calvin cycle, again the Calvin cycle has four stages.
One is carbon fixation. In carbon fixation, carbon dioxide is being fixed by converting carbon dioxide into organic forms. So here in carbon fixation, the RUBP will react with carbon dioxide to form a usable form.
Then follows the second stage which is the reduction stage wherein the fixed carbon dioxide will be reacted with ATP and NADPH to form a G3P molecule. Now, stage 2 will repeat until they gather enough G3P molecules to proceed. Then, when the G3P molecules are now enough, some G3P molecules will be used to form the carbohydrate or to form the glucose. And some will be used for the last stage which is the regeneration stage.
So here in the regeneration stage, the G3P molecules will be converted back to RUBP. Then again, RUBP will react with carbon dioxide again to start the cycle once more. Okay, so again in this discussion I did not focus on much details of the cycle.
I just discussed the general process of each stage for us to have a better understanding. So I did not also discuss the specific molecules and that's okay. What's important here is that we know what's happening.
in general, in the Calvin Cycle. So let's go back to the photosynthesis formula. So again, water and sunlight were used in the light-dependent reaction or in the light reaction to produce oxygen. And not only oxygen, but also some were produced as ATP and NADPH.
But we didn't write it here because We will use the ATP and NADPH produced in the light reaction in the next stage, which is the dark reaction. In the dark reaction, carbon dioxide were used together with ATP and NADPH in order to produce sugar. alright now in summary This image summarizes the whole process of photosynthesis.
As we can see, the inputs here are the reactants. We have water, which is used in the light reaction, and carbon dioxide, which is used in the Calvin cycle. The products of the light reaction is Oxygen which is released by the plant through respiration, it releases the oxygen that we breathe in.
And after the Calvin cycle, the sugar or glucose is produced. So, Sir Noy, you haven't mentioned about the glucose yet in the process. I have been mentioning it, but we're done with G3P molecules.
I just said earlier that there are other G3P molecules that are used to make sugar. Now, when G3P molecules are produced, they will assemble together and they will become the... the glucose or sugar later on.
Now to continue with this illustration, ATP and NADPH that are produced from the light reaction, we will use it in the Calvin cycle. And when it is used, it will become ADP, Adenosine Diphosphate and NADP+. So they will go back to the light reaction so that they can be energized again and become ADP and NADPH. So what does the process tell us?
So what does the story of photosynthesis tell us? First, photosynthesis makes us realize how plants can give us much energy than meat. Because in the food pyramid, Mas madaming energy yung binibigay sa atin ng mga plant-based food. Rice, carbohydrates, sobrang raming energy ang binibigay niya compared to meat, dairy products. Although some dairy products can make us fat, pero in general, The carbohydrates that we see in plants are the number one reason why we gain weight.
And this is the reason why, I mean, much of the reason why is because they produce the energy themselves. The second thing that photosynthesis tells us is that plants are really very important. Without plants... our our atmosphere will continuously be polluted okay especially nowadays where global warning and the amount of carbon dioxide in our atmosphere is very high so we have to plant a lot of trees and plants as well okay in general although there are other organisms that can that can produce oxygen pero the benefits of plants are still different.
So to end this lesson, the bottom line of discussing photosynthesis is its importance to us as a person and the importance of plants in the ecosystem. Let's not confuse their importance. Because When plants are removed from the ecosystem, everything will collapse. The ecosystem will be destroyed.
So let's go back to our objectives that we relayed earlier. So these were our objectives for this lesson. So first, we were able to differentiate light and dark reaction in terms of the process. the importance and the location where they are happening. Second, we were able to identify the reactants and the products in the light reaction and the dark reaction and through animations and the flow chart and diagrams, we were able to sequence the event in both reactions.
Then finally, na-explain natin yung effect sa plants when they're active. of the photosynthesis were removed. What will happen if there is no light?
Will the light reaction proceed? What happens when there is a carbon dioxide deficiency? Will the dark reaction. Okay?
So, that's it. These were our objectives. And thank you very much for reaching this time, reaching this part of the video. It means that we have finished our discussion on photosynthesis. And I hope that there is...
we find plants not only as creations of God but we see them as important agents in our ecosystem. From this point on, as STEM students or as bio students, we are able to appreciate plants in a more detailed scientific manner through the process of photosynthesis. And I hope that next time that we see plants, we can plant them, we can be inspired.
magtanim ng halaman ng puno, okay? Kahit sa bakuran lang natin. And that is our simple way of giving back to their benefits. And with that, thank you very much for watching and listening to this lecture video. And I hope to see you in the class, in the classroom, for a more deepening, life-changing sessions.
Have a great day, everyone.