In this video, we're going to talk about photosynthesis. Just a brief introduction into it. Now, what is photosynthesis?
Let's think about the word photo and synthesis. Photo means light. Synthesis means to build something.
And that's what we're doing. We're using the energy provided by light to build something. In this case, carbohydrates.
Here we have the net equation for photosynthesis. We're combining six water molecules with six carbon dioxide molecules and using light to build simple sugars like glucose C6H12O6 and we're also going to get oxygen gas as a product as well. Now on the left side of this equation are the reactants and on the right side are the products. The picture on the left tells you where these molecules enter and leave the plant.
Water enters the plant through the roots. So the plant pulls up water and minerals that it needs through the roots from the soil. Carbon dioxide enters the leaves of the plant through tiny openings known as stomata.
And oxygen, it leaves the plant through those same openings. So it's important to understand that water and carbon dioxide, they go into the plant. and the plant releases oxygen during photosynthesis.
Now, let's move on to the chloroplasts. The chloroplasts is the organelle that carries out photosynthesis. On the other hand, the organelle that is responsible for cellular respiration is the mitochondria.
Now, these two processes, they are exact opposites of each other. Photosynthesis converts carbon dioxide and water with the help of light energy, into glucose and oxygen gas. On the other hand, cellular respiration takes glucose and oxygen gas and converts it back into carbon dioxide and water, releasing energy in the process. So you need to be familiar with these two processes.
Now the pigment that is responsible for absorbing light energy is known as chlorophyll. And chlorophyll is found in the thylakoids, which you can see in this picture. One stack of thylakoids is known as the granum.
If you have multiple stacks, this is called grana. So that's the plural form of the word granum. The lumen is the fluid inside of the thylakoid, and the stroma is the fluid inside of the chloroplasts. The chloroplast has two membranes, the inner membrane and the outer membrane, and between that you have the intermembrane space.
So make sure you understand that the chloroplast is the organelle in which photosynthesis is carried out. Now you also need to know that chlorophyll absorbs blue light and it absorbs red light. However, it reflects green light.
This is why most plants appear green. Photosynthesis can be broken down into two stages. The light dependent reactions and the light independent reactions.
The light dependent reactions occur inside of the thylakoids within the chloroplasts. The light independent reactions, which is basically the Calvin cycle, also known as the dark reactions, they occur in the stroma. the chloroplasts.
Now the reason why they're called light independent or dark reactions is because they can proceed without the assistance of light energy. The light dependent reactions oxidizes water into oxygen gas and remember oxidation involves the loss of electrons so whenever an oxidation reaction occurs there has to be a reduction reaction. NAD plus I said that wrong, NADP plus rather, picks up those electrons and is reduced into NADPH. Now some of the energy that is transferred by light is used to make ATP from ADP and phosphate. ATP is produced by chemiosmosis using an enzyme called ATP synthase.
So for the light dependent reactions, you need to know that the products are oxygen gas, ATP, and NADPH. The reactants are water, NADP plus ADP and phosphate. Now let's move on to the light independent reactions or the Calvin cycle. The Calvin cycle takes in carbon dioxide and reduces it into sugars such as glucose.
Now because that's a reductive process, oxidation has to happen somewhere. NAD NADPH is oxidized back into NADP+. So NADPH, it gives up its electrons, turning into NADP+. Carbon dioxide ultimately receives those electrons and eventually turn it into glucose.
ATP is used to power that process. As ATP converts back into ADP and P, It energizes the Calvin cycle, giving it the energy it needs to convert CO2 into glucose. So the reactants of the Calvin cycle are carbon dioxide, ATP, and NADPH.
The products are sugars such as glucose, NADP+, ADP, and P. So let's begin our discussion now with the light. dependent reactions. So what we have here is the electron transport chain inside of the thylakoid membrane.
Now the first thing that's going to happen is a light particle will strike photosystem 2 and it's going to excite the electrons in chlorophyll. So chlorophyll is going to lose its electrons which will flow into this mobile electron carrier called plastoquenone. Now because chlorophyll lost electrons, it needs to replenish those lost electrons. And so it's going to take the electrons from water, oxidizing it into oxygen gas. So one water molecule produces one oxygen atom, and it's going to give off two hydrogen ions and also two electrons.
So water will ultimately lose two electrons to photosystem II. Photosystem II is also called P680 because that's the wavelength of light that has the highest absorption. It's 680 nanometers.
Now, Plastoquinone is going to carry the electrons to cytochrome B6F complex. And as the electrons pass through that complex, what's going to happen next is it's going to pump protons from the stroma. That is from... outside of the thylakoid into the lumen or the inner thylakoid space.
So the proton concentration in the stroma is going to decrease while the proton concentration inside the thylakoid is going to increase, producing a concentration gradient. Now the electrons will continue the journey from cytochrome B6F to this particular peripheral protein, plastocyanin. This is a copper-containing protein, and it's going to transfer the electrons to Photosystem I. Now, from this journey, the electrons have lost some of its energy. And so what's going to happen here is the electrons will gain more energy by being struck by another photon of light in Photosystem I, also known as P700. So this particular photosystem has its maximum absorption at a wavelength of 700 nanometers.
So after the electron is struck by a photon of light, that photon will impart its energy to the electron. The electron will get excited and having more energy it's going to go to another peripheral protein called ferredoxin, which is an iron sulfur protein, and that's going to carry the electrons to NADP reductase. Now NAPD reductase. It's a peripheral protein, but it's also an enzyme.
You can see the word ACE. This enzyme is going to reduce NADP+. So the electrons are going to leave, and they're going to meet up with NADP+.
Once NADP+, accepts the electrons along with a hydrogen ion, it's going to be reduced into NADPH. So this reaction also reduces the hydrogen ion concentration in the stroma. which will favor the production of ATP soon.
So because there's a buildup of hydrogen ions on the inner thalakoid space, and there's very little hydrogen ions in the stroma, what's going to happen here now is these hydrogen ions, due to this concentration gradient, they will begin to flow through an enzyme called ATP synthase in a process known as chemiosmosis. As those hydrogen... excuse me, as those hydrogen ions flow through the enzyme, this protein will rotate in such a way that it's going to combine ADP and phosphate to make ATP.
So that's how the electron transport chain works in photosynthesis. So as we can see here, water is oxidized into oxygen gas. NADP reductase produces the electron carrier NADPH, and ATP synthase is used to produce ATP.
So those are the major products of the light-dependent reaction. And keep in mind that photosystem II, not photosystem I, but photosystem II is what converts water into oxygen gas. You might be tested on that.
Now let's talk about the other part of photosynthesis, that is the Calvin cycle, or the light-independent reactions. The Calvin cycle can be broken down into three parts. The first part is the fixation of carbon dioxide.
The second part is reduction. And the third part is the regeneration of RUBP, ribulose 1,5-biphosphate. So the first thing that happens is that carbon dioxide enters the cycle.
Carbon dioxide is going to react with ribulose-biphosphate. catalyzed by the enzyme rubisco, and that's going to turn into 3-phosphoglycerate, represented by the symbol PGA. So that's carbon fixation. RUBP is a molecule with five carbon atoms. On carbon 1, we have a phosphate group, and on carbon 5, we have another phosphate group.
So that's RUBP. Now there's three of them. and they're going to react with three CO2 molecules.
Now when one molecule of CO2 reacts with one molecule of RuBP, we're going to get initially a six carbon molecule, but that's going to be broken up into two three carbon molecules. So PGA is a three carbon molecule, and it's going to have one phosphate group. So if we keep track of a total number of carbons, if each RuBP has 5 carbons, 3 of them has 15 carbons. And then we're going to add 3 carbons from the 3 CO2 molecules, so we should have a total of 18 carbons. So thus there's 6 3-phosphoglycerate molecules.
Now, the enzyme kinase, when you see that, that enzyme is used to transfer a phosphate group to a molecule. And so this next step requires ATP. We're going to use six ATP molecules to phosphorylate 3-phosphoglycerate. And so now we have 1,3-biphosphoglycerate. So we still have a 3-carbon molecule, but now we have a phosphate group on carbon 1 and carbon 3. So as we can see here, PGA kinase is an enzyme that catalyzes the conversion.
of 3-phosphoglycerate into 1,3-biphosphoglycerate. So ATP gives up a phosphate in order to become ADP, and that phosphate is transferred to this molecule. As you can see, we now have two phosphate groups. So anytime you see a kinase enzyme, it catalyzes the transfer of a phosphate group from one molecule to another.
Now in the next step, we are going to use NADPH. I put a P first. We're going to use this molecule to reduce 1,3-biphosphoglycerate into G3P, glyceraldehyde 3-phosphate.
So NADPH is going to convert into NADP+. Now we need six molecules of NADPH to do this. This reaction will be catalyzed by the G3P dehydrogenase enzyme.
This enzyme, as it suggests the words dehydrogenase, it removes hydrogen from NADPH. Now at this point we're going to get six G3P molecules. One of the six G3P molecules is going to be used to produce sugars like glucose and fructose and things like that. The other five G3P molecules is used to regenerate the three ribulose biphosphate molecules. So keep in mind the total number of carbons here is 15. Each G3P molecule has three carbon atoms and it has a phosphate group on carbon 3. So 5 times 3 gives us a total of 15 carbons, which is what we started with.
So notice the net result. The Calvin cycle converts three molecules of CO2 into one molecule. of G3P. That is the net result of the Calvin cycle.
Now let's summarize what we've just considered. In the Calvin cycle, we saw that three molecules of CO2 was converted into one molecule of G3P, and that required the use of nine ATP molecules and six NADPH. molecules. Now in order to make one molecule of glucose we're going to need six molecules of CO2 and once that enters into the coven cycle that's going to produce two molecules of G3P which can be used to produce one molecule of glucose and so we need to double the numbers.
That's going to require 18 molecules of ATP and 12 molecules of NADPH. We just got to double everything. And so that's a simplified review of photosynthesis and the two processes that make it up. That is the light dependent reactions, which occur inside. of the thylakoid membrane and the light independent reactions with the Calvin cycle, which occurs in the stroma of the chloroplasts.
So keep that in mind. And that's it for this video. Thanks again for watching, and don't forget to subscribe.