Understanding Photosynthesis and Its Processes

We have been talking about cellular respiration and fermentation, methods by which animals derive energy from foods. But now we're going to actually start talking about how organisms actually make food. And it's going to be... split into two sections, photosynthesis 1 and photosynthesis 2. Okay, let's begin. We can take organisms on Earth and divide them into two groups, ototrophs and heterotrophs. Ototrophs are self-feeders. They sustain themselves without eating anything derived from other living organisms. and they produce organic molecules from carbon dioxide. Biologists call autotropes the producers of the biosphere. Here are some examples of autotrophs on the left. Plants. Nearly all plants are autotrophs. Algae. Protists. Prokaryotes. There's algaes. Protists. Some cyanobacteria. And the defining feature is that they use light as a source of energy. Heterotrophs, to which we belong, are unable to make their own food. There are other feeders unable to make food, and we eat other organisms to sustain ourselves. In this lecture and in the following lecture, we are going to focus on ototrophs and how they use light as a source of energy in a process called photosynthesis. Let's talk about where actually photosynthesis occurs in plants. So here is a leaf of a plant and it's green and that's a telltale sign of chloroplasts where photosynthesis occurs. So chloroplasts exist in all green parts of a plant. And there are about a million chloroplasts in a little chunk of leaf which has a surface area of about a square millimeter. Leaves are chocked full of chloroplasts. And chloroplasts are found mainly in the mesophyll, the tissue of the interior of the leaf. Mainly found... the mesophyll right here the inside is called a mesophyll and there are stomata these little openings on the bottom of the leaves to allow CO2 to enter and oxygen to leave during photosynthesis. and it can open and close. Veins are what delivers water from the roots to the leaves. But it is also the system where sugars are actually exported to the roots in other non-photosynthetic parts of plants. And you might know this, to make maple syrup, one actually taps into these veins to draw the sugars that the maple tree has made. And it's stored in the roots. And in the springtime, it shoots it up to the branches so that it can grow leaves, so that photosynthesis can happen. So the green cells in the mesophyll are called mesophyll cells. And in them, you find chloroplasts, where again, photosynthesis occurs. There are about 30 to 40 chloroplasts per mesophyll cell. And chloroplasts, like mitochondria, have two membranes. And they're filled with what's called stroma. And there, inside the chloroplast, there is a third membrane system called a thylakoid. Okay? These thylakoids are the third membrane system. mitochondria only have two, right? And the space inside is called thylakoid space. And these are usually stacked, as you can see, like poker chips, but they're stacked and the stacks are called grana. And it is in the thylakoids where this green pigment lives, which gives the plants its green color. And those pigments are called chlorophyll. They're the green pigments in the thylakoid membrane. And it's the chlorophyll that captures the energy from light. So let's talk about photosynthesis. As you might have guessed, photosynthesis is also a redox reaction. And the general process of photosynthesis is energy from light plus six carbon dioxide and six water molecules results in a glucose molecule and byproduct of photosynthesis six oxygen. And the carbon from carbon dioxide gets reduced. It accepts the electrons. And where do the electrons come from? Well, they come from the splitting of water molecules. So water, or the oxygen in the water, becomes oxidized. If you noticed, this is almost exactly the opposite electron flow from cellular respiration, right? Where glucose in the presence of oxygen yields or gets oxidized, meaning gives up its electrons, to become the carbon dioxide, and the oxygen molecule gets reduced to form water plus energy. Remember, the oxygen is the final electron acceptor down the electron transport chain. So in cellular respiration, the direction of electron flow yields energy, right? Therefore, reversing the electron flow in the case of Photosynthesis requires energy. Therefore, energy is on this side. And where does it come from? It comes from light. So let's start talking about the two stages of photosynthesis at a very general level. The two stages of photosynthesis are what's called light reaction and the Calvin cycle. The light reaction is the steps of photosynthesis that convert solar energy to chemical energy. This is where water is split and O2 is a waste product. As you can see water is brought in, it's split, and we have waste product of oxygen. And light is absorbed by the chlorophylls, and that derives the transfer of electrons and hydrogen to NADP+. The electrons are handed off to NADP+, to make into NADP+, to convert into NADPH. Chlorophyll. transfers electrons to NADP plus to make NADPH. And NADP stands for nicotinamide. adenine dinucleotide phosphate. It's almost exactly the same as NAD plus in cellular respiration, but it just has a phosphate group attached to it. And this is also a step where ATP is generated. But more importantly, no sugars are made here. No sugars made. And as the picture depicts, it occurs in the thylakoid. The Calvin cycle, the second part of photosynthesis, incorporates CO2 from the air into organic molecules. And this step is called carbon fixation, because it takes carbon from the air and fixes it into complex molecules. And it does it by reducing carbons to carbohydrates by adding the electrons. Reduces carbons. carbohydrate sugars by NADPH. So NADPH is the electron source where it brings it into the Calvin cycle by where carbon dioxide or the carbon is fixed into becoming sugars. And ATP is required to fix carbons as well. And this cycle is also called the dark cycle because it doesn't require sunlight directly to run. It does not require sunlight directly to run. But it doesn't mean that it happens at night, right? That's not what the dark cycle means. And this occurs in the stroma of the chloroplast, as this depicts in the stroma of the chloroplast. And in this lecture, we're going to talk about the light reaction. And in the following lecture, we're going to talk about the Calvin cycle. Okay? So in order to start talking about the light reaction, we need to first start by talking about the absorption of light by chlorophyll. Okay? So the molecules that actually absorb light are called pigments, photopigments absorb light of certain wavelength. And plants are green because that is a color that is reflected or transmitted through the plants. So the photopigments in plants are chlorophyll that are in the membrane of the thylakoids. And they use all wavelengths of light except for green. What does that mean? Well, we can actually generate what's called an absorption spectrum. Absorption spectrum for chlorophyll, and there are different types of chlorophylls, but it's the graph plotting absorption versus wavelength chlorophyll a is the primary photo pigment that participates in absorbing light there are other pot of pigments in the leaves But this on the y-axis is the absorption of light. And let's just focus on chlorophyll A for now. As you can see, the wavelength of light right around the purple-blue area is absorbed really high. But as soon as you reach the green wavelength... there's very little absorption and the absorption again picks up again in the red spectra so chlorophyll a which directly participates in light reaction of photosynthesis. It shows that the violet blue spectrum wavelength and red light work best for photosynthesis. since they are absorbed and green is the least effective color. When we actually see a color, like a red car for instance, the pigments in the paint in the red car absorb all other wavelengths of light except for red. So it's a reflecting red color. That's why we see it as red. Chlorophylls are exactly the same. It's absorbing every single wavelength except for green. That's why we see chlorophyll as green. Okay? and the action spectrum confirms this, confirms this observation. As you can see, here's the rate of photosynthesis on y-axis. It's high and it dips for green and comes back up for red. So photosynthesis is high in blue-purple and in red, but extremely low in the green. Okay, action spectrum. And this is what chlorophyll actually looks like at the molecular level. There is a light absorbing head of the molecule called the porphylen ring. There is a magnesium ion right in the middle of it. The two different flavors of chlorophyll differs only in this one molecule, right here. In the second part of this chlorophyll molecule, there is a hydrocarbon tail And this is what interacts with the proteins inside the membrane of the thylakoid. It buries itself into the membrane. Okay, light absorbing and membrane tethering. So how is the energy from the light actually captured by this molecule? It's captured by this molecule by exciting the electrons in the chlorophyll. Okay, so here's... cartoon picture of a photon of light coming captured by the chlorophyll molecule and the electrons in its outer shell gets excited and it jumps up absorbing the energy jumps up to what's called an excited state And really quickly afterwards, these electrons decay back down, back down to the ground state, releasing the energy in both the form of heat, as well as in a photon of energy, fluorescence energy. So let's write that down. Absorption. of light energy, electrons are excited and jump up to a higher orbital. into what's called an excited state. But in a billionth of a second, electrons come back down to what's called a ground state. Billionth. of a second and when it loses that energy it emits energy in the form of heat and light Right? This is why cars, if left out in the sun, gets hot. The pigments get excited, and then when they relax back down to the ground state, it's releasing energy. It becomes hot. That's why white cars... are cooler to the touch than black cars because white reflects all visible spectrum, black absorbs all visible spectrum of light. Anyways, so when it relaxes, it releases photons, like I mentioned, right? Light. And this is how fluorescent light bulbs work. And here is an artificial preparation of a beaker full of chlorophyll. If you hit it with UV light, ultraviolet light, it fluoresces in this red-orange color. And it's this what is being seen here. But of course this is an artificial system, right? It does no use for a plant to be fluorescing. So in vivo, or in the actual plant, in live preparation, this energy is captured by other molecules. in what's called the photosystem. Okay? So this energy is captured. So what does a photosystem look like, and what do they do? The photosystem, which is this entire thing, is composed of the reaction center complex and the light harvesting complex composed of reaction center complex and the light harvesting complexes. So the photopigments, the pigments in the light harvesting complexes, capture the energy and that electron is passed from pigment to pigment to pigment until it finds a special pair of chlorophyll molecules, which then ultimately hands off that electron to what's called the primary electron acceptor. So the pigment captures the photon and energy is transferred from pigment molecule to pigment molecule. Much like a beach ball in a concert. And then ultimately it is passed to the reaction center complex, which captures the electrons. and becomes reduced. This is called the primary electron acceptor. And of course this is a redox reaction. because electrons are being passed, becoming reduced. And this is the cartoon version up here, and this is the crystallized structure of the photosystem 2. You can see in the red reaction center complex and surrounding it is going to be the light harvesting complexes. And there are two types of photosystems. There are photosystem 2 and photosystem 1. Two types of photosystems. PS2 and PS1. And PS2 actually functions first in the light reaction, but it's called PS2 because it was discovered after PS1 was. So the numbering scheme is historical. And this is called the linear electron flow. In a bit we'll talk about a circular electron flow, but this is how majority of photosynthesis occurs. How it occurs. So step one, the light hits the photopigment in PS2. PS2. And then the electrons get to pair a specialized chlorophyll a called p680. p680 chlorophylls. Then step two electrons are transferred. from the excited P6AD to the primary electron acceptor. When P6D gives up its electron, P6AD becomes oxidized. to make P680+. P680+. The oxidized form of P680 is one of the strongest biological oxidizing agents known. So it wants to grab electrons from anything possible. And that helps, actually, this third step of splitting water and grabbing a pair of electrons. Water is split. by an enzyme to give p680 its electrons. Strongest biological oxidizer. And then. The primary electron acceptor hands the electron down the electron transport chain, much like cellular respiration. Electrons are passed along the electron transport chain. to cellular respiration. And each time the electron is transferred down, and this exergonic fall of electrons from high energy to low energy drives protons to be pumped. And that, just like cellular respiration, yields in ATP production. So, exergonic fall of electrons to lower energy state produces ATP via proton. being pumped into the thylakoid lumen. And this ATP production is called chemiosmosis. Right, ATP is being made. And finally, instead of oxygen being the final electron acceptor in cellular respiration, these electrons actually Enter photosystem 1. Electrons passed to photosystem 1 and P700 flavor of chlorophyll. And then, with the help of PS1 capturing another photon of light, it is able to then donate electrons to its primary electron acceptor. And passes it to primary electron acceptor using... energy from light and then the electron runs down another set of electron transport chain to to NADP reductase, which is an enzyme that transfers electrons to NADP to produce NADPH. So seven is electron transport chain. Last but not least, electrons transfer to NADP plus to make NADPH. And this molecule is then, as you can see here, it's fed into the Calvin cycle to help fix the carbons from the atmosphere. This seems very complicated, but essentially what it's doing is that it's using a photon of light in PS2, Photosystem II, to energize, to excite the electrons, and as the electrons lose energy, use that to synthesize ATP, and then Photosystem I captures another photon of light to excite that electron up And then that is used to then synthesize NADPH. So I think this picture does, explains this very beautifully. So this is what's called a linear electron flow. It's going from PS2 going on to PS1. Linear. There's also what's called a circular electron flow. A circular electron flow does not use photosystem II and no NADPH is made. just ATP. And you can see from this figure right here, the electron transport chain is coming down, bringing the electrons down to photosystem I, where the primary electron acceptor gives it back to the electron transport chain between photosystem II and photosystem I. So the electron just flows in a circle, only producing ATP. So this is a very specialized kind of photosynthesis, right? And there are mutant plants that cannot do this cyclic electron flow. which gives us a good idea of why such a process even exists. These plants actually do not grow well in places where there's intense amount of sunlight. which suggests that the cyclic electron flow may be photoprotective to these plants, which can do it. And this process of cyclic electron flow will become important when we talk about a particular adaptation of photosynthesis, called C4 photosynthesis. It will become important. I'm talking about... C4 plants. And we'll talk about C4 plants in the next lecture. So let's talk about this production of ATP for a moment. Remember that it's called chemiosmosis. Let's compare cellular respiration and photosynthesis. They're very similar. Proton gradients formed by electron transport chain. It uses proton gradient to generate ATP, but there are slight differences. And that arises from which compartment where protons are being pumped into? In the mitochondria, protons are pumped into the intermembrane space, right? Mitochondria protons pumped into intermembrane space. In the chloroplasts, protons are pumped into the thylakoid, called the thylakoid space. Right, as you can see from this cartoon drawing, the higher pH, the more acidic area where the protons are being pumped into, is shaded dark. So the intermembrane space is dark here. In the chloroplast, instead of the space between the membrane, because it has a third membrane system, it acidifies the thylakoid, the inside of the thylakoid, called the thylakoid space. And then the rest of the same. The electron transport chain is going to move protons from area of low concentration to area of high to make it acidic. And then this chemiosmosis. is a process by which protons moving back down as electrochemical gradient using that to generate ATP using that as an energy source. Okay again for the chloroplast it's the thylakoid space in the mitochondria it's the intermembrane space. Protons moving across this membrane generates ATP. So that brings us to this last figure of the light reaction and chemiosmosis all in one. Again, we're only talking about this half of photosynthesis today. So let's summarize. Light is captured by pigments in photosystem II. It bounces around like a beach ball. And P680 gives up its electrons to the final electron acceptor. And P680, when it's oxidized, is an extremely powerful oxidizing agent. When P680 becomes oxidized, it's really wanting to look for electrons to balance that out. That's where water comes in, where water is split by an enzyme, and electrons are given up. And captured by the 680. And this is a source of oxygen, the byproduct of photosynthesis, and that'll go out the stroma. And of course the protons from the water is going to be the source of some protons in the thylakoid space. Now the electrons are going to hand it down the electron transport chain down to photosystem 1, to the, and it'll bounce around again, to P700 chlorophylls, and it'll use another photon of light, the energy from it, to hand that electron to the primary electron acceptor, and the electron transport chain will eventually, with the use of this enzyme called NADP plus reductase, to reduce NADP into NADPH. where now the NADPH is going to then enter the Calvin cycle, which we're going to talk about in the next lecture. Between the photosystem II and photosystem I, the electron transport chain is actually going to pump protons into the thylakoid space, again, making the inside acidic, and the ATP synthase, the same molecule that we found in mitochondria is going to use this proton gradient to drive the synthesis from ADP into ATP. And the ATP is also, again, fed into the Calvin cycle because carbon fixation actually requires ATP molecule. Okay? So this is everything that we talked about in a big, grand picture. All right, that's it for today.

Ototrophs are self-feeders. They sustain themselves without eating anything derived from other living organisms. and they produce organic molecules from carbon dioxide.

Biologists call autotropes the producers of the biosphere. Here are some examples of autotrophs on the left. Plants. Nearly all plants are autotrophs.

Algae. Protists. Prokaryotes.

There's algaes. Protists. Some cyanobacteria. And the defining feature is that they use light as a source of energy.

Heterotrophs, to which we belong, are unable to make their own food. There are other feeders unable to make food, and we eat other organisms to sustain ourselves. In this lecture and in the following lecture, we are going to focus on ototrophs and how they use light as a source of energy in a process called photosynthesis.

Let's talk about where actually photosynthesis occurs in plants. So here is a leaf of a plant and it's green and that's a telltale sign of chloroplasts where photosynthesis occurs. So chloroplasts exist in all green parts of a plant.

And there are about a million chloroplasts in a little chunk of leaf which has a surface area of about a square millimeter. Leaves are chocked full of chloroplasts. And chloroplasts are found mainly in the mesophyll, the tissue of the interior of the leaf. Mainly found... the mesophyll right here the inside is called a mesophyll and there are stomata these little openings on the bottom of the leaves to allow CO2 to enter and oxygen to leave during photosynthesis.

and it can open and close. Veins are what delivers water from the roots to the leaves. But it is also the system where sugars are actually exported to the roots in other non-photosynthetic parts of plants. And you might know this, to make maple syrup, one actually taps into these veins to draw the sugars that the maple tree has made.

And it's stored in the roots. And in the springtime, it shoots it up to the branches so that it can grow leaves, so that photosynthesis can happen. So the green cells in the mesophyll are called mesophyll cells. And in them, you find chloroplasts, where again, photosynthesis occurs. There are about 30 to 40 chloroplasts per mesophyll cell.

And chloroplasts, like mitochondria, have two membranes. And they're filled with what's called stroma. And there, inside the chloroplast, there is a third membrane system called a thylakoid.

Okay? These thylakoids are the third membrane system. mitochondria only have two, right? And the space inside is called thylakoid space. And these are usually stacked, as you can see, like poker chips, but they're stacked and the stacks are called grana.

And it is in the thylakoids where this green pigment lives, which gives the plants its green color. And those pigments are called chlorophyll. They're the green pigments in the thylakoid membrane. And it's the chlorophyll that captures the energy from light. So let's talk about photosynthesis.

As you might have guessed, photosynthesis is also a redox reaction. And the general process of photosynthesis is energy from light plus six carbon dioxide and six water molecules results in a glucose molecule and byproduct of photosynthesis six oxygen. And the carbon from carbon dioxide gets reduced.

It accepts the electrons. And where do the electrons come from? Well, they come from the splitting of water molecules. So water, or the oxygen in the water, becomes oxidized.

If you noticed, this is almost exactly the opposite electron flow from cellular respiration, right? Where glucose in the presence of oxygen yields or gets oxidized, meaning gives up its electrons, to become the carbon dioxide, and the oxygen molecule gets reduced to form water plus energy. Remember, the oxygen is the final electron acceptor down the electron transport chain.

So in cellular respiration, the direction of electron flow yields energy, right? Therefore, reversing the electron flow in the case of Photosynthesis requires energy. Therefore, energy is on this side. And where does it come from?

It comes from light. So let's start talking about the two stages of photosynthesis at a very general level. The two stages of photosynthesis are what's called light reaction and the Calvin cycle.

The light reaction is the steps of photosynthesis that convert solar energy to chemical energy. This is where water is split and O2 is a waste product. As you can see water is brought in, it's split, and we have waste product of oxygen. And light is absorbed by the chlorophylls, and that derives the transfer of electrons and hydrogen to NADP+.

The electrons are handed off to NADP+, to make into NADP+, to convert into NADPH. Chlorophyll. transfers electrons to NADP plus to make NADPH.

And NADP stands for nicotinamide. adenine dinucleotide phosphate. It's almost exactly the same as NAD plus in cellular respiration, but it just has a phosphate group attached to it. And this is also a step where ATP is generated.

But more importantly, no sugars are made here. No sugars made. And as the picture depicts, it occurs in the thylakoid. The Calvin cycle, the second part of photosynthesis, incorporates CO2 from the air into organic molecules.

And this step is called carbon fixation, because it takes carbon from the air and fixes it into complex molecules. And it does it by reducing carbons to carbohydrates by adding the electrons. Reduces carbons.

carbohydrate sugars by NADPH. So NADPH is the electron source where it brings it into the Calvin cycle by where carbon dioxide or the carbon is fixed into becoming sugars. And ATP is required to fix carbons as well. And this cycle is also called the dark cycle because it doesn't require sunlight directly to run.

It does not require sunlight directly to run. But it doesn't mean that it happens at night, right? That's not what the dark cycle means. And this occurs in the stroma of the chloroplast, as this depicts in the stroma of the chloroplast. And in this lecture, we're going to talk about the light reaction.

And in the following lecture, we're going to talk about the Calvin cycle. Okay? So in order to start talking about the light reaction, we need to first start by talking about the absorption of light by chlorophyll.

Okay? So the molecules that actually absorb light are called pigments, photopigments absorb light of certain wavelength. And plants are green because that is a color that is reflected or transmitted through the plants.

So the photopigments in plants are chlorophyll that are in the membrane of the thylakoids. And they use all wavelengths of light except for green. What does that mean?

Well, we can actually generate what's called an absorption spectrum. Absorption spectrum for chlorophyll, and there are different types of chlorophylls, but it's the graph plotting absorption versus wavelength chlorophyll a is the primary photo pigment that participates in absorbing light there are other pot of pigments in the leaves But this on the y-axis is the absorption of light. And let's just focus on chlorophyll A for now. As you can see, the wavelength of light right around the purple-blue area is absorbed really high. But as soon as you reach the green wavelength...

there's very little absorption and the absorption again picks up again in the red spectra so chlorophyll a which directly participates in light reaction of photosynthesis. It shows that the violet blue spectrum wavelength and red light work best for photosynthesis. since they are absorbed and green is the least effective color. When we actually see a color, like a red car for instance, the pigments in the paint in the red car absorb all other wavelengths of light except for red.

So it's a reflecting red color. That's why we see it as red. Chlorophylls are exactly the same.

It's absorbing every single wavelength except for green. That's why we see chlorophyll as green. Okay? and the action spectrum confirms this, confirms this observation.

As you can see, here's the rate of photosynthesis on y-axis. It's high and it dips for green and comes back up for red. So photosynthesis is high in blue-purple and in red, but extremely low in the green. Okay, action spectrum. And this is what chlorophyll actually looks like at the molecular level.

There is a light absorbing head of the molecule called the porphylen ring. There is a magnesium ion right in the middle of it. The two different flavors of chlorophyll differs only in this one molecule, right here.

In the second part of this chlorophyll molecule, there is a hydrocarbon tail And this is what interacts with the proteins inside the membrane of the thylakoid. It buries itself into the membrane. Okay, light absorbing and membrane tethering. So how is the energy from the light actually captured by this molecule?

It's captured by this molecule by exciting the electrons in the chlorophyll. Okay, so here's... cartoon picture of a photon of light coming captured by the chlorophyll molecule and the electrons in its outer shell gets excited and it jumps up absorbing the energy jumps up to what's called an excited state And really quickly afterwards, these electrons decay back down, back down to the ground state, releasing the energy in both the form of heat, as well as in a photon of energy, fluorescence energy.

So let's write that down. Absorption. of light energy, electrons are excited and jump up to a higher orbital.

into what's called an excited state. But in a billionth of a second, electrons come back down to what's called a ground state. Billionth.

of a second and when it loses that energy it emits energy in the form of heat and light Right? This is why cars, if left out in the sun, gets hot. The pigments get excited, and then when they relax back down to the ground state, it's releasing energy. It becomes hot. That's why white cars...

are cooler to the touch than black cars because white reflects all visible spectrum, black absorbs all visible spectrum of light. Anyways, so when it relaxes, it releases photons, like I mentioned, right? Light.

And this is how fluorescent light bulbs work. And here is an artificial preparation of a beaker full of chlorophyll. If you hit it with UV light, ultraviolet light, it fluoresces in this red-orange color. And it's this what is being seen here. But of course this is an artificial system, right?

It does no use for a plant to be fluorescing. So in vivo, or in the actual plant, in live preparation, this energy is captured by other molecules. in what's called the photosystem.

Okay? So this energy is captured. So what does a photosystem look like, and what do they do?

The photosystem, which is this entire thing, is composed of the reaction center complex and the light harvesting complex composed of reaction center complex and the light harvesting complexes. So the photopigments, the pigments in the light harvesting complexes, capture the energy and that electron is passed from pigment to pigment to pigment until it finds a special pair of chlorophyll molecules, which then ultimately hands off that electron to what's called the primary electron acceptor. So the pigment captures the photon and energy is transferred from pigment molecule to pigment molecule.

Much like a beach ball in a concert. And then ultimately it is passed to the reaction center complex, which captures the electrons. and becomes reduced. This is called the primary electron acceptor. And of course this is a redox reaction.

because electrons are being passed, becoming reduced. And this is the cartoon version up here, and this is the crystallized structure of the photosystem 2. You can see in the red reaction center complex and surrounding it is going to be the light harvesting complexes. And there are two types of photosystems. There are photosystem 2 and photosystem 1. Two types of photosystems. PS2 and PS1.

And PS2 actually functions first in the light reaction, but it's called PS2 because it was discovered after PS1 was. So the numbering scheme is historical. And this is called the linear electron flow. In a bit we'll talk about a circular electron flow, but this is how majority of photosynthesis occurs. How it occurs.

So step one, the light hits the photopigment in PS2. PS2. And then the electrons get to pair a specialized chlorophyll a called p680.

p680 chlorophylls. Then step two electrons are transferred. from the excited P6AD to the primary electron acceptor. When P6D gives up its electron, P6AD becomes oxidized. to make P680+.

P680+. The oxidized form of P680 is one of the strongest biological oxidizing agents known. So it wants to grab electrons from anything possible.

And that helps, actually, this third step of splitting water and grabbing a pair of electrons. Water is split. by an enzyme to give p680 its electrons.

Strongest biological oxidizer. And then. The primary electron acceptor hands the electron down the electron transport chain, much like cellular respiration. Electrons are passed along the electron transport chain. to cellular respiration.

And each time the electron is transferred down, and this exergonic fall of electrons from high energy to low energy drives protons to be pumped. And that, just like cellular respiration, yields in ATP production. So, exergonic fall of electrons to lower energy state produces ATP via proton.

being pumped into the thylakoid lumen. And this ATP production is called chemiosmosis. Right, ATP is being made. And finally, instead of oxygen being the final electron acceptor in cellular respiration, these electrons actually Enter photosystem 1. Electrons passed to photosystem 1 and P700 flavor of chlorophyll. And then, with the help of PS1 capturing another photon of light, it is able to then donate electrons to its primary electron acceptor.

And passes it to primary electron acceptor using... energy from light and then the electron runs down another set of electron transport chain to to NADP reductase, which is an enzyme that transfers electrons to NADP to produce NADPH. So seven is electron transport chain.

Last but not least, electrons transfer to NADP plus to make NADPH. And this molecule is then, as you can see here, it's fed into the Calvin cycle to help fix the carbons from the atmosphere. This seems very complicated, but essentially what it's doing is that it's using a photon of light in PS2, Photosystem II, to energize, to excite the electrons, and as the electrons lose energy, use that to synthesize ATP, and then Photosystem I captures another photon of light to excite that electron up And then that is used to then synthesize NADPH.

So I think this picture does, explains this very beautifully. So this is what's called a linear electron flow. It's going from PS2 going on to PS1. Linear. There's also what's called a circular electron flow.

A circular electron flow does not use photosystem II and no NADPH is made. just ATP. And you can see from this figure right here, the electron transport chain is coming down, bringing the electrons down to photosystem I, where the primary electron acceptor gives it back to the electron transport chain between photosystem II and photosystem I.

So the electron just flows in a circle, only producing ATP. So this is a very specialized kind of photosynthesis, right? And there are mutant plants that cannot do this cyclic electron flow. which gives us a good idea of why such a process even exists.

These plants actually do not grow well in places where there's intense amount of sunlight. which suggests that the cyclic electron flow may be photoprotective to these plants, which can do it. And this process of cyclic electron flow will become important when we talk about a particular adaptation of photosynthesis, called C4 photosynthesis.

It will become important. I'm talking about... C4 plants.

And we'll talk about C4 plants in the next lecture. So let's talk about this production of ATP for a moment. Remember that it's called chemiosmosis.

Let's compare cellular respiration and photosynthesis. They're very similar. Proton gradients formed by electron transport chain.

It uses proton gradient to generate ATP, but there are slight differences. And that arises from which compartment where protons are being pumped into? In the mitochondria, protons are pumped into the intermembrane space, right? Mitochondria protons pumped into intermembrane space. In the chloroplasts, protons are pumped into the thylakoid, called the thylakoid space.

Right, as you can see from this cartoon drawing, the higher pH, the more acidic area where the protons are being pumped into, is shaded dark. So the intermembrane space is dark here. In the chloroplast, instead of the space between the membrane, because it has a third membrane system, it acidifies the thylakoid, the inside of the thylakoid, called the thylakoid space.

And then the rest of the same. The electron transport chain is going to move protons from area of low concentration to area of high to make it acidic. And then this chemiosmosis. is a process by which protons moving back down as electrochemical gradient using that to generate ATP using that as an energy source.

Okay again for the chloroplast it's the thylakoid space in the mitochondria it's the intermembrane space. Protons moving across this membrane generates ATP. So that brings us to this last figure of the light reaction and chemiosmosis all in one. Again, we're only talking about this half of photosynthesis today. So let's summarize.

Light is captured by pigments in photosystem II. It bounces around like a beach ball. And P680 gives up its electrons to the final electron acceptor. And P680, when it's oxidized, is an extremely powerful oxidizing agent. When P680 becomes oxidized, it's really wanting to look for electrons to balance that out.

That's where water comes in, where water is split by an enzyme, and electrons are given up. And captured by the 680. And this is a source of oxygen, the byproduct of photosynthesis, and that'll go out the stroma. And of course the protons from the water is going to be the source of some protons in the thylakoid space. Now the electrons are going to hand it down the electron transport chain down to photosystem 1, to the, and it'll bounce around again, to P700 chlorophylls, and it'll use another photon of light, the energy from it, to hand that electron to the primary electron acceptor, and the electron transport chain will eventually, with the use of this enzyme called NADP plus reductase, to reduce NADP into NADPH.

where now the NADPH is going to then enter the Calvin cycle, which we're going to talk about in the next lecture. Between the photosystem II and photosystem I, the electron transport chain is actually going to pump protons into the thylakoid space, again, making the inside acidic, and the ATP synthase, the same molecule that we found in mitochondria is going to use this proton gradient to drive the synthesis from ADP into ATP. And the ATP is also, again, fed into the Calvin cycle because carbon fixation actually requires ATP molecule. Okay? So this is everything that we talked about in a big, grand picture.

All right, that's it for today.

Transcript for:Understanding Photosynthesis and Its Processes

Transcript for:
Understanding Photosynthesis and Its Processes