Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

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Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.


Most biology textbooks that I have seen, often have a diagram that look kind of like this. where they have some kind of warm and fuzzy picture where there is a plant and it's giving off the glucose and oxygen and, here is the fuzzy bunny. And he is eating the glucose in oxygen and breathing out the carbon dioxide and water. Creating this nice lovely cycle of life kind of thing.

Thing is this really implies that the plants are doing this for the rabbit. But would they really want to? No, because we eat them. If you’ve ever given somebody a bouquet of flowers, remember that’s a bundle of several plant genitalia. And if you eat corn on the cob, you are eating hundreds of boiled corn babies.

Would you be feeling kindly towards any creature that did that to us? No. So why are the plants doing those whole photosynthesis thing? They are doing photosynthesis in order to get the energy to make the building materials, the glucose, etcetera, so that they can make the building materials they need to make stuff like the cellulose, cell wall, etcetera. Then they are also using it to get the energy that they need to stay alive, and they will then store that starch down in their roots.

You got to remember photosynthesis is one of the key processes in biology. It’s the main way in the ecosystem that new carbon is added into the system, to make the organic molecules that all living things depend on. Whether it's a tree needed an organic molecules to build new branches, or animals like us needing to make hair of skin cells, so that we steal it from things like plants when we eat their babies.

Plants also do photosynthesis in order to gain more energy for themselves. And also because all of these animals keep eating them, it adds more energy to the ecosystem. So like I said it's one of the key things in biology and that’s why out of the 12 official AP labs, two of them are all focused on photosynthesis.

So you really got to get this in order to do well on the AP test. So what I’m going to do is, I’m going to begin with going over the structure of the chloroplast, the organelle in the cell, that does photosynthesis. As well as going over a little bit of what the heck is energy. And I’ll over view the two steps of photosynthesis.

Then I’ll go into depth on that first step which is called the light reactions, which is all about gathering in the energy. Then I’ll finish off by going through the last step of photosynthesis, which is called the Calvin cycle. Which is using that energy to grab carbon dioxide from the air, and squish it together to form glucose.

Before I get too into this, I need to make sure that you understand what energy is. Scientists define energy as the ability to do work or cause a change. That means that energy is this kind of nebulous thing. But it can be stored in lots of ways. Energy can be stored in something's position, how high it is. The higher I lift this ball, the more energy it has.

I can store energy in stretching something or putting pressure on it. So I’ve stretched this and now I’m going to transfer some of the energy from the stretched spring to the pencil, to create a change. I can even store energy, or there is energy stored in molecules in their bonds. And we can just release that energy in all sorts of ways.

Now again how is energy stored in molecules? It’s stored in the bonds that make up the molecules. And you guys know that electrons are what make up bonds of chemicals.

So there are electrons in high energy bonds that are called high energy electrons. And there is electrons that make up low energy bonds, and those would be low energy electrons. To give you an example of a molecule that has fairly high energy bonds, wood. If I burn wood, it turns into carbon dioxide gas and water and the bonds of carbon dioxide have low energy bonds, low energy electrons. And that’s because the energy is being transferred to make the heat in the flame or the light that you see when you burn wood. So that under bonds let’s go deeper into photosynthesis.

So plants do photosynthesis, as well as all other sort of other photosynthetic creatures like algae and bacteria. They all do photosynthesis to get the building materials that they need to make their cellular organelles and other organic molecules, as well as to get the energy that they need to stay alive by minute, second by second. They do that using photosynthesis.

What goes on during photosynthesis can be summarized in this equation. Here you see the energy of light comes in. What do the plants use that for? They use the energy of light to force these carbon atoms, from carbon dioxide together, to form a six carbon molecule called glucose. C6 H12 O6. They use the hydrogens that are from the water to make the 12 hydrogens that you see there in glucose. So 6 Carbon dioxides plus 6 waters, using the energy of light, forms a glucose molecule.

It takes 6 carbon dioxides in order to make that one glucose molecule. And when this happens, you wind up because you are also using 6 waters. Take a look at all those oxygens, you have a bunch of left over oxygens. So the oxygen gas that’s given off, is not something that the plant is doing on purpose to provide the oxygen that we need. In fact, when photosynthesis is first evolved, it was kind of just an unfortunate by-product, it’s kind of like you are fatting.

How can you remember it's 6 plus 6 1 and 6. When I was first teaching this, I had a kid who was a little bit odd and he said, "6-6-6 ah plants are the antiChrist." I don’t really think so, but if it helps you remember this, then let’s go forward. So 6 carbon dioxides plus 6 water, plus the energy of light, gives you a glucose molecule and 6 oxygens.

Now this organelle that does this in the plant, is called the chloroplast. You need to know the basics of the chloroplast structure for the AP bio test. Now it has an outer membrane and an inner membrane. When you have this kind of double membrane structure that’s called an envelope. Within the main portion of the chloroplast, you also have a series of a third layer of membranes, that are folded over to form this flattened disks. Each flattened disk is called a thylakoid. So the thylakoids what they do is, by having all these folds, they're increasing the surface area to the volume ratio, that allows the plant to have more surface to absorb the sunlight energy.

Now you’ll see that the thylakoids are arranged in stacks. One stack of multiple thylakoid membranes, is called a grana, or a granum grana is plural. Each thylakoid has inside of it a space called the lumen. And the lumen of each thylakoid is actually connected to the lumen of its neighboring thylakoids. In fact one granum's lumen is connected to the lumen of a neighboring granum by these neighboring connections called lamellae. Outside the thylakoid you have the space here called the stroma, and that’s where a lot of chemical processes happen.

Let’s go into the steps of photosynthesis and take a look at the first half of photosynthesis called the light reactions. So in the light reactions, what’s going on is, see here we have a thylakoid membrane.


Embedded within the phospholipid bio-layer of the thylakoid membrane you have a bunch of special proteins and other molecules. And their job is to grab the energy of light. And then they use that energy of light and they give it to some of the electrons that make up a molecule called chlorophyll.

That chlorophyll's electrons have so much energy now that that they can leave the chlorophyll, and be passed on to the other members of the special proteins, etcetera, that I mentioned in the membrane. When that happens, they need to get replacement electrons, and they get that from water. Ultimately the energy of those electrons is used to do two things.

One thing is to suck in hydrogen ions from this stroma, that surrounding fluid, and shove it inside the lumen of the thylakoid. Ultimately, that high concentration of hydrogen ions will force its way out through a special molecule called CF1 particle on ATP synthase. Which is simply a molecule, a specialized enzyme that can use the energy of these hydrogen ions going out, to synthesis ATP. And that’s what we see here.

The other use of these high energy electrons, is that they are put on to a career called NADP+. The NADP+, when it gains the electrons, gathers a couple of hydrogen ions, or gathers a hydrogen ion becomes an NADPH.

So ATP is carrying the energy that used to be from the high energy electrons, and NADPH is also carrying high energy electrons. So here we have two electron molecules. Now again, the water is used to provide replacement electrons since they are going away on the NADPH. And when you break apart of the water, you ultimately fart out some oxygen gas.

ATP is the energy currency of the cell. It’s adenosine molecule a kind of DNA on it nucleotide with 3 phosphates.

Each phosphate has a negative charge, so they repel each other. So to get them close to each other, bonded to each other, takes a lot of energy. Just like when I was setting that rat trap, it took a lot of energy and that energy is stored in the molecule.

And ATP like I said, is like the energy currency of the cell. It can be used for lots of things in the cell. Making proteins, pumping things into the cell or out of the cell, whatever the cell wants to do, it essentially pays for it using ATP. It’s like $1 bills.

NADPH also is carrying energy but it’s not used in as many processes as ATP is. So you may be thinking, why bother going through this next step when you’ve got your energy? Well the thing is, when can the plant do this? Only when there is light. Night time, light goes away. If the plant doesn’t have some way to get the ATP every second of its existence, it’s going to die. And that’s why we are going to go through the Calvin cycle. That’s a way of using the energy you have, to build a larger molecule that can store that energy.

Imagine ATP was a $1 bill, you go to work and you get paid 1000 dollars. You have 1000 $1 bills. If you want to keep that around for longer than it takes for the people to rob you of it, you want to deposit those 1000 $1 bills into the bank or get say several hundred dollar bills. And that’s the function of the Calvin cycle. It takes the energy in this immediate form, and translates it into longer term storage process.

So let’s take a look at what happens in the Calvin cycle. What the Calvin cycle does is, it uses the ATP’s energy and the high energy electrons being carried by the NADPH, to force carbon dioxide molecules together to fit into carbohydrates sugars like glucose, as they stay a typical example.

So you can kind of think of it as ATP is paying their enzymes as you do these process. While the high energy electrons that are being carried by NADPH, are the nails that are nailing together the carbons together to form these long chain molecules, like glucose or rather carbohydrates. Once the NADPH is dropped off its electrons that the hydrogen is carrying, it goes back immediately as NADP+. And since its in the stroma, the liquid that’s right outside the thylakoid, it's right there. And it can pick up more electrons and deliver.

I sometimes call NADP+ an electron taxi because it’s just taxing or carrying the electrons to the Calvin cycle. And the ATP breaks apart into ADP and Phosphate ion and goes back and they get recharged, and keeps cycling back and forth. That’s photosynthesis.

With that big picture or view in mind, let’s now dive in to the details of the light reactions. So we are going to take a look at the thylakoid membrane structure right now. So if you take a look at the thylakoid membrane, again you see that phospholipid bio-layer that’s common to all membranes in the cell.

And embedded within the thylakoid membrane, are these clusters of proteins and other molecules. These two here are called Photosystems. They are large massive collections of specialized pigments or molecules that can absorb light, called primarily chlorophyll. And there is other kinds of pigments besides chlorophyll, they are typically called accessory pigments.

In plants, the major pigment is called chlorophyll A. There is other versions of chlorophyll obviously called chlorophyll B or chlorophyll C. Some goes of those accessory pigments are called things like carotenes or xanthophylls or urathrophylls. In the Photosystems I and II, unfortunately they are not named in order of use, but they are named in order of discovery.

You will see these little blue things those represent those pigments. There will be in the centre, something called the reaction center. All the other pigments around it are called antenna pigments. Their job is to harvest light, to gather in light energy. Because you have different kinds of accessory pigments, that makes the Photosystems able to range or absorb a broader range of colors of light. Photosystems to reaction center has added central position its reaction center can best absorb light that’s at a wave length of 680 nanometers. That’s why it’s called P680, that reaction center.

Whereas Photosystem I’s reaction center absorbs 700 nanometer light best. So it’s called P700. These things called PQ and PC, these are special proteins that are embedded within the membrane, that can carry high energy electrons from the Photosystem here, Photosystem II and ultimately transfer to Photosystem I. Because they form this electron transport system, they are called non electron transport system, or sometimes called electron transport chain. You can think of them essentially like wires, that conduct up electricity or electrons from one place to another.

This thing in the middle of the cytochrome complex is a special protein that can use the energy of electrons, high energy electrons that is, to shove hydrogen ions from one side of the membrane, to the other, from the stroma into the lumen. Kind of like your fan uses the energy of high energy electrons to shove air from one side of the fan to the other. It’s the exact same principle.

Over here we see a special protein called NADP+ reductase, which is that an enzyme that reduces NADP+. Because if you don’t know what reduction is, you need to know that there is oxidation and reduction, and they are all about what’s happening with electrons.

Oxidation is when a molecule loses electrons. While reduction is when a molecule or an atom, gains electrons. I know a bunch of you are saying, reduce means to lose, right? Why is it gaining? Blame Benjamin Franklin because he is the guy who kind of habitually said positive negative and he didn’t know that he’d become more and more negative reducing your charge, when you are gaining electrons.

So how can you remember that losing electrons means oxidation, and gaining electrons means reduction? Remember L.E.O Lose Electron, Oxidation. L.E.O is Leo the lion. So Leo goes Ger L.E.O lose electrons, oxidation, Gain Electrons, Reduction Leo goes ger will help you remember this. I’ve taken many years of chemistry as soon as somebody told that to me, it stuck in my head and I didn’t have to go through all the complex processes I come up in college to remember that.

So down here, we have a special molecule called ATP synthase. This entire thing here, this bob is pushing down there is called the CF1 particle. I think the C probably stands for chloroplast, and the mitochondria have very simple thing called the F1 particle.

So let's get started with the steps of the light reactions, sometimes called the light dependent reactions. In the first step we see here, Photosystem II and tender pigments absorb a full turn of light. A little chunk of light and they pass the energy of that light to the central reaction center the P680. And the energy of that light is used to throw off a couple of electrons. Those electrons have the energy, so we call them high energy electrons. And they are passed on to the electron transport system.

Now remember electron form bonds are what hold the atoms and molecules together. If Photosystem II keeps losing electrons, it’s going to fall apart. So it needs replacement electrons, and that’s where water comes in.

Water is made put of two hydrogen attached to a central oxygen. By breaking off the bonds, the electrons that was holding this hydrogen ions, those hydrogens onto the oxygen, they become hydrogen ions. And the Photosystem II can use those electrons, which are low energy electrons to replace the ones it just lost.

We are using this kind of brownish color to represent low energy electrons, and yellow to represent high energy electrons. When this happens, you wind up creating some hydrogen ions here inside of the lumen. And the oxygens ultimately form O2 gas as the oxygen from one broken water molecule, combines with a broken off oxygen, from another water molecule. And that forms O2 gas. O2 gas is small because each oxygen is equally electronegative, you wind up having a covalent bonds so it’s not polar.

So it just defuses out through the membrane. Membrane can’t stop it and it ultimately may leave the plant, or get used by the plant’s mitochondria. Now we started to build up some of the hydrogen ions here inside of the lumen. And remember that, because this is going to be important later. Just a little side note, in an AP biology essay, if you happen to mention this is called a photolysis. Photo means light, 'lysis' means to break. We are using the energy of light to break water. You mention photolysis, you got yourself one out of the ten points you were looking for. You're on your way to getting a good grade. And remember if you can get 3 or 4 out of 10 on an essay, you are probably in the range of getting a passing grade

So let’s take a look at what happens to the energy of this high energy electron. As it goes through the electron transport system here, we see this protein that goes all the way across the membrane, uses the energy of the electron, to shove another hydrogen ion from outside in the stroma, into the lumen.

So we are starting to build up inside of our lumen more and more hydrogen ions. And that again, is going to be important. So we're transferring the energy from the electrons to the changing position of the hydrogen ions. Now you notice that makes the electrons now low energy electrons. Just like if I drop a ball from up here, to the floor, it translates or transfers its energy to sound and other things, and now it’s a low energy ball.

Luckily, Photosystem II needs some energy electrons because it too will get hit by light. And so when light hits Photosystem I, you see the Photosystem I uses that energy to energize one of its electrons, actually a pair of electrons. And that pair of electrons are passed on to NADP+ reductase.

Now what does NADP+ reductase do? You can guess. It reduces, helps gain electrons, it reduces NADP+ into NADPH. Just again, to go to how to pump up your score on the essays, by mentioning all of these structures, you're probably going to get another 2 or 3 points. And again, now just without even remembering all the processes, just by describing the structures that are within the chloroplast, you are probably getting a passing grade.

So this is a lot easier than kids will make it put to be. So the high energy electrons from Photosystem I are used to reduce NADP+ into NADPH. One of the two things that the Calvin cycle needs to provide the energy to be glucose. And you can see the NADPH there is drifting away defusing away to the Calvin cycle in the stroma. Wait a second NADPH that has an H on it, where did that come from? Well see that we are giving electrons to NADP+ we give 1 to NADP+, and it becomes NADP with no charge.

The plus and the minus has been cancelled out. We actually give a pair of electrons, so it becomes NADP-. That attracts a positively charged hydrogen charging ion and it becomes the NADPH. It’s taking the hydrogen ion from outside in the stroma. We’ve been subtracting hydrogen ions from in the stroma. We’ve been pumping hydrogen ions from the stroma and to the lumen, and we're creating hydrogen ions in the lumen. This means that the concentration of hydrogen ions in the lumen, can reach upwards of 1000 times that which is outside in the stroma.

That’s a good thing that we are making this high concentration in the lumen. Because that high concentration of hydrogen ions, you guys know that means its acidic. It’s got a very low PH. And PH is one of the things that interferes with proteins or enzymes, if it gets too low. And so Calvin cycle which is depended on a bunch of enzymes won’t be interfered with.

So we build up this high concentration hydrogen ions inside the lumen. And we are going to use this to power the formation of ATP. To help you understand this, I’m going to use an analogy. This build up of concentration of hydrogen ions in the lumen is kind of like, me building a concentration of a molecule inside this balloon.

I have created a high pressure inside and outside, there is much lower pressure. We call a difference between two things, in science we call that a gradient. So this would be called a pressure gradient. The hydrogen ions were building up this pressure because there’s lots of them. But they are also, notice that they are positively charged. All of these hydrogen ions repel each other. And because we’ve been removing hydrogen ions from the outside from the stroma, the outside or the stroma, is kind of negative compared to the inside which is very positive.

So these hydrogen ions are repelling each other and they have been attracted to the outside. So there is this strong driving force. And we can use that.Just like I can use this driving force, the pressure gradient here, I can use to force this balloon from me to somewhere else.

We are going to use this difference in concentration of hydrogen ions, from one side of the membrane to the next, to create ATP. This difference in concentration of hydrogen ions across the membrane, is called the chemiosmotic gradient. So we are going to use this chemiosmotic gradient to make ATP.

So as the hydrogen ions go through this channel over here, the end portion has that enzyme I told you about before, an ATP syntase. What happens in reality, is as they got through this channel they force the channel to spin. And that spinning, that rotational movement, is used to move some proteins. And those proteins can grab adenosine di-phosphate there and a phosphate ion in the other protein, and then they ram them together. And the energy of their ramming together is transferred, is used to form the bonds between that third Phosphate. So now we have our ATP molecule.

And so the hydrogen ions going through. It’s much like if I had held that balloon next to a fan or something, I could let the air blowing out. Turn the fan, and I could use that fun to generate electricity. That’s how a windmill works.

And so now we are forming the ATP right outside the thylakoid membrane. And right outside the thylakoid membrane, we call that the stroma. There is the Calvin cycle and now we’ve made the ATP we need, and we’ve made the NADPH we need.

Now because we are using chemiosmosis to provide this ATP, this is called chemiosmotic phospholation. To phospholate something is to put a phosphate on it. And sometimes they could even call it chemiosmotic photophospholation or simply photophospholation, since light is providing the energy to do this phospholation. Now we are ready to go into the Calvin cycle.

Like I said, we are now ready to get into the Calvin cycle. We’ve maybe ATP, and we’ve made the NADPH, the Calvin cycle requires from the light reactions. Now we're in the stroma and floating around in the stroma are a bunch of enzymes. You don’t need to know all of their names except for one of them which I’ll get into in a moment.

So let’s take a look at what’s going on. So the first step of the Calvin cycle, and this is the key step of the Calvin cycle. And sometimes this is called carbon fixation, is when a carbon dioxide molecule. And we are representing just the carbons because they are the key thing. Is combined with a 5 carbon molecule called Ribulose bisphosphate.

The enzyme that does this, is the only one of the Calvin cycle whose name you need to know. And that’s Ribulose bisphosphate caboxylase. Which is a really big word but it tells you what exactly it does. Ribulose bisphosphate is combined with carbon dioxide. And I see the two Oxygens on it attached to the rest of the molecules is called a carboxyl group.

So we are doing carboxylation to Ribulose bisphosphate and 'ase' at the end means it’s an enzyme. So Ribulose bisphosphate carboxylase carboxylates Ribulose bisphosphate. The short name of Ribulose bisphosphate, and thank God there is a short name, is RuBisCo. And just think RuBisCo sounds like Nabisco what does Nabisco make? Carbohydrate cookies.

Rubisco makes carbohydrates. So RuBisCo combines the carbon from carbon dioxide together with the five carbons of Ribulose bisphosphate to make a 6 carbon molecule, that we call nothing. Because it immediately falls apart to form a three carbon molecule called phosphoglycerate. I could do this one carbon at a time, but you remember that I’m trying to make glucose. And how many carbons are in glucose? 6. So I’m going to add 6 carbons in this cycle to 6 Ribulose bisphosphate at one time.

Remember, in the stroma you’ll have thousands of these things happening every second. So don’t get too focused on the numbers. So here we have Ribulose bisphosphate combined with carbon dioxide to form phosphoglycerate. Now the phosphates that we have on either end of the Ribulose, 'bis' which means 2, phosphate, make it easier to do that, because phosphates each have a negative charge. And so those ends that are repelling each other, stressing the bonds at the curving chain in between.

Now you'll notice phosphoglycerate only has one phosphate on it. Why? 6 of these guys, 6 times 2 is 12. They have 12 phosphates, so there’s only 1 phosphate per glycerate that we’ve got. So if we want to do some changes here, how about we add the phosphate and that’s what we are going to do. We come along and we add the phosphate adenosine triphosphate. We reap off one of its phosphates, and stick it on to the phosphoglycerate to make this phosphoglycerate. And this makes it much easier to manipulate the carbons there. And that’s what we do now, when we go to glyceraldehyde-3-phosphate.

Now to make glyceraldehyde-3-phosphate we need to add in some high energy electrons. And that’s what we have on NADPH. So NADPH comes along, delivers its high energy electrons that knocks one of the phosphates off. And we have this three carbon molecule called glyceraldehyde-3-phosphate. You may be wondering, what’s the difference between glyceraldehyde-3-phosphate and phosphoglycerate? There is a bunch of other atoms involved here; oxygens and hydrogens. But I don’t care and the AP Bio people don’t care that you know the difference between them structurally. You just need to know their names.

And to make your life simpler, you can even refer to glyceraldehyde-3-phosphate simply as G3P. And phosphoglycerate as PG and biphosphoglycerate is BPG. I’ve also seen it referred to as DPG for di and save that.

Now glyceraldehyde-3-phosphate is an extremely useful molecule. It can be used to make lots of things. In fact you break off that phosphate and you can make yourself the glycerol backbone of a triglyceride. You can use glyceraldehyde-3-phosphate or G3P to help make amino acids. But if we take two of them and ram them together, you can get yourself glucose. So let’s take a look at that.

If we plot two 3GP’s that gives us glucose. Hey we’ve got 12 of them, why don’t we just use all of them and make ourselves a ton of glucose, because that’s the who point right? That will be kind of like if you had a lemonade stand, and you spent one day, you bought say $10 worth of lemons and sugar and stuff, and you made yourself. "Wow I’ve got a ton of money, I’ve got $10 yay!" Then you go, and you spend it all on movie ticket and maybe one piece of candy ticket prices nowadays.

Well, that means that you are out of money and you can’t keep doing your lemonade stand. Instead what you should do is skim off the $2 profit, and that’s what’s going on here. We are skimming off the profits because of the plant. Goes to here, we’ve not got a cycle, and we want to cycle because this allows the plant to continue doing this process over and over.

So how many G3Ps would we have left? We took out 2 from 12, that leaves us 10. Now if you’ve been falling down with the math, you can see that 103GPs, I see here these are 3 carbons and that’s a 5 carbon. That’s kind of complicated, how do we do that? Because each of these steps 5 plus 1 equals 6, divide by 2 that’s 3, everything is fine. Here we have 10 going to 6 that’s not easy math.

I can show you the process, and if you are really curious about the process you can read some really challenging complicated textbooks. But unless you have a Bachelors of Science in Biology or Biochemistry, you don't need to know this. In fact the AP biology people don’t want you to know this. So instead, magic happens. I mean a bunch of enzymatically catalyzed reactions occur, and we convert the 103GP’s into a Ribulose bisphosphate molecules here. You notice, we are going to need some phosphates to do this, and that’s where the last of the ATP is used.

So the ATP drops off some additional phosphates, and now we are ready, and we've got ourselves back at the beginning. So this is a cycle. Every time you add in the 6 carbon dioxides, you ultimately pop out a pair of G3P’s, which can slam together to form the glucose that you want or whatever other organic molecules you need.

We’ve now used up the ATP that came out of the light reactions, and they go back as ADP to get recharged. And we’ve delivered the high energy electrons from NADPH, and then the NADP goes back and goes to the light reactions to get recharged. That’s the Calvin cycle.

I’d advice going through some of the animations that in your bonus materials folder and watching a couple of times. You'll start to understand this stuff a little bit better, and you will realize it’s not hard. There is also work sheet in your bonus materials folder that will take you through a number of websites, that will go through the logic of this whole process. And also help you learn some of the details that I've kind of glossed over.

Again, if you can get the underline logic of this, it’s not that hard of the process. So let’s go through that one last time. So with photosynthesis, the light reactions are all about grabbing the energy of light, using that energy to create high energy electrons.

And this is something that I’ve seen in the Rubrics for the AP biology essays, using sunlight energy to transfer into chemical energy. So you are making high energy electrons. The energy of those high energy electrons, is used to pump hydrogen ions into the lumen, creating that chemiosmotic gradient. The high energy electrons are also put on to NADP+ reducing NADP+ into NADPH.

The chemiosmotic gradient is also used to phospholate, using some of those vocabulary words, to phospholate adenosine di-phosphate into the adenosine tri-phosphate. The ATP and then NADPH are then sent off to the Calvin cycle. To get replacement electrons, water is broken to provide hydrogen ions and also those low energy electrons. This process is, remember, photolysis, you got yourself a point on the AP Bio essay and that generates O2 gas.

So we’ve made through chemiosmotic, phospholation, another point in the essays. To make our ATP and NADPH, that goes off to the Calvin cycle. Using the energy of the ATP, and the high energy electrons being carried by electron taxis, the NADPH, the Calvin cycle goes through the process of carbon fixation. Got yourself another point on the AP biology essay.

And we are shoving carbon dioxide onto Ribulose bisphosphate using RuBisCo. Got yourself another point on the essays. And we go through the Calvin cycle harvesting out every time we go through it, we skim off a few of those G3P’s that are being produced, to produce our carbohydrates or glucose sugars.

The NADP+ now that it's delivered its electrons, goes back here to the light reactions to regain to be reduced yet again. And the adenosine diphosphate and phosphate ions go back to the thylakoid membrane to also be phospholated again. Each one providing what’s required for the other.

And that’s photosynthesis, it’s not that hard. Spend the time, go through this a number of times and you’ll get it. And once you get this, you’ll be able to ace that part of the AP exam. And again, it’s a big chunk of it, so master this, you’ve mastered the AP biology exam.

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