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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Photosynthesis vs. Respiration

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.


I'm going to start this off by breaking the Science teacher code of silence, on one of our favourite trick questions asked about photosynthesis and respiration. Kids have been taught since elementary school that plants do photosynthesis and animals do respiration. And the organelle of the photosynthesis is the chloroplast, and the organelle for aerobic respiration is mitochondria. So far so good, so what's the trick? Well the trick is to ask questions like this; Which of the following are found in plant cells, cell wall, mitochondria, chloroplast, centriole. Kids who have been paying attention in class go on, "Plants, they don't have bones, oh they have a cell wall. There's one and do photosynthesis or chloroplast, so it must be I and III." That's the trick.

Remember, photosynthesis is all about grabbing energy and using that to build sugar, to store the energy. Well, don't the plants want that energy? In order to break that sugar down, they have to do a aerobic respiration, so they actually need mitochondria. So the correct answer is C. This highlights one of the many reasons why kids often report to me that, photosynthesis and respiration are hard.

Now, not only are they going on at a microscopic level, so kids can't see them happening in front of them, but generally, teachers teach them separately. So it's really hard for kids to see the connections. They're really not that hard to understand, especially if you keep in mind the fact that they are very similar processes, just run in reverse. It's kind of like putting on your clothes, it's very similar to how you take them off.

So I'm going to start this off by doing a quick overview of photosynthesis, followed by an overview of aerobic respiration. Then I'll finish off by going through comparing and contrasting those 2 processes, because that's kind of essay questions that will typically throw kids for a loop on the AP Biology question or the essay question. So I'm going to get you ready for that.

In order for you to get this whole thing, I'm going to have do a little bit of Chemistry. But don't worry, I'm not going to push too hard. All I need you to know is that, things are made out of molecules. Molecules are put together by atoms. And if I pull some atoms off of the molecule, and use them to build new molecules, that's called a chemical reaction. Not tough. Now where I will get into a little bit of 'don't ask too many questions' is this whole idea of energy.

Now hopefully you know that, energy is simply the ability to work, or cause a change. Now there is many ways that energy can be stored, and one example of where an energy can be stored, is in a book. If you were standing next to a bookshelf and there is a book that was on a low shelf, versus a high shelf, which one would be more worried of falling on your foot. The high one, because the low one doesn't have a lot of energy to cause change, like rearranging the shape of your bones in your feet. Or if one that's up here if it falls, that has a lot more energy.

Just to make sure you're all clear on this, photosynthesis is a process done by plants, in order to grab energy from the air. And it's also done by other creatures like algae or photosynthetic bacteria. Now, the basic idea of this is that, it uses the energy of light to grab carbon dioxide from the air, and water and put them together in order to create some glucose and oxygen gas. I'm going to get all scientific on here and I'm going to put up one of those scientific reactions.

And here we see 6CO2 + 6H2O + light, yields C6H12O6 + 6O2. Now before you start thinking I'm in AP calculus, there is Math, it's not hard Math. First thing to do is notice, all the numbers are basically 6 or 2, or 6 times 2.

See here we have 6 carbon dioxides over here. Notice you have 6 carbons in glucose. We have 6 waters over here, and that's got 6 times 2, 12 hydrogen over there. We have 6 oxygens over here and that takes care of the 6O2s that were being brought in by the carbon dioxide. And we can use these oxygen here to make those 6 guys over there. Now, I was teaching this many years ago and a student who had watched one too many horror films was looking at this and going, "If 6 6 6, plans of the antiChrist!"

Now after all the button classes I've been forced to take in college, I'm no big fan of plans, but I've never accused them of causing doomsday. But if that helps, then hey, sure, why not.

So if you just rattled off this equation on an AP Biology essay about photosynthesis, you've already gotten yourself a point. And if you can get two or three more, you're getting an average of passing grade. But if you want to get more points, then what you need to do is you need to start talking about the structure of the chloroplast. Well let's take a look at that.

Here we see a chloroplast and it has a double outer membrane, with an inner third membrane ranging these little folded disks, called thylakoids. Now you'll see the thylakoids are arranged in these stacks called grana. An individual stack is called a granum. Around them, within the double membrane or envelope of the chloroplast, is the watery liquid called the stroma, which has obviously some water as well as various enzymes and other things floating around in it. And it's in those thylakoid disks where the first half of photosynthesis, called the light reaction, occurs. Let's take a look at that.

In the light reaction, it obviously requires light. And what happens is that, light hits the thylakoid membranes here this grana.

In the thylakoid membrane are embedded a number of different molecules, including something called chlorophyll. It's a plant pigment. Chloro means green, phyl means leaf. It's called that because it's found in leaves that are green. So this chlorophyll molecule absorbs the energy of light. And it absorbs so much energy that it gives to some of its electrons. And those electrons have enough energy to spin and fly away from the chlorophyll molecule. Those electrons don't just go shooting off as some kind of weird lightning ball, instead they are passed one to next along a chain of special molecules also in the thylakoid membrane, called the electron transport system.

The energy of those high energy electrons, is used to shelve hydrogen ions from the stroma, into the inner space of the thylakoid disk called the lumen. When those hydrogen ions ultimately pour back out of the thylakoid, they have to pay back the energy that was used to put them in ther. And they are used to create a energy molecule called the ATP, that's adenosine triophosphate. You make an ATP by taking an adenosine diophosphate, ADP, and the single phosphate ion, and you slam them together to get your ATP. So that's one of the products that we need.

The next thing that happens is even more light is absorbed. And that light is given to another chlorophyll molecule. And that electron from the chlorophyll goes off to a high energy electron carrier, called NADP+. We see here, the NADP+ comes along and it picks up the electrons that are being given off by the chlorophyll, and becomes something called NADPH. So it's reduced to NADPH. Now, you may be thinking chlorophyll is giving away a lot of electrons. Electrons are the things that hold atoms together, they form the bonds. If we keep giving away all the electrons that make up chlorophyll, why isn't chlorophyll falling apart. That's a good question, it needs to get replacement electrons.

What's very obviously present in here, in all living cells there is a lot of water. So what the plant does, is that it uses water to get those replacement electrons. Remember, water is H2O, there is a pair of hydrogen attached to an oxygen. You pluck off the bonds that were holding them there, take away the electrons, give them back to chlorophyll. Chlorophyll is fine, but the water is now broken apart and it forms oxygen gas. Where is the hydrogen? There is my hydrogen being carried away.

So, what happens next is now we've got the energy temporarily stored in ATP, and the high energy electrons being carried by the NADPH, kind of like a high energy electron taxi. They go do a process called the Calvin Cycle.

Now the Calvin Cycle is a series of biochemical reactions occurring in the watery stroma of the cell as enzymes, using the energy provided by ATP and using the high energy electrons being provided but the NADPH. They grab carbon dioxide from the air, and assemble it into sugars like glucose. And that's what we see here. Carbon dioxide comes in and carbohydrates come out. So you just can keep doing this process. Now, once this has happened, what happens to the used up energy of the ATP and the high energy electrons? Well the energy is now in the glucose. If this processes keep happening though, what comes out? Well let's take a look.

We see here that the ADP and the phosphate go back to the light reactions, and the NADP+ goes back also. And so you can just keep cycling, energy is absorbed and delivered here. Then that energy is used to create the sugar that stores that energy. Not only is it useful for storing energy, but glucose is also a very handy building material for the cell.

So now that we've seen how energy is used to make glucose, let's take a look at how we can then get that energy back out by breaking apart glucose. So aerobic respiration is all about how you break apart that glucose, using oxygen in the air. Now, pretty much every kind of cell that's on this planet, does respiration in one form or another. So, there is things like bacteria, protista, plants, the fungi. And we animals, we can all do aerobic respiration. Be aware there is a few things that do specialised version of this called anaerobic respiration. But I'm going to focus on those of us who can do aerobic respiration, which is the majority of creatures on this planet.

Let's take a look at its chemical equation. And here you see this looks pretty familiar. Remember previously with photosynthesis, we had carbon dioxide, water plus energy, as the reactants, now they are the products, the stuff produced. Over here, we have glucose and our 6 oxygen molecules, which are now the reactants, where before were the products. So again, if you just remember that they are the same things just running reverse, your life will be simpler. Now of course there are some details that are a little bit different. We do aerobic respiration so we can do things like walk talk or hug or think. We don't do it to glow. So in photosynthesis, the energy is in the form of light. Here it's in the form of ATP, which can be then used by the cell to do anything it wants, like protein synthesis or sending signals.

Let's go through the steps of aerobic respiration, now that we've over viewed the equation. The first step in aerobic respiration, is glycolysis. If you look at the word glyco means sugar, lysis means to split and this is what's happening. We take our glucose and we split it in half. Now to do this, this is happening in the cytosol or cytoplasm of the cell. The watery liquid which fills the cell inside the cell membrane.

There, enzymes are used to put some energy in to glucose, that destabilises it and then some other enzymes can break apart the glucose, ultimately creating a pair of three carbon molecules, called pyruvic. Again glucose has 6 carbons, we snap it in half, and we now have our 2 pyruvics.

When you do this you get out some ATP. We spend two you get out four. We also are able to take a couple of high energy electrons that were in the original glucose out, and put them on to another high energy electron carrier, some of the NADP+ called NAD+. And this forms NADPH.

Next, we take that pyruvic and we go off to the mitochondria. Now let's take a quick look at the mitochondria so that we can make sure you understand its structure, much like you understand the structure of the chloroplast. The mitochondria, much like the chloroplast, has more than one membrane. It has an outer membrane it's sort of an envelope like the chloroplast, and then a folded inner membrane. Much like the folded thylakoid membranes of the chloroplast. Inside this folded membrane, we have a watery liquid, that sounds familiar. This one is called the matrix.

Now the folded inner membrane of the mitochondria, is called the cristae. Let's take a look at the next step and this shows what's happening to that pyruvic. It goes to the Matrix, that watery liquid of the mitochondria. And there, the pyruvic goes through a series of biochemical events much like the Calvin Cycle as a whole series of biochemical events. And here, enzymes start plucking off the carbon dioxides that were used to ultimately build that glucose in that pyruvic originally. As the carbon dioxides are being plucked off, some of the high energy electrons that were holding it on, are being delivered to NAD+, making NADH. We see that happening here, we see that happening here.

You also can get some ATP and another high energy electron carrier called FAD. That becomes FADH too. So we've taken our glucose, through glycolysis we've spilt in half, and now we're inside the mitochondria. In the mitochondria, we start breaking up the pyruvic, and we're getting out some ATP, and a bunch of these NADH and FADH to high energy electron carriers.

If you can remember to mention this high energy electron carriers, during a essay on aerobic respiration, you got yourself another point. Remembering that it's the Krebs cycle that does this following glycolysis, a couple more points. Now what do we do with those high energy electrons? Let's take a look.

Remember high energy electrons earlier in the chloroplast? They were being used for a couple of purposes. One to be sent off to build glucose, the other purpose of those energy electrons, remember there was something I mentioned about pumping hydrogen ions and making ATP? Here is the high energy electrons. They get dropped off and here we see, just like in the thylakoid membrane, an electron transport system is in the cristae membrane of the mitochondria. And so the high energy electrons are dropped off and the energy of these electrons is used to shelve hydrogen ions into that space, between the outer membrane and the cristae.

Ultimately, those hydrogen ions go back across this special molecule here called the ATP synthase. Which is just like the ATP synthase that's on the thylakoid membrane, and resynthesise ATP. That's why it's called ATP synthase. 'Ase' means enzyme, so this is an enzyme that synthesises ATP.

Now, you may be thinking, there's these high energy electrons, their energy has been used, so the last thing that happens is that we wind up taking the electrons. Now their energy is gone, they are low energy electrons.

Let's put them back on to water and that's what the oxygen is used for. It absorbs the electrons, grabs some hydrogen ions, and there is no water. And we're done with aerobic respiration.

Well I know there was a lot of ground cover. Hopefully you're starting to realize it's pretty easy, especially if you know how repetitive it is. The two equations for example, they are essentially the same just run in reverse. Let's take a closer look at that.

Again, here we see the reactants of photosynthesis, are the products of aerobic respiration. While the products of photosynthesis, are the reactants of aerobic respiration. Just mentioning that, will get you a point. Just focus in on this sort of stuff, it's not that hard. Again, it's all about storing energy or releasing the energy.

Now, while animals do have mitochondria and that's it, plants, remember have both. They have the chloroplast and the mitochondria. During the day, both are running. During the day light energy comes in and photosynthesis, the plant starts kicking out glucose and oxygen. During the day, the plant is using some of that carbohydrates and oxygen in order to make the ATP it needs to run the various processes of the cell. Protein synthesis like I mentioned, or pumping stuff out of the cell. And that releases the carbon dioxide and water either into the air or straight back into the chloroplast. During night time however, can't do this, so this is the only process that's occurring.

This is one of the ways that you can actually measure the rate of these two different things. Because if you have a CO chamber, you can put a plant cell in there and then measure how much carbon dioxide is being absorbed during the light. And then when you turn off the light, you can then measure how much carbon dioxide is given off. That carbon dioxide being given off is being caused by aerobic respiration.

Earlier when there was light, and the carbon dioxide was going down, here is the trick. It was going down because of photosynthesis. Back up a little bit, because of aerobic respiration. The true way to photosynthesis is the absolute difference between what you measured in the light, what you measured in the dark. That's what sometimes called gross primary productivity versus net primary productivity, got yourself another point.

Some of the other things that are going on, in both of these processes they are using an electron transport system, because it's very efficient at making ATP. Here it's in the thylakoid membrane, here it's in the cristae membrane. And this is all about electron transport system of high energy electrons. In the production of ATP, those high energy electrons are being used to shelve hydrogen ions across the membrane. While, when they then come out, they make the ATP.

What are the electron carriers involved? In photosynthesis, there is NADPH and in aerobic respiration there is NADH. Now one problem I've had with my students before, is that they can't remember which one is used photosynthesis, and which one is not. If only there were some clues that plants do photosynthesis using NADPH. In alternate aerobic respiration there is that FADH. If only there is a way to figure out that is only used once? It's kind of like a fad.

The last thing I'm going to go into is the compartmentalization of these processes. Here, you notice there is this compartment, this inner compartment here and compartments there. With aerobic respiration, remember those hydrogen ions being pumped by the electron transport system, they are being pumped into this space here.

With the chloroplast, the hydrogen ions are being pumped into the inner space of thylakoid membrane. Why is that significant? Those of you who know something about chemistry may be saying, "Hey hydrogen ions, lots of hydrogen ions means acids." That's right. And if you know anything about enzymes, acids and enzymes usually don't work too well together.

Notice that they're being pumped away in this case from the matrix, and in this case away from the stroma. Where are all the enzymes involved in anaerobic respiration photosynthesis? Here is where the Krebs cycle is. Here is where the Calvin cycle is.

So the Krebs cycle and the Calvin cycle are both on the safe side of the membrane. Again, these things are like the same, just run in reverse. I mean just think, what does photosynthesis begin with? Electron transport system and it ends by doing a bunch of biochemistry as your simple molecule. Aerobic respiration it begins with this biochemistry to take apart the molecule, and then it ends with the electron transport system to make the ATP. That's it. Just remember those sorts of things and, points on the essays.

As I keep saying, if you can keep in mind these two processes are essentially the same thing just running reverse, learning and studying this stuff will just be a whole lot simpler. Bare minimum, remember the equations. Remember there is 6O2s, 6H2O, oxygen and a single glucose. For photosynthesis you're putting the CO2 and H2O together, along with energy, to produce the glucose and oxygen. Whereas with respiration, you're taking the glucose and oxygen, and tearing it apart to make carbon dioxide, water and get that energy back out.

Photosynthesis begins with the light reactions which involves electron transport system that uses high energy electrons, to make ATP, by pumping hydrogen ions.

Just as a side note, that's called Chemiosmosis. mention that and, one more point on your essays. And then it continues by making high energy electrons that it puts on to a high energy carrier called NADPH. That goes off to the Calvin's cycle which uses the ATP, high energy electrons from NADPH to build your glucose. Aerobic respiration starts with glycolysis, taking that glucose, reaps it in half then runs through the Krebs cycle. Break apart the pyruvic, giving out a little bit of ATP and some high energy electrons on NADH and FADH too. It finishes off with chemiosmosis, using electron transport system to pump some hydrogen ions and kicking out a ton of ATP and putting those used electrons under the oxygen to make some water.

So a lot of steps to recover but if you can keep it straight, with what I've been going over in this, you'll do fine on the multiple choice section. To really sharpen up, I strongly recommend that you though however watch my episodes where I focused in on the details of photosynthesis, and the details of respiration. Do that and you'll master it and you'll kick a major portion of the AP Biology exam.

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