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Biology: Photosystem 1


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About this Lesson

  • Type: Video Tutorial
  • Length: 9:48
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 104 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Photosynthesis (18 lessons, $26.73)
Biology: The Light Reactions (4 lessons, $6.93)

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

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Once chlorophyll gets excited, it must be passing its energy on somehow. And that's what I want to talk about now, the role of some of those parts of that thylakoid membrane called photosystems. And what is it that those photosystems do? And where do those proteins fit into this whole story? And do we really make ATP, because we don't want to hear about making ATP? That's mitochondrial stuff, right? No it is not. It's chloroplastic stuff too, and we're going to take a look right now.
Now first of all I want to talk to you about this diagram that I'm going to be referring to repeatedly as we go through the light reactions of photosynthesis. And I want you to see something that is not a random event. See we've put this in here so that there are two places where light seems to be hitting. Now that's not just artist happenstance. This, in essence, is going to be a block of material that we have to talk about. And there are two places that are going to loom huge, one in particular, during this discussion. And these two places, these two areas are called photosystems. And it is in these photosystems where those reaction centers I told you about are going to be found. And it is in these photosystems where all of the electron excitation is going to take place. And it is in these photosystems where electrons are going to be passed.
Here's the thing. What's going to happen here is that they are going to have names. And we have to refer to them as something. Their names are going to be simple. This one here is called photosystem 1. This one here is going to be called photosystem 2. Now I have to warn you in advance. A lot of books start out a discussion of photosynthesis at photosystem 2. You're saying, "Well that's dumb." Well it's not so dumb really. And when we put this whole story together, you'll see a good logic behind that. But we're going to start at photosystem 1. We're going to take a look at what happens here. We're going to a look at an alternative thing that can happen there. And we're going to see the role of the photosystems by looking at photosystem 1. By the way, it kind of makes evolutionary sense, because guess what. The reason this is called photosystem 1 is it was the first one discovered. And number two, evolutionarily, it indeed is the first photosystem that evolved. So we have to start talking about photosystem 1.
Nothing is going to change much. The bottom line is that in photosystem 1, chlorophyll is still going to get excited. And let's take a look briefly at what we have here. In photosystem 1, here's what we're going to see. We're going to see chlorophyll. And chlorophyll indeed is in the reaction center. Now let's get some vocabulary into the way, or perhaps out of the way.
First of all, we know that we have the light-harvesting antenna. You know that, been there done that. The light harvesting antenna is going to have the chlorophyll A, which eventually is going to be the culprit, if you will, that gets the electron out to that large distance away from the ground state. And therefore we can have a readily made oxidation. Chlorophyll doesn't give its electron directly to anything except one chemical. And that chemical is called the primary electron acceptor. So chlorophyll will pass its electron to the primary electron acceptor. So the electron goes there. From there, in photosystem 1, and remember that's what we're seeing here, photosystem 1, chlorophyll will pass it to the primary electron acceptor, and now to a membrane protein. But before I show you what happens there, gee what's up with this thing P700?
Well P700 stands for the wavelength of light that this particular photosystem can absorb. You're going to see in photosystem 2, it's going to be called something else, not P700. Let's not muddy the waters yet, but you know that now. That's why we're referring to this as P700, photosystem 1, P700. Here we go. Watch how elegant this is going to be.
So electron gets excited. The electron is grabbed by the primary electron acceptor, and it in turn is going to be passed to ferredoxin. Let's put it this way. It's going to be passed along an electron gradient, if you will. In other words, high-energy electron from chlorophyll passed to the primary electron acceptor, passed to ferredoxin.
What's going to happen now is the essence of photosynthesis. You'll really appreciate this at a later time. But it's important now. Here you have an electron in a high-energy state. Wouldn't it be wonderful to somehow be able to capture that energy and, perhaps, get it somewhere else? And here's the thing. On the outside of the thylakoid membrane, so out here is your NADP. It is your NADP that can now grab that electron from the ferredoxin and become NADPH. And actually that happens twice to give me NADPH+H. So what we are going to have is NADPH+H formed by this process. We have just reduced a coenzyme. And in reducing that coenzyme, life is going to be wonderful, because now we have a way to carry that energy.
I have to tell you that sometimes, though, there is a very cool control mechanism that's going to kick in here. Sometimes there's a build up of NADPH. Sometimes there's not enough NADPH, or the ratio of NADPH to NADP is way too high for that to happen. And so now a very interesting thing occurs, a process called the cyclic reactions. Now why do I want to tell you about the cyclic reactions? Well we've just shown you how you make NADPH. I want to show you what else can happen. Here's my electron. So let's not forget there's a chlorophyll down here, CHL. Light hits it. Up goes the electron. It passes to ferredoxin. But there's an imbalance of NADP. So there's maybe not enough NADP to reduce, maybe the ratio is off. But the bottom line is the electron now takes an alternate pathway. Notice we have this on a gradient so you can see it, and it eventually goes back to chlorophyll. So it's a cycle. And that's why these are called the cyclic reactions.
But here's the thing. You want to know what the cyclic reactions do? They don't make NADPH. So you're saying, "What a useless piece of junk this is." But it does something else. It actively transports. The energy of that is used to build a hydrogen gradient. Have you ever heard of a hydrogen gradient before? I know you have, because now we're going to start to build up a battery. And in building up that battery, look what can happen. When we build up a battery of hydrogens inside of here by pumping them in. One way we pump them in--you're going to see other ways. One way we pump them in is by the cyclic reaction. We build up hydrogens inside. Now we have a chemiosmotic imbalance. We can now do a very cool thing. We can let these hydrogens--are you okay with that? Does it make sense? Okay the hydrogens are in here. We can have those hydrogens leave the thylakoid membrane. So I'm going to draw the thylakoid here. Here's my ATP synthase right there. Now my hydrogen gradient is right here. My hydrogens can go down this gradient, and they can take ADP, NP and phosphorylate ADP with a high-energy bond. You just saw a process called photophosphorylation. Photo, light, one word, phosphorylation, the adding of a phosphate group, ADP, by the energy of the photon.
Now the question is why are we making ATP, question number one? And question number two, and this is even more puzzling, back to that making of NADP, if you think about that, our friend chlorophyll, something horrible happened to him. This is a sad story. Chlorophyll now has an electron hole. It's lost an electron. What are we going to do about that hole? Stay tuned. You'll find out.
The Light Reactions
Photosystem 1 Page [1 of 2]

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