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Biology: Photosystem 2


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About this Lesson

  • Type: Video Tutorial
  • Length: 9:05
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 97 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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The poor photosystem 1, it's lost an electron. It's been all excited. You got the photosystem all excited. You got its electron just shot right out there. And then, BAM, NADP takes it away, and now you have an electron hole. How are we going to fill this hole? Well remember something, folks, that there's a whole lot more going on here than just photosystem 1.
Remember that there's photosystem 2. And if you just look at this diagram, you can probably figure out what's coming next, because it looks to me like photosystem 1, indeed, does give up its electron to the ferredoxin, which eventually gets it to make the NADPH. But it looks like that hole is going to get filled. And it looks like it's going to come from these molecules, PC and this blue molecule here. In fact, I can tell you that there is a chain of proteins in here very much like the chain of proteins in the mitochondrial membrane. Some of them are exactly the same. You've heard of a chemical called cytochromes, these proteins, and some of these cytochromes are exactly the same as in the mitochondrial membrane. These same proteins that pass electrons from higher to increasingly lower energy levels each time being used to pump, perhaps, protons and to cause a gradient, but each time being passed from high to low. That being said, where are these guys getting their electrons from? Well it looks like PQ is getting its electron from, indeed, photosystem 2.
So let's take a look at what's going on here. Here's the thing. Photosystem 1, we'll start at the far right here and see if I can make some kind of legible drawing for you. Photosystem 1 is going to lose its electron. So it gets excited, and its electron gets shot out to NADP, which becomes NADPH. Photosystem 1 now has lost an electron. Where is it going to get its electron from? Well on down the membrane is photosystem 2. Now I have to tell you, this is sometimes referred to as P680. You know what that means. You know that that corresponds to the wavelength of light, slightly different than the wavelength of light that photosystem 1 can absorb. So here's the thing. Photosystem 2 gets its electron excited. And that electron is now going to be passed. And it's going to be passed from protein, to protein, to protein. And a very cool thing happens. One of the things that can occur here is we are going to form--now remember all of this is happening in a membrane. And the passage of this electron is going to allow us to pump hydrogens. So here is another way we can get hydrogens concentrated in the lumen of the thylakoid. Because remember we are going to want a low concentration of hydrogens out here, and a high concentration inside. And as those hydrogens go shooting across that ATP-ase, they have to be re-brought back into the thylakoid. So the hydrogens out here, as they chemiosmose across, are then going to be pumped using the energy of this. How cool is that?
But wait a minute. You know enough about photosynthesis right now to be asking a very important question. Go ahead. Ask it. That's right. If this loses its electron, we just created another hole. Photosystem 2 lost an electron. So photosystem 1 is happy now. But photosystem 2 lost its electron. The thing is going to stop. And in fact, it's lost two electrons, because remember you eventually want to reduce NADP with two electrons to make NADPH+H. How are we going to do that?
By the way, here's a quiz for you. What's the name of the enzyme that reduces NADP? Now are you remembering? NADP reductase, remember that. All of the reductases are the things that reduce these coenzymes. I had to throw that in there. Every once in a while I've got to keep you awake. Especially if it's lunchtime, you might be getting a little sleepy, got to keep you awake with those questions.
Now where are we going to get the electrons from? Where can we do this? Sitting in the thylakoid--and now comes the most amazing thing in photosynthesis, the thing that blew the early scientists away. And it's this. Here's the membrane. I don't know why I'm hesitating. This is easy. And there's my chlorophyll molecule. I'll make it green. And we'll put it in the membrane like that. With this thing sitting there in the membrane, and having lost an electron, it is now electron hungry. It desperately wants an electron. It has been oxidized. And remember chlorophyll likes its electrons. Sitting down here in the thylakoid is a molecule that, up until now, was pretty cocky. It held its electrons pretty well. A molecule that looks something like this. A molecule called water.
Now things start getting interesting, because chlorophyll used to have a pretty strong attraction for that electron before the photon came in and tickled it a little bit and got it out of there. But now it's lost an electron. It has a very strong electronegativity. It's looking for an electron, and here in the thylakoid is water. What would you do if you were chlorophyll? I know what you would do. You would take that and you would split it. And here's what you would gain. When you split that water, here's what you would do. You would rip, remember two electrons went through. What do you know? There are two hydrogens. Let's rip their electrons off. So we are going to rip the electrons off of hydrogen. And what we are going to get is we're going to get two hydrogen ions. Hey, what do you know? Another way to make it positive inside. So not only are we pumping these bad boys from out here to be a battery, we're making our own by dissociating water, which by the way has a name. The dissociation of water is called photolysis, the breaking of water, -lysis, using photons. But that's not all. HHO, well guess what, for every two molecules of water you split open, you're going to get, it just so happens, a molecule of oxygen. From one water molecule, you get what we're going to term half of an O[2]. Put two of them together and you get a full O[2]. Do you think that's important? Of course that's important. That's the source of oxygen for this planet. Oh this is so exciting I can't stand it.
So anyway, here we are. Here it is. Water split, two hydrogen ions, half of an O[2], the hydrogen ions are in there. What are we going to use those for? The battery. What are we going to use that battery for? To generate ATP. So what do we have when all is said and done? When all is said and done, we have ATP. We have, when all is said and done, reduced coenzymes, NADPH+H. And when all is said and done, we have a waste gas called oxygen. And now I have a question for you. Where is the glucose? I don't know. We'll just have to see.
The Light Reactions
Photosystem 2 Page [1 of 2]

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