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Biology: Photorespiration

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About this Lesson

  • Type: Video Tutorial
  • Length: 8:07
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 88 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: Photorespiration (3 lessons, $4.95)

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 http://www.thinkwell.com/student/product/biology. 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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So you're saying to yourself, "Photorespiration? First we have respiration, or second we have respiration. Now we have photosynthesis. Is there still a third process? No more biochemistry please." Well you know I have to warn you. All is not well in Photosynthesis Land some days. And photorespiration is not a new process that benefits anybody. In fact, it's a problem that plants have. And they often have it in conditions where it's hot and dry. And I want to tell you all about that.
So we are about to talk about photorespiration, a problem for plants. Well we have to go back and talk a little bit about leaf structure. Let's take a look at a leaf. And now I want to focus not so much on this mesophyll, but these stomates, these holes in the bottom of the leaf. And I want you to understand that the stomates, we talked about their role in transpiration and the loss of water and sucking water up from the roots, but that, in itself, can cause a problem, because of these things called guard cells, their control. Guard cells generally close down, as I told you, in the night and open during the day by turgor pressure within the cells. So they can open up and water can come through. Well it turns out that guard cells have another kind of cool adaptation. They actually, in warm, hot, drying conditions behave like it's nighttime. And what they do is they close. And here's the reason why. The evaporation of water from the leaf in a hot, drying condition would be so severe that the plant would eventually die. That the plant would suck too much water out of the ground, and could not keep up with its water needs. And eventually the soil would dry out. The plant would die. So you'd lose more water than you could possibly gain through evaporation.
Well that being said then, stomates will close during the day. Well why is that a problem? The problem is rubisco. You see rubisco is not real fussy. Rubisco will bond with more than carbon dioxide. Rubisco will bond with oxygen. Is this upsetting you? Let me tell you why this is going to be a problem. Let's take a look at it. Okay, here's the thing. In the atmosphere, CO[2] is about one in 10,000 parts. So about one out of every 10,000 air molecules is CO[2]. Okay. Well think about it. When those stomates are closed, the Calvin cycle is going to starve. You're not going to be able to make any carbohydrates. But even worse is this problem with rubisco, because in addition to all of this, rubisco seems to be equally attracted to carbon dioxide and oxygen. So rubisco will bond to oxygen. Its active site seems to bond also with oxygen.
Well let's look at the ramifications of that. If you do something like that, here's what's going to happen. RuBP, which rubisco uses as one member of its substrate, plus CO[2], well you know that story. When you mix RuBP and CO[2], you get a six-carbon substance, which will give me two three-carbons. And life is good. But if you were to take RuBP and bond it to oxygen instead, well remember RuBP is a five-carbon, now you're going to get a five-carbon substance. And now instead of two three-carbons, you're going to get something else. You're going to get a three-carbon substance and a two-carbon substance. Now you see this is a problem. You're not going to be able to generate any. You're lucky if you can recycle to your RuBP. You're not fixing carbon because of photorespiration. And in fact, you're getting nothing accomplished.
In fact, let me show you what happens to those materials. The three-carbon substance that you make, that ends up going and being replaced into the Calvin cycle. So that's going to be a three-carbon substance that's going to go back into the Calvin cycle. And in essence, you just took RuBP and turned it into a three-carbon substance, and ended up losing two of its carbons. That's counterproductive. So that's going to end up going to the Calvin cycle. Big deal, your two-carbon substance, it's even worse.
Your two-carbon substance is going to end up going into a peroxisome. As I'm writing this, take a look at this picture. You could see the peroxisome in that picture with its little grid work in there. And right next to it is the mitochondria. And right next to that is a chloroplast. Think about this. The peroxisome, it's going to diffuse from the chloroplast, into the peroxisome, from the peroxisome, into the mitochondria, where, best case scenario, CO[2] is going to be generated. So you get to start over again if you're lucky. So this whole idea of photorespiration is a drag for plants. In fact, you know what? It's been calculated that soybeans lose about 50% of their efficiency in hot, drying conditions. And soybeans are a staple crop. So in terms of agriculture, this is something we need to think about. We're literally siphoning materials away from the Calvin cycle.
Now there is a solution to this, but before I tell you the solution to this, I want to ask you a question. What's this doing here? I'm kind of stressing this whole thing of evolution. And now I'm talking about something that's counterproductive. Nobody knows, but we're never without a hypothesis. This could be an evolutionary leftover. Remember vestigial structures? I don't want to call this a vestigial metabolic pathway, but perhaps it's an evolutionary remnant. Well when would this have been productive? Probably never, but I want you to remember ancient atmospheres. The ancient atmosphere didn't have oxygen in it. So as rubisco was selected naturally for CO[2] bonding, when rubisco first evolved, there was no question of whether it would be selected or not, because there was no oxygen to act as a selective pressure against this looseness of the rubisco molecule. And it wasn't until oxygen entered the atmosphere that rubisco's role became kind of divalent, if you will, became to the point where sometimes it caused more problems. I don't want to say that plants are losers. Look around you. I mean most organisms on this planet are photosynthetic in one way or another, or many of them are, certainly.
So what would you predict? Well maybe a way to get around this. Maybe with this leftover rubisco looseness, maybe plants have come up with a way to get around photorespiration. We'll have to take a look at that in another lesson.
Photosynthesis
Photorespiration
Photorespiration Page [1 of 2]

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