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Biology: The Leaf: Adaptations for Photosynthesis


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About this Lesson

  • Type: Video Tutorial
  • Length: 8:43
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 93 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: Adaptations for Photosynthesis (5 lessons, $8.91)

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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Remember one thing. Well you've actually got to remember a whole lot more than one thing, but right now I want to say one thing that's very important. Structure follows function. And as we begin to delve and dig into the layers of photosynthesis, we have to start at the top. We have to take a look at how plants are adapted for photosynthesis. I can't really tell you about photosynthesis until we take a look at how and where a plant does its photosynthesis.
You probably know that plants do photosynthesis in their leaves. And, you know, leaves take on a whole lot of different structures. I'm particularly keying in on leaves, but remember there are groups of organisms that do photosynthesis that don't have leaves. So keep in mind this is kind of a generic look at some of the structures. But if you take something say like a photosynthetic bacterium, well there's not too many leaves in a one-celled organism. So a lot of the structures are going to be different. But we've got to start somewhere. So let's start here at the leaf.
And let's take one of these leaves and slice it in cross section. So if you understand what a cross section means, it's like taking a piece of pepperoni and making a slice right through it. And you're looking in that slice. So you're looking at the cross section, the face of it. And as we look at a leaf, whether it's a--this particular one might be the leaf of a maple tree. But remember there are some leaves that don't look like your prototypical leaf like this. A pine needle, for example, is a leaf. But we want to look at this one. Here's the adaptations that are going to be key. And I want to review the basic structures of a leaf.
First of all leaves are waterproof, at least this particular leaf. And you might be saying to yourself, "Well that's kind of dumb. Why would a leaf be waterproof if it needs water for photosynthesis?" And in fact on the top of every leaf there is this structure called the upper epidermis. It's a layer of cells. And on top of the upper epidermis is quite often a waxy layer called the cuticle. That's what gives this its waterproof waterproofness, waxy cuticle. Well we'll get back to that. Why would the top of a leaf be waterproof? But I don't want to talk about that until we get to the bottom of the leaf. More on that in a second.
Now if you look at this particular leaf, you're going to see a layer, and this particular layer of cells we've highlighted. And the reason we've highlighted this layer of cells in color is because this is the business end of the leaf. This is where photosynthesis occurs. And we call this the middle layer. But we're not just going to call it the middle layer. We're going to call it the mesophyll, the middle layer. And what we want to do is I want to talk about that mesophyll in a little more detail in a second, but I want to work my way down through the leaf.
Now below the mesophyll, remember that's your business end of the leaf, we have a lower epidermis. And now we can start talking about the waterproofness of a leaf. You and I both know leaves need water. Well where do they get their water from? Well they don't get it from the air per se. They get it from a series of veins that pass through their body, the plant body itself. And the veins start in the very tip of the roots, are continuous through the main portion of the root, are continuous through the stem, whether it's the stem called the tree or the stem of a flower, and they're continuous through there. They eventually go out the branches, down the twigs, through the twiglets, into the petiole, which would be this structure right here, and eventually into the leaf. And so we have actually drawn for you a vein right here, this circular thing. Boy I have a lot of great things to tell you about veins. Unlike your veins, a plants veins are solid, because they have no pump to pump their sap, their blood around. But that's another story altogether. We're here to talk about photosynthesis. So then how does a plant get water? Well plants get water because it comes up through the veins. Well if they don't have a pump, how does water get up there?
Well you see here's the thing. Plants, at the base of their leaves, do not have a waterproof layer. Now why would there be a waterproof layer on top, and not the bottom? Well generally speaking, on the top of a leaf is where most of your heat and evaporative energy is going to be--you know. Leaves sit out there and the top of their leaf faces the sun. That's where the evaporation is going to occur. On the other hand, the bottom will not be in direct sunlight, the bottom of the leaf. And so therefore the waterproofness is not going to be that crucial on the bottom as it is on the top. In fact, there are even little holes on the bottom of the leaf. We call these holes stomates. And stomates have these specialized cells, which we'll talk again about later on in the course called guard cells. And the guard cells allow the stomates to be open during the day and closed at night. Well why is that? Think about it. What you know about photosynthesis? When does a plant need water? When it is doing photosynthesis. When does a plant do photosynthesis? It does photosynthesis during the day. So therefore, what's the purpose of these stomates? The suction of water through what we call evaporation. So water is going to be evaporated out the stomate. And since it evaporates out the stomate, it's going to be evaporated out of the mesophyll. And since it's going to be evaporated out of the mesophyll, it's going to be pulled from the vein, from the stem, from the root, from the ground. How cool is that? I have even more stories to tell you about that, but that's not what we're here to talk about. We're here to talk about the mesophyll.
The mesophyll--now you get a real treat. I'm going to draw it for you. Look at these tightly packed cells right here and then these kind of cells that seem to be a little spongy. There are lots of air spaces. And you'll notice that there are even spaces above the lower epidermis that seem to have air spaces in them. Watch this. Let's do a great big enlargement of this. If you take a look at the mesophyll, you'll see that those tightly packed cells and these spongy cells down here are continuous with air spaces. And what do you think goes into those air spaces? That's right, air. And now if we draw my lower epidermis like so, notice I left a gap. There is my stomate. Well guess what, stomates aren't just for water evaporation, CO[2] can go in there. So that is a way for a plant to get the CO[2] of photosynthesis. Structure follows function. Oxygen can diffuse in and out through here. You see, structure follows function. And one last thing, if CO[2] is in here, and we can get all sorts of water out here too from veins, and osmosis out of the veins, well let me ask you a question. What do you think is going to be found in each one of these cells? And I know you know the answer to this, little round green things. And these little round green things, maybe about 30 or 40 of them per cell, are the working part of the cell. These are going to be the things that are going to do the photosynthesis. These are going to be the things that have this pigment in them called chlorophyll. And the green, those little structures, are chloroplasts. So the mesophyll is the business end of the leaf, and they have the chloroplasts in it. What's in the chloroplasts? Well that is the secret to photosynthesis.
Adaptations for Photosynthesis
The Leaf: Adaptations for Photosynthesis Page [1 of 2]

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