Hi. It's Mr. Andersen. And welcome to AP Biology Lab 9 walkthrough. This lab
is on transpiration. Whenever I hear the word transpiration I immediately think of leaf
evaporation. And so that's the best way to think about it. And you'll have a better understanding
what transpiration is. Basically what's happening is that water is flowing up a tree. And the
way it flows up a tree is the sun. You can see the sun up here is shining. Basically
it's heating up the water in the leaf. And the water is evaporating. Now it's leaving
the leaf through something called a stomata. So if this is a cross-section in a leaf, there's
going to be these tiny little holes. Almost like little mouths. They're called stomata.
And basically water is going to go evaporate out of the stomata. So as it evaporates up
here, and again the sun is providing the energy, there's going to be a tube that goes all the
way from here back through the xylem. All the way down the tree. All the way down into
the roots. And so as there's this connected tube of water all the way up to the top. So
basically as the water evaporates off the leaf, this whole column is going to move up.
Because all of those water molecules are attracted to each other because of hydrogen bonding.
And so in this lab what we're going to do is measure the rate of that. Now it's hard
to measure an actual tree so we'll be using something called a potometer. A potometer
looks like this. What you do is you take one leaf. And that one leaf is going to be connected
to a tube. You have to make sure that it's air tight right here. And then that tube is
going to be filled with water all the way around over to here. So it's basically going
to line up. And then we're going to have a pipette on this end. So if we zoom in you're
going to have pipette that looks just like that. So basically what happens is as water
is going to evaporate off of the leaf on this side, that's going to pull water up through
the stem. That's why it has to be air tight here. And as it pull the water like this,
this is going to move that water is going to move down. And so we can measure the rate
of that movement. And we can measure the rate of transpiration. Now one thing you have to
kind of watch out for is that we want to make sure that all leaves are going to be a little
bit different. So one thing you can do is you can put this whole thing on graph paper.
This is really bad graph paper. Like this. And then you can trace the leaf on the graph
paper like that. You can calculate the area of the leaf inside it. And so we can make
sure that all leaves are going to be measured as a per surface area. But let me kind of
show you what we have. So this would be our standard lab set up. Basically set up in your
lab ready to go. We then compare that over time to the following. We have another one
where we put that leaf inside a bag and then we put mist inside the bag. So you may be
thinking in the top of your head like, what's going to happen there? Next we have another
lab set up. We'll call this lab set up three, where we have the leaf in front of a light
bulb. Now one important thing that I'm not showing here that you have to have is that
you put the light bulb like this. So the light is shining in this direction. We'll also put
some water. And we call that a heat sink. Because we don't want to measure the temperature
and how that effects it. So we'll put a heat sink between the light bulb and then we'll
put the leaf over like that. So just the light is moving through. So that would be set up
three. And then set up four, what we're going to have is a leaf and then a fan blowing on
it. And so what you should think in your head is, well how are each of these going to effect
it? And so basically what we find, this is data that was collected in class. So at time
3, 6, 9 all the way through 21 minutes. We should start at 0 as well. What we get is
a fairly linear relationship. Again this is data that was collected in class. So it's
pretty linear. The slope of this line, since we are going over time, the slope of that
line, since we're measuring millimeters per meter squared, basically the slop of that
line is going to give me the rate. So slope equals the rate. In other words, the steeper
the slope, the faster the rate. So let me clear all of my scribbling out of the way.
Which of these is going to be the fastest? Well the light is going to be the fastest.
Followed by the fan. Followed by the room. Followed by mist. And that's hopefully what
you were thinking. In other words, if you just think of that leaf like a wet towel that's
sitting there, what are the following going to do to it? Well in summary, basically if
you increase the temperature we're going to get an increase in the amount of movement
of kinetic energy. And so it's going to increase it. If we increase the humidity, there's going
to be an indirect relationship. In other words the more humidity we put, the less evaporation.
And the reason why is that they simply, the molecules can't jump off and evaporate. If
we increase air, there's going to be a direct relationship because it's going to move more
of those water molecules away. And light intensity as well. It's going to increase the temperature
of those molecules. It's going to speed up their kinetic energy and they're going to
transpire more quickly. And so that's kind of a summary. It's pretty straight forward
lab. But I hope that's helpful.