Hi. It's Mr. Andersen. Welcome to the AP Biology Lab 4 walkthrough. This
is on plant pigments and photosynthesis. Photosynthesis remember is how plants use energy of the sun
to convert carbon dioxide into usable carbon. Sugar is one thing that we need from them.
So again, it's broken down into two parts. You've got the light reaction. And then you
have the Calvin cycle. In the light reaction we're taking in water. We're splitting that
giving off oxygen. But we're using that hydrogen to add energy in the Calvin cycle and eventually
produce things called sugar. And so I don't want to go into the specifics of photosynthesis,
but know this. Photosynthesis wouldn't be able to work if we didn't have plant pigments.
Especially chlorophyll A. And so we start this lab by doing a little bit of chromatography.
In this lab basically you're going to take some chromatography paper. So it's like filter
paper. You cut it out. This shape right here will kind of restrict the color, the pigments
as they flow up. You're then going to dip it in solvent. So you're going to put some
alcohol solvent down here on the bottom. And basically as that solvent moves up, it's going
to carry the pigments with it. Now I use spinach. So I'll take a little bit of spinach. Put
it down here across this filter paper. Then I use a quarter to grind it into the paper.
And then we're going to run the solvent through it. So basically its going to start to migrate
up like this. I remember when I was a little kid I did this with water soluble markers.
So basically it's going to move up like this. The solvent is kind of hard to see but you'll
see a faint line up here. The solvent is going to be that liquid that moves up. So we'll
have the distance that the solvent moves. We could maybe measure this in centimeters.
And then we're going to see each of the different pigments move up a different amount. And so
basically one thing that sometimes you have to calculate is what's called the rf value.
And so the rf value is going to be the ratio that they move. The ratio of the pigment divided
by the solvent. And so this first one up here is going to be carotene. If I wanted to figure
the rf value I'd figure out how far the pigment moved. So let's say the pigment right there
moves around we'll say 12 centimeters. And then I figure out how much the solvent moved.
The solvent moved, we'll say 13 centimeters. And then I just divide 12 by 13. And I get
a number. So it's going to be a ratio. Let's say it's going to be, I have no idea, 0.9.
Something like that. If we look at that, carotene. The then next one, this would be actually
chlorophyll A. This would be chlorophyll B. This would be xanthophyll. And so basically
what you can do using chromatography is to separate it into the different pigments. Remember
chlorophyll A is that magical pigment that actually starts photosynthesis. The other
ones can contribute energy to chlorophyll A. So what do we do in the lab set up? Well
in the lab set up we're going to cut holes or leaf chads out of a leaf. We're going to
punch those with a hole punch. We're then going to immerse them into water. But that
water is going to have baking soda inside it. The function of the baking soda is to
provide carbon dioxide. So carbon dioxide is going to be here. We put our little leaf
chad in the bottom. And then we're going to put it underneath the light. You can see right
here in this diagram I'm using a heat sink because we don't want to increase the amount
of heat. Because that could be another variable that we don't want to have effect our results.
So basically what's going to happen is this leaf chad down here on the bottom is going
to absorb energy from the light. It's going to convert the H2O in to O2. And those O2
bubbles are going to cause this to eventually become more buoyant. And it's eventually going
to float up to the top. And now I've got the release of oxygen. It gives me something,
a reactant or a product that I can measure. And so I can measure the rate of photosynthesis.
Especially the light reaction. And so what do we find when we do this? Well each of the
students in my class are doing a number of different things. But let's start with color
for example. So if we did R O Y G B I V. What I would find is that I would have a higher
photosynthetic rate on the reds and the blues then I would in the greens. Because again,
they're not good at absorbing that amount of, that color of the spectrum. What about
the distance to the light? Well the farther they are away from the light the slower it's
going to occur. What about temperature? We'll find that if we increase the temperature we're
going to get an effect. Carbon dioxide as well. Or the type of the leaf. So there's
a bunch of things that you should measure or could measure. We could graph those as
histograms. We could graph them if it's like increasing temperature as a line graph. But
I want you to look over here because this is going to be our dependent variable. This
is what we're measuring. And we're going to measure the photosynthetic rate in floats
per second. In other words, that leaf chad is eventually going to float to the top. And
the sooner it floats to the top, the faster the rate is. And so we measure it in float,
how one thing floating divided by seconds. And so we can get the rate. And so that's
basically the lab. It's a quick way to measure the light reaction. Now one thing you should
be ready for is that the College Board uses different type of lab. And in this lab what
they do is use a chemical called DPIP. DPIP is a chemical that takes the role of NADP+.
So basically what it's doing is it's taking the role of this chemical right here. As it
accepts hydrogen it's going to change from a bluish color right here. In other words
as it's reduced to a clear color. And so it's going to turn to a clear color like that.
And so you can measure that using one of these devices down here, a spectrophotometer. In
other words the faster it changes to that color, the faster the photosynthesis is occurring.
And so it's just another thing that you should understand another way we could measure it.
They suggest you try chloroplast. Chloroplasts that are boiled. A lot of different things
that we can see and how that affects the rate of photosynthesis. But again it's something
that you should understand because photosynthesis is really cool. And I hope that helps.