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.
