Hi. It's Mr. Andersen and this is AP Biology Science Practice 1. What are
science practices? Well there are seven of them. And they're overarching skills and knowledge
that you should have to do well as a scientist. Why is that important in an AP Bio class?
Well if you're a teacher, if your'e an AP Biology instructor, practices are skills and
knowledge that you want to build in your students throughout the year. And if you're a student,
these are the practices that you want to pick up. Because when you take the AP Biology test
in the spring, they're going to ask you to apply the knowledge that you've picked up
throughout the year using science practices. And so you want to understand what a model
and what a visual representation is, because it's going to allow you to do better on the
test. And it could also allow you to do better as a scientist. And this right here is a picture
of DNA. So this is bacterial DNA under an electron microscope. And you might fool yourself
into thinking that we're looking at the double helix. That we're looking at DNA. But that's
not really what it is. If we zoom in as close as we can see, that's not what we see. In
fact the DNA is wrapped around histone proteins which are wrapped around more DNA and more
histone proteins. And we eventually get to something that looks like this. We call it
a fiber of DNA and that's what you're looking at in this picture. And so it's weird to think
that we've never seen a double helix. We've never seen DNA at this level, especially at
this level. And so how do we know that that's what it looks like? Through careful experimentation.
Watson and Crick developed a model. And a model is going to allow us to understand how
DNA works. It's a visual representation of what's going on inside the genetic material
of a cell. And so if I were to ask you, think about this, how is the DNA eventually become
a protein in a cell? Well in your brain you're going to start coming up with all of these
mental models of how the DNA maybe becomes messenger RNA and then is somehow translated
in the cytoplasm. That's your mental model. But it's still not a model. It's still not
a visual representation because it's not shared by everyone. And so once we have a picture
of how it works, now we're at the level of a conceptual model and that's what this science
practice is really about. And so throughout AP Biology, remember there are four big ideas
that we're going to talk about. Evolution, Free Energy, Information and then finally
Systems. And I came up with four models that would be typical in each of these different
big ideas. And so if we're talking about evolution, this is a nice model that shows natural selection.
So we've got bacteria, we've got a selective process when we're choosing these bacteria
and then this is a finally population. And maybe we're thinking about bacteria and so
this is resistance levels. And so the ones that are able to survive are going to be the
ones that have the highest resistance. And so by visually making natural selection apparent
to us, it's easier to deal with questions. Or let's say we're looking at free energy
and how free energy is transferred. This is a nice visual representation of photosynthesis.
So it shows the light reaction in the Calvin Cycle. It shows the reactants and the products
of each. And it also shows these carrier molecules of NADPH and ATP. What if we're looking at
information flow? Remember that deals with things like genetics and cell communication.
This would be a great example of a model. This shows you how an operon works. And so
this is going to be our RNA polymerase and we have a repressor here. Or maybe if we're
looking at systems a great model could be this pyramid of energy showing carnivores,
herbivores and plants. And so this gives you an idea of what a model looks like. And how
it can be applied in an AP Biology class. But they're asking that you can do five things
using models and visual representations. And so they first of all want you to be able to
create models and representations. And so you can think of each of these questions,
where's the first one, like a question you might experience on the AP Biology test in
the spring. In other words they're asking you to apply the knowledge that you've built
using a science practice. In this case you would have to build or create a model of representation.
And so you could pause the video, I've got five of these, and you could try to do this
and then you could watch me explain it. And so pause the video now and let me go through
it. So we've got a hypothetical population of beetles. There's wide variation in color
matching the range of coloration of the tree trunks. Create a graph that show how the beetle
population would change as a result in changes in the environment that darken the tree trunks.
And so what are some first things that I would look at? So we've got a beetle population.
There's differences in color. But they're saying that we have a variety of different
colors. And so we're going to represent that with a graph. We want to show the frequencies,
but we're going to have a normal distribution. In other words we could be put beetle color
here along the x axis, from light to dark and we're going to get a normal distribution.
In other words some of the beetles are really light. Some are really dark. But most of them
are going to be in the middle. What are they then asking us to do? They want to show us
evolution. They want to show us how they're changing as the bark becomes darker and darker.
So what's going to happen? Well as the bark becomes darker and darker, all of these lightly
colored beetles are going to die because the birds are going to see them. And they're going
to show up. And so they're going to die on this side of that curve. And so this would
be pre-evolution and then this would be post-evolution. And so what we're going to see is directional
selection. And so it's neat. I could look at that. We now have a visual representation
of a concept and this is what they're getting at. Can you build a model like this? Or, if
you were given four options in a multiple choice portion, could you choose the one that
reflects this hypothetical change? Let's go to the next thing they'd like you to be able
to do. They want you to be able to describe a model or a visual representation. Well here's
a question. What will happen to the water molecules in dissolved salts over time? So
we have a U-tube over here on the side. You could look right here that we've got water,
which is going to be this bluish color and we have these dissolved salts. And so they're
going to ask you what would happen over time? One other piece of evidence is that we've
got a semi permeable membrane down here. What does that mean? It's only going to allow certain
things through. In this case it's only going to allow water to go through. So what would
happen over time? Well we're now dealing with diffusion. And so these salt molecules are
going to be randomly bouncing around and they would always want to move from an area of
high concentration to low concentration. They would always want to move from the left side
of the U-tube to the right side of the U-tube. But they can't, because there's a semi permeable
membrane here. And so the water is the only thing that can move. So let's look at the
water now. Well the water is going to have a higher concentration of water on the right
side then the left side. And so the water is going to start flowing through this semi
permeable membrane. So the level of the water would magically move up on this side and it's
going to move down on this side. How long is it going to do that? Until the concentration
of salt molecules to water molecules is going to be the same on either side. Now does the
water stop moving? No. It's still going to move back and forth, it's just that it's going
to be at an equilibrium. So you can see now that I'm giving you a model and then I'm asking
you to describe the model or what's going to happen over time. The third thing they
want you to be able to do in this science practice is to refine a model or representation.
And so they could give you a model and then they could ask you questions based on that.
So I've got a model over here to the right and what I'm asking is how will changes in
the messenger RNA sequence effect the properties of the newly born protein? Okay now I'm asking
you to refine the model that I have given you. And so right here you can see that we've
got translation going on. So we've got messenger RNA. It's moving through a ribosome. And as
it does, we've got our tRNA. So the tRNA, which is going to be this molecule right here
is going to arrive at the A site and it's going to contribute it's one amino acid. And
so that would be just describing this model. But they want you to refine it. In other words,
what would happen if we would change the messenger RNA sequence? Well if we change that sequence
here, it's going to change the amino acids that come in and therefor it's going to change
the proteins. What's going to happen if we change the proteins? Well remember, or excuse
me, the amino acids? Every amino acids is going to be the same except for the R group
that hangs off to the side. And so if we change those R groups, we're going to change the
chemical interactions between all of those R groups and so we're going to get a protein
that folds differently. In other words its secondary and tertiary structure is going
to be different. And so now I'm not answering a question based on this model, I'm saying
if we could refine it what else do I know. Next thing they want you to be able to do
is use models and representations. And so right here they're saying the digram to the
right shows transduction in bacteria. How does genetic variation in bacteria result
from this process? So they're going to ask you to use the model. In this case we've got
a bacteriophage, remember it's a virus that's infecting a bacteria. But that's not what
the question is about. They're asking you to say how that would effect the genetic variation
in the bacteria. Now remember, bacteria don't have sex. They don't do meiosis and produce
sperm and egg. And so how could we get variation in it? Well let's look at what's going on.
So the bacteria is injected with DNA from the bacteriophage. That's basically programming
that bacteria to make more viruses. Except there's one thing going on right here. At
this point, instead of this virus being packaged with viral DNA that's created by the bacteria,
it's packaged with bacterial DNA. And so as these viruses spread out, this one virus is
injecting bacterial DNA into another bacteria. So it's transferring DNA from one bacteria
to another. What is that going to give that new bacteria? It's going to give it variation.
And so again I'm applying my knowledge to a model or a visual representation. And then
the last one is we need to be able to reexpress models and representations. And so in this
one we're looking at signal transduction. So you can see right here that we've got an
insulin receptor and then we've got glut or we've got a glucose transport. So what's the
question asking? How can changes in key elements of signal transduction alter cellular response?
And so again now they're asking you to apply the knowledge that you have. In this case,
why is insulin important? Insulin is going to dock with an insulin receptor, but what
it's really going to open up are going to be these glucose transports. In other words
it's going to open up this gate and so glucose can get into the cell. And so what are some
questions we can ask from that? Well let's say there's no insulin. If there's no insulin
here, there's going to be no signal transduction and it's not going to open up and it's not
going to allow glucose to come in. So if you're a type I diabetic, if you don't have insulin,
they you're out of luck. But let's say you're a type II diabetic. Where's the problem going
to be there? Now we've got a problem with the insulin receptor itself. We're creating
insulin but it's not docking properly with the insulin receptor. What's that going to
do? We still don't have a signal transduction. We still don't have a glucose transport opening
up. And so again these are all questions that you might find on an AP Biology test in the
spring. In other words they're asking you to use models, create models. But models are
neat. They allow us to make sense of a mental model. And lots of times it gets visual and
it gets much easier to understand. And they're used by scientists. What's the most famous
model of all? That model was developed by James Watson and Francis Crick, right here.
They're shaking hands with McCarty, who is one of the scientists who had figured out
that is was DNA that was actually doing the transforming inside bacteria. But they were
able to build a model. They were able to build a model because they knew what DNA was made
up of. They knew that it was made up of phosphates, sugar and then these nitrogenous bases. They
knew the ratio of the bases. But until they could physically build it, they couldn't visualize
it. And so models and visual representations are incredibly important. In AP Biology it
will help you do better on the test and I hope that was helpful.