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.