AP Biology Practice Exam: Section II, Long Free Response Questions

Welcome to Educator.com. I am Dr. Carleen Eaton.0000

In this lesson, I am going to be continuing the review of a practice AP Biology Examination.0004

And in this section, I will be doing section 2, which are the long free response questions.0011

You can find this practice test in the book Barron's AP Biology 4th Edition, and this is model test 1.0019

When you approach the long free response questions or any free response question actually,0031

remember that you are given points for correct statements, but you do not lose points for saying things that are incorrect.0037

Also, make sure that you address every part of the question.0048

These questions we are going to be doing today have multiple parts to them.0052

So, the first question talks about cell communication and has four different parts.0057

Some major points to cover for part A are that both multicellular and unicellular organisms need to use cellular communication.0065

Multicellular organisms such as plants and animals must coordinate the functions of groups of cells so that organs and tissues can work together.0079

Unicellular organisms also need to communicate because they can act in a coordinate manner at times.0131

So, they use communication to act in a coordinated manner. For example, bacteria can recognize population density.0147

So, they can recognize the density of the population that surrounds them through a mechanism called quorum sensing.0173

And quorum sensing plays a role in biofilm formation.0200

Now, you can break cell signalling into two broad types: cell signalling through direct contact and cell signalling over distances.0211

Some examples of cell signalling that occurs through direct contact are plasmodesmata found in plant cells and gap junctions found in animal cells.0233

And in both of these, there is contact between the cytoplasm of adjacent cells.0257

This means that molecules from one cell can pass directly into another cell, so cytoplasmic molecules can go directly from one cell to another.0281

Cells can also communicate locally and over relatively long distances.0307

So, talking about cell signalling based on distance, autocrine signalling is a type of signalling in which0313

the cell secretes a chemical messenger that binds to receptors on the same cell initiating a response.0328

So, this is autocrine signalling.0352

There is also paracrine signalling. In this type of signalling, molecules are secreted by cells and diffuse and bind to receptors on nearby cells.0353

So, this is a form of local signalling or local communication.0386

Finally, there is endocrine signalling. In endocrine signalling, cells can communicate with distant cells.0393

For example, hormones such as thyroid hormone, estrogen, aldosterone are released by cells and travel through the bloodstream to target organs.0415

And these organs can be some distance away.0438

OK, so, this covers part A.0450

Part B of this question, first of all, one thing you could discuss is ligands.0454

A ligand is a molecule that binds to another molecule, and they are specific to their receptor.0463

So, a ligand binds to a particular receptor or group of receptors.0482

OK, ligands are specific to their receptors, and they induce a conformational change in the receptor.0489

The two types of receptors are cell membrane receptors and intracellular receptors.0516

Most ligands are hydrophilic. They are, therefore, water soluble and not permeable to the plasma membrane.0549

Therefore, they must bind to cell membrane receptors.0572

Actually, let me correct that. I meant to say that hydrophilic ligands are not permeable to the cell membrane, so they bind to cell membrane receptors.0583

Hydrophobic receptors are permeable to the cell membrane, and therefore, they can enter the cell and bind to intracellular receptors.0595

So, hydrophilic ligands would not bind to cell membrane receptors, whereas hydrophobic ligands - again ligands - can bind to intracellular receptors.0620

To give you some examples of cell membrane receptors, G protein-coupled receptors.0639

And these bind to a wide variety of hormones and neurotransmitters.0660

So, what happens is a ligand binds the G protein receptor, and it causes a conformational change in the receptor.0666

And as a result of that conformational change, the receptor binds to the nearby G protein.0690

The protein then releases GDP and binds GTP, so the G protein binds GTP, and that puts it in its active form.0702

And then, the G protein can bind an enzyme that activates the enzyme.0717

That activated enzyme may then bind another enzyme, and what you have here is a cascade.0726

This cascade is called a signal transduction pathway, so the signal is transduced from outside of the cell.0737

The ligand binds the receptor on the surface of the cell and sends a message to the interior of the cell.0748

Another class of receptors, so another example, are protein kinase receptors.0764

And one well-known example of that is receptor tyrosine kinases or TKs, and these are receptors that are also enzymes.0769

And tyrosine kinases often activate multiple transduction pathways and elicit numerous responses at the same time.0791

So, they can activate multiple pathways and elicit numerous responses.0801

And this is important because it allows the cell to coordinate activities such as growth.0816

So, the cell can coordinate growth and other activities.0825

Another type of receptor or a class of receptor are ligand-gated ion channels.0837

In this type of receptor, binding of a ligand to the receptor opens this receptor, which is also a channel.0851

So, it opens the channel, and ions can pass through.0863

When the ligand dissociates from the receptor, the gate closes.0871

So, these were some types of cell membrane receptors.0877

Intracellular receptors are located in the cytoplasm or the nucleus of the cell.0881

And as I mentioned, you would, therefore, need a hydrophobic ligand that is permeable to the cell membrane and can enter the cell.0899

So, signal molecules enter the cell, and they bind the receptor in the cytoplasm.0907

And then, the receptor molecule complex can travel to the nucleus.0920

Or in some cases, the receptors are already in the nucleus, and the signal molecule goes to the nucleus and binds to a receptor there.0925

So, they bind the receptor, and these are often hormones.0932

And then, this ligand or receptor complex turns genes on or off, and it acts directly as a transcription factor.0935

Again, hormones, thyroid hormone, testosterone, those are some examples of ligands that can act in this manner that bind to intracellular receptors.0962

OK, so, we have talked about the various types of receptors, and now, in part C,0974

we are going to talk about what happens when the receptor binds, when the signal is transmitted to the cell, what the effect is.0983

So, the result of binding of the receptor by ligand is transduction of the message to the interior of the cell if it is a membrane receptor.0992

If it is an intracellular receptor, then, the message can be brought directly into the cell.1016

And this initiates a response, and the response can be that a gene is turned on or off, as I mentioned in the previous section actually.1028

And again, in some cases, when there is an intracellular receptor, a hormone receptor complex can act directly as a transcription factor.1046

Whereas with the cell membrane receptor, genes are turned on and off, but it is not a direct action. It is an indirect action.1066

And the result will be that there will be an increase or decrease in mRNA synthesis.1078

And cell membrane receptors act indirectly as transcription factors.1089

Cell signalling, pathways may also act at the level of the protein.1099

So, instead of blocking or increasing production of a protein, it can affect the activity of a protein.1113

So, increase or decrease the activity level of a protein, so it could activate a protein.1121

Finally, section D. There are common features of cell communication processes found among a wide variety of organisms.1135

And these reflect a shared evolutionary history.1146

For example, bacteria, yeast, animals and plants all use signal transduction, and the details of these pathways are remarkably similar.1184

And this suggests that cell signalling evolved early in the history of life.1213

OK, so that completes all the sections of question 1.1240

I am going to go on to question 2, and the topic of this talks about biomes, weather, climate.1245

So, starting out with section A, climate describes a region in terms of temperature, precipitation, wind, sunlight,1254

factors like that over the long term- to the temperature, precipitation, wind, the amount of sunlight over the long term.1268

By contrast, weather describes conditions over the short term such as on a particular day.1304

You are asked to describe a biome in section B, and I actually chose a different biome1326

that is given in the book just to show you another way that this question could be answered.1332

So, the biome I am going to describe is the desert biome, and deserts are often found in bands around 30° north and south latitude.1337

They are found in North America, South America, Africa, Asia and Australia.1355

They are typically on the interior of continents, but there are actually coastal deserts.1377

The climate is typically hot and dry, but it can get cold at night.1406

However, there are exceptions. There are some cool or even cold deserts.1420

Vegetation: it includes low shrubs, cacti and small trees.1432

The plants that you will find in deserts are drought-resistant, and they have minimal leaf area so that they can retain water more efficiently.1452

Animals that live in the desert include reptiles, rodents and various small mammals, birds, insects.1475

Many of these animals are nocturnal and have adaptations that allow them to conserve water, OK?1499

So, that covers part A and B.1515

In section C, there are numerous examples that you could use of a disturbance.1519

Some are fire, volcanic eruptions, severe storms, winds, floods. I am going to focus on fire.1525

So, periodic fires occur in many biomes. So, periodic wild fires actually occur in many biomes such as grasslands and forests.1535

They can also be human caused.1556

When a fire occurs, there will be plants destroyed over a very large area.1570

Animals may be killed directly by the fire or lose their habitat and be displaced.1589

Secondary succession will occur because the soil is still present, and some seeds or plants may survive underground.1601

So, plants will reinhabit the area, and animals will also move back into the area.1632

The results of this is that there will be an increase in biodiversity because1647

after a long time without a disturbance, certain species may end up dominating.1660

And what a disturbance does is it can result in a fresh start where other plant or animal species can get a foothold.1665

So, after a long period without a disturbance, certain species dominate.1683

And you can look at this as a fresh start to allow other species to compete for resources.1703

In the final section of this question, you are asked to talk about the effect of a geographic cline of evolution of an organism.1720

Now, when we talk about geographic clines, what we mean is a gradient of the characteristic across a geographic region.1730

A classic example is the fur color of rabbits in North America.1755

If you observe rabbit species in the North and then, travel farther South,1769

what you will see is that going from North to South, the fur color goes from white to brown.1775

And this is because in the North, a white rabbit would better blend in with a region where there is a lot of snow.1782

And in a more southern region where there is very little or no snow,1792

the rabbit is going to blend in better with the surrounding soil or the trunks of trees.1798

You can also see an effect going from North to South with body size. You will see a larger body size in cooler areas.1808

So, you will see that body size goes from larger to smaller as you go from a cooler to a warmer region.1826

And this is because larger body size means that there is a smaller surface to volume ratio, which helps to conserve heat.1837

And it is also postulated that an organism with a larger body size will have greater fat stores.1858

So, these are two examples of the effect of a cline on characteristic.1868

So, that concludes this lesson, as well as the review of the AP Biology practice test.1874

Thank you for visiting Educator.com.1881