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

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

In this lesson, I will be continuing the review of a practice AP Biology Exam, and in this section, I am covering the short free response questions.0004

You can find this practice exam in Barron's AP Biology 4th Edition, and this is model test 1.0018

Question 3 is the first of the short free response questions, and the topic is eutrophication.0028

So, some of the points that you could cover for this are that the release of excessive0039

quantities of nutrients into a body of water leads to eutrophication.0051

In particular, nitrogen or phosphorus are two nutrients that lead to this process.0083

And nitrogen and phosphorus are found in fertilizer, and they can be carried by runoff into lakes, streams and other bodies of water.0090

Phosphors is also found in dishwasher detergent. Nitrogen and phosphorus are found in various other products as well- household products.0114

If the waste water makes its ways into rivers and lakes, that can also lead to eutrophication.0124

Now, some properties of a eutrophic lake. Eutrophication stimulates primary production.0131

So, this leads to an increased biomass of phytoplankton and zooplankton.0138

So, eutrophication leads to increased primary production resulting in increased biomass of phytoplankton and zooplankton.0143

Another property of a eutrophic lake is that there may be increased algal blooms, and some of these algae can be toxic.0173

So, the toxic algae can be harmful to consumers including consumers farther up on the food chain.0187

Another point that you could cover, a property of eutrophic lake is that there is an increase in the turbidity of the water as a result.0203

In addition, the depth of the lake water may be decreased.0221

Dead algae and plants accumulate at the bottom of the lake and decomposers, so microorganisms that0237

break down dead organic matter - decomposers in other words - increase, so increase in the population of decomposers.0261

And this increase leads to a decrease in the dissolved oxygen because they consume oxygen.0278

So, the result is decrease in dissolved oxygen causes fish kills and reduces the diversity of organisms in the lake.0287

So, you are asked in this question just to name two properties of a eutrophic lake,0313

but I gave you multiple that you can choose from, points that you could have covered.0317

Question 4 focuses on changing salinity levels and how those challenge the survival of organisms.0324

Osmoregulation is more challenging in an environment where the salinity levels change.0337

So, recall that water potential determines the direction of movement of water.0370

And water moves from higher to lower potential, so from regions from higher water potential to lower water potential.0396

So, if we look at organisms that are in a higher salinity environment, sea water,0422

sea water has lower water potential, and that is because of the solutes in it.0429

This means that organisms exposed to salt water lose water to their environment.0444

And so, they must constantly replenish it, lose water and must replenish it. By contrast, organisms in fresh water gain water.0461

Water is going to move into these organisms and must remove it, and one way they can do this is by pumping it out, OK?0481

So, this question or this response discusses challenges encountered by organisms that are faced with changing salinity levels.0495

The next topic talks about microRNAs, and we know that only about 2% of the human genome contains sequences that code for proteins.0507

Many of the non-coding sequences regulate gene expressions, so they are non-coding sequences that regulate gene expression.0539

MicroRNAs are small segments of RNA that do not code for proteins, and they are involved in the regulation of gene expression.0559

And these microRNAs are very widely found in organisms.0597

They are found in organisms from unicellular algae all the way up through humans, so very widespread in life.0604

And microRNAs regulate gene expression at the post transcriptional level by binding to complimentary mRNA sequences.0627

So, microRNAs bind to complimentary mRNA sequences.0638

And in doing so, they can either cause the mRNA that they are bound to degrade or block translation.0657

So, either by causing mRNA to degrade or stopping translation from occurring,0675

the end result is that microRNAs silence the expression of their target genes.0681

Question 6: stabilizing selection. Stabilizing selection favors the intermediate form of a phenotype over more extreme forms.0703

And I am going to give you a couple of different examples of this.0744

One is the one that is also given by the book, and that is birth weight in humans.0747

Both high and low birth weight babies encounter more difficulty than those of intermediate weight.0762

So, selection has favored an intermediate way for human offspring.0796

So, that is one example, but a different example, just to give you an idea of what else you could have put as an answer,0807

there are many things that you could have put, but another is plant height.0811

In some plants, selection has favored intermediate plant heights,0818

and that is because a plant with a short stem may not get enough sunlight because it is blocked by taller plants.0822

The sunlight is blocked by taller plants.0846

A very tall plant, so a plant with a long stem, maybe more susceptible to damage by wind and be less structurally stable.0851

So, intermediate plant height can be favored.0873

So, these are two examples of stabilizing selection.0883

In question 7, you are asked to give differences between different types of immunity.0893

Looking first at innate immunity, so innate immunity is also called non-specific immunity, so it is often referred to as non-specific immunity.0899

And that is because it consists of defense mechanisms that do not distinguish between a particular invader.0918

So, this type of immunity does not distinguish invaders. It does not go after a particular type of invader.0925

The first line of defense in the immune system is a type of innate immunity, and these are barrier defenses.0939

Many of these are barrier defenses- includes barrier defenses.0950

The skin, that is a barrier defense, and it blocks any type of invader. It is not specific for a certain type.0963

So, it blocks viruses, chemicals, dirt, bacteria.0971

There are also mucous membranes that are part of the first line of defense.0976

And these line body surfaces that open to the outside such as the respiratory tract in the gastrointestinal tract.0981

Tears are also part of the first line of defense.0991

Lysozymes breakdown bacteria, and these are found in mucous in tears.0996

pH: stomach, urine and skin all have low pH, and this helps to stop the growth of bacteria or kill bacteria.1005

So, innate immunity is non-specific. It includes the first line of defense and the second line of defense.1018

The second line of defense consists of both cellular defenses and proteins.1035

And as part of the second line of defense, we mount what is called an inflammatory response.1047

So, if an invader gets passed the skin, the mucous membranes, these various first lines of defense, then, the second line of defense attacks.1054

And the inflammatory response is triggered by histamine and causes blood1062

vessels to dilate and to become more permeable, so increased permeability.1083

As a result, fluid and phagocytic cells will enter the area, and these phagocytic cells will engulf and destroy invaders.1091

Example of a phagocytic cell is a macrophage, so this was innate or non-specific immunity.1123

The second type of immunity is known as adaptive immunity. This is still part of question 7, but now, I am discussing adaptive immunity.1139

Comparing it with innate immunity, adaptive immunity is also called specific immunity.1155

And it takes longer to become activated than innate immunity, but it is a very powerful part of the immune system.1166

And we call it specific immunity because the adaptive immune system, the cells in that system only target specific invaders, specific antigens.1189

This is sometimes known as the third line of defense, and it consists of B cells and T cells.1213

B cells produce antibodies. T cells are responsible for what is called cell-mediated immunity.1225

And cytotoxic T cells destroy cells that are infected by pathogen or other cells that are abnormal in some way.1241

Helper T cells assist in activating B cells so that the B cells produce antibodies, and they also stimulate cytotoxic T cells.1260

And remember a particular B cell or T cell will only respond to a particular antigen. It is specific to an antigen.1285

Antigen-presenting cells known as APCs process and present antigens to T cells stimulating a response or eliciting a response, OK?1295

So, this gives you some of the comparisons and differences between innate and adaptive immunity.1328

The final question in this short free response section is talking about the Calvin cycle and light-dependent reactions.1336

So, as part of light-dependent reactions in photosynthesis, ADP is phosphorylated to form ATP.1348

NADPH - excuse me - NADP⁺ not NADPH, is reduced to form NADPH. OK, so, that is the light-dependent reactions.1371

Meanwhile, the Calvin cycle or the light-independent reactions, ATP is used as an energy source.1394

So, ATP is broken down to ADP, and NADPH is oxidized back to NADP⁺.1405

So, if a poison inhibits the Calvin cycle, ADP and NADP⁺ will not be regenerated.1425

And these molecules are needed for the light-dependent reactions.1451

So, a poison that inhibits an enzyme in the Calvin cycle would actually have an indirect effect on the light-dependent reactions as well.1468

That concludes this lecture for Educator.com.1478