2020 Practice Exam 2 Frq Ap Bio

Navigating the complexities of the AP Biology exam can feel daunting, especially when facing challenging Free Response Questions (FRQs). The 2020 Practice Exam 2 FRQs present a valuable opportunity to hone your analytical and problem-solving skills in the context of biological principles. Let’s delve into the intricacies of these FRQs, dissecting each question, and providing detailed explanations to enhance your understanding.

FRQ 1: The Case of the Mysterious Microbe

This FRQ probes your ability to analyze experimental data, propose hypotheses, and understand the ecological roles of microorganisms. Imagine you are a research scientist studying a newly discovered microbe in a remote hot spring. The question revolves around designing experiments to determine the microbe's metabolic processes and its role in the hot spring ecosystem.

Understanding the Scenario

Before diving into the specific prompts, let's establish a solid understanding of the scenario. We are dealing with:

• A novel microbe: This implies that its characteristics are not well-defined, and we need to investigate its properties.
• A hot spring environment: This suggests that the microbe is likely a thermophile, an organism that thrives in high-temperature conditions. This ecological context is crucial for formulating hypotheses and experimental designs.

Part (a): Identifying the Mode of Nutrition

The first part of the FRQ asks you to design an experiment to determine whether the microbe is an autotroph or a heterotroph. Understanding the distinction between these two modes of nutrition is fundamental.

• Autotrophs produce their own food from inorganic sources through photosynthesis or chemosynthesis.
• Heterotrophs obtain their nutrition by consuming organic compounds.

Here's a potential experimental design:

1. Preparation:
   • Prepare two sterile culture media:
     • Medium A: Contains only inorganic salts, a nitrogen source (e.g., ammonium chloride), and water. This medium lacks any organic carbon sources.
     • Medium B: Contains the same inorganic salts and nitrogen source as Medium A, but with the addition of a specific organic carbon source like glucose.
   • Ensure both media are free of any pre-existing microbial contaminants by autoclaving them.
2. Inoculation:
   • Obtain a pure culture of the unknown microbe.
   • Divide the culture into two equal portions.
   • Inoculate Medium A with one portion and Medium B with the other. Ensure equal initial cell densities in both cultures.
3. Incubation:
   • Incubate both cultures under optimal conditions for the microbe, considering it's from a hot spring. A temperature of 50-70°C might be appropriate.
   • Provide consistent lighting conditions to rule out light as a limiting factor.
4. Observation and Measurement:
   • Monitor the growth of the microbe in both media over a period of days.
   • Measure the cell density in each culture at regular intervals using a spectrophotometer (measuring turbidity) or by direct cell counts under a microscope.
   • Record any visible changes in the media, such as color changes or precipitate formation.

Expected Results and Interpretation:

• If the microbe grows in Medium A (inorganic salts only): This indicates that the microbe is an autotroph. It can synthesize its own organic compounds from inorganic sources.
• If the microbe grows in Medium B (with glucose) but not in Medium A: This indicates that the microbe is a heterotroph. It requires an organic carbon source to grow.
• If the microbe grows in both Medium A and Medium B: This could suggest that the microbe is a facultative heterotroph, capable of utilizing both inorganic and organic carbon sources. Further experiments could be designed to investigate its preference or efficiency in utilizing each type of carbon source.
• If the microbe does not grow in either medium: This suggests that the microbe might require specific nutrients or conditions not provided in either medium. This would necessitate further experimentation to identify these requirements.

Control Group:

A crucial component of any well-designed experiment is a control group. In this case, a suitable control would be:

• A third sterile culture medium (Medium C) identical to Medium A (inorganic salts only), but without any microbe inoculation. This control verifies that any growth observed in Medium A is indeed due to the inoculated microbe and not from contamination.

Part (b): Investigating the Microbe's Role in Nitrogen Cycling

Nitrogen cycling is a critical process in ecosystems. This part of the FRQ requires you to propose a hypothesis about the microbe's role in nitrogen cycling and design an experiment to test your hypothesis.

Background on Nitrogen Cycling:

• Nitrogen fixation: Conversion of atmospheric nitrogen (N2) into ammonia (NH3).
• Nitrification: Conversion of ammonia (NH3) into nitrite (NO2-) and then into nitrate (NO3-).
• Denitrification: Conversion of nitrate (NO3-) into gaseous nitrogen (N2), returning it to the atmosphere.
• Ammonification: Decomposition of organic matter, releasing ammonia (NH3).

Hypothesis:

Based on the hot spring environment and the presence of other microbes, a reasonable hypothesis could be:

• Hypothesis: The microbe plays a role in nitrification by converting ammonia (NH3) into nitrite (NO2-) or nitrate (NO3-). Hot springs often have high ammonia concentrations due to geothermal activity and decomposition. Nitrification would help regulate nitrogen availability in the ecosystem.

Experimental Design:

1. Preparation:
   • Prepare a sterile culture medium containing a known concentration of ammonia (NH4Cl).
   • Divide the medium into two equal portions:
     • Experimental Group: Inoculate with the pure culture of the unknown microbe.
     • Control Group: Do not inoculate (sterile control).
2. Incubation:
   • Incubate both the experimental and control groups under optimal temperature and other conditions for the microbe.
3. Measurement:
   • At regular intervals (e.g., daily), measure the concentrations of ammonia (NH3), nitrite (NO2-), and nitrate (NO3-) in both the experimental and control groups.
   • Use specific ion electrodes or colorimetric assays to accurately measure these nitrogen compounds.

Expected Results and Interpretation:

• If the ammonia concentration decreases in the experimental group, while the nitrite and/or nitrate concentrations increase: This supports the hypothesis that the microbe is involved in nitrification. The microbe is likely converting ammonia into nitrite and/or nitrate.
• If the ammonia, nitrite, and nitrate concentrations remain relatively constant in both the experimental and control groups: This suggests that the microbe does not play a significant role in nitrification under the tested conditions. Other nitrogen cycling processes or different environmental conditions might be influencing nitrogen transformations.

Controls:

• Sterile Control (mentioned above): Ensures that any changes in nitrogen concentrations are due to the microbe and not due to spontaneous chemical reactions or contamination.
• Heat-killed Microbe Control: Inoculate a culture medium with heat-killed (non-viable) microbes. This control helps determine if the observed nitrogen transformations are due to the microbe's metabolic activity or due to abiotic processes.

Part (c): Impact of Temperature on Growth Rate

This part of the FRQ asks you to predict how temperature might affect the growth rate of the microbe and provide a rationale.

Prediction:

• Prediction: As temperature increases from a low point to an optimum level, the growth rate of the microbe will increase. Beyond the optimum temperature, the growth rate will decrease.

Rationale:

• Enzyme Activity: The microbe's metabolic processes are driven by enzymes. Enzyme activity is highly temperature-dependent. At low temperatures, enzyme activity is reduced, slowing down metabolic reactions and thus limiting growth. As temperature increases towards the optimum, enzyme activity increases, leading to a faster growth rate.
• Protein Denaturation: Beyond the optimum temperature, enzymes and other proteins within the microbe can begin to denature. Denaturation disrupts the three-dimensional structure of the proteins, rendering them non-functional. This leads to a decline in metabolic activity and a decrease in growth rate. In extreme cases, protein denaturation can lead to cell death.
• Membrane Fluidity: Temperature also affects the fluidity of the cell membrane. At low temperatures, the membrane becomes less fluid, hindering the transport of nutrients into the cell and waste products out. At high temperatures, the membrane can become too fluid, disrupting its integrity and leading to leakage of cellular contents.
• Homeostasis: Organisms need to maintain homeostasis for optimal functioning. Temperature plays a crucial role in maintaining internal stability. Extreme temperatures disrupt homeostasis, impacting the growth and survival of the microbe.

Key Takeaways from FRQ 1

• Experimental Design: Clearly define your hypothesis, controls, variables, and expected results.
• Ecological Context: Consider the environment in which the organism lives.
• Biological Principles: Relate your answers to fundamental biological concepts like enzyme activity, protein structure, and nutrient cycling.

FRQ 2: Genetic Mutations and Phenotypic Consequences

This FRQ focuses on your understanding of molecular genetics, specifically how mutations in a gene can lead to changes in protein structure and function, ultimately affecting the phenotype of an organism. The scenario involves a gene responsible for pigment production in a hypothetical plant.

Understanding the Scenario

We are given that:

• A gene codes for an enzyme involved in pigment production.
• Mutations in this gene can lead to different phenotypes (variations in pigment color).

Part (a): Explaining the Central Dogma

This part of the FRQ requires you to explain how a mutation in the gene can alter the structure of the enzyme. This relates to the central dogma of molecular biology: DNA → RNA → Protein.

Explanation:

1. DNA Mutation: A mutation is a change in the nucleotide sequence of DNA. Mutations can occur spontaneously or be induced by mutagens (e.g., radiation, chemicals).
2. Transcription: The mutated DNA sequence is transcribed into messenger RNA (mRNA). The altered DNA sequence will result in a corresponding change in the mRNA sequence.
3. Translation: The mRNA is translated into a protein (in this case, the enzyme). The ribosome reads the mRNA codons, and each codon specifies a particular amino acid. If the mRNA sequence is altered due to the mutation, the resulting amino acid sequence of the enzyme will also be altered.
4. Altered Enzyme Structure: The amino acid sequence determines the three-dimensional structure of the enzyme. A change in the amino acid sequence, even a single amino acid substitution, can alter the folding and conformation of the enzyme.
5. Consequence: This altered structure can affect the enzyme's active site, substrate binding affinity, and catalytic efficiency. In the context of pigment production, a mutated enzyme might have reduced activity or be completely non-functional, leading to a change or absence of pigment.

Types of Mutations:

Briefly mentioning different types of mutations can strengthen your answer:

• Point mutations: Single nucleotide changes (substitutions, insertions, or deletions).
• Frameshift mutations: Insertions or deletions that shift the reading frame, leading to a completely different amino acid sequence downstream of the mutation.
• Nonsense mutations: Mutations that introduce a premature stop codon, resulting in a truncated and often non-functional protein.

Part (b): Impact on Phenotype

Next, you need to explain how the altered enzyme structure affects the phenotype of the plant.

Explanation:

1. Enzyme Function: The enzyme is responsible for catalyzing a specific reaction in the pigment production pathway. If the enzyme's structure is altered due to a mutation, its ability to catalyze this reaction will be affected.
2. Pigment Production Pathway: Pigment production often involves a series of enzymatic reactions. If one enzyme in the pathway is non-functional or has reduced activity, the pathway will be disrupted.
3. Phenotype Change: The disruption of the pigment production pathway can lead to:
   • Reduced pigment production: The plant might have a lighter color or a different shade.
   • Complete absence of pigment: The plant might be albino (white).
   • Accumulation of pathway intermediates: If the enzyme is blocked, the substrate for that enzyme might accumulate, leading to a different color phenotype.
4. Example: If the wild-type enzyme converts a colorless precursor into a purple pigment, a mutation that inactivates the enzyme might result in a plant with a colorless (white) phenotype.

Part (c): Environmental Factors

The final part of the FRQ asks how environmental factors can influence the expression of the mutated gene and the resulting phenotype.

Explanation:

1. Gene Expression Regulation: Environmental factors can influence gene expression by affecting transcription, translation, or protein stability.
2. Temperature: Temperature can affect enzyme activity and protein folding. Even if the enzyme is mutated, a specific temperature range might allow it to function partially, leading to some pigment production. At other temperatures, the enzyme might be completely non-functional.
3. Light: Light can influence the expression of genes involved in pigment production. In some plants, light is required for the transcription of genes encoding enzymes in the chlorophyll biosynthesis pathway. If a mutated plant is grown in the dark, it might not produce any pigment, regardless of the mutation.
4. Nutrients: Nutrient availability can affect enzyme synthesis and activity. For example, a lack of certain minerals (e.g., magnesium) can inhibit the synthesis of chlorophyll, even if the plant has a functional chlorophyll biosynthesis pathway.
5. pH: pH can affect enzyme activity and protein folding. Extreme pH levels can denature enzymes, even if they are not mutated.
6. Example: A plant with a mutated enzyme that produces a pale yellow pigment might exhibit a darker yellow color when grown in soil rich in certain minerals compared to when grown in nutrient-poor soil.

Key Takeaways from FRQ 2:

• Central Dogma: Understand the flow of information from DNA to RNA to protein.
• Mutation Types: Know the different types of mutations and their potential effects.
• Enzyme Function: Relate enzyme structure to its function.
• Environmental Influences: Recognize how environmental factors can affect gene expression and phenotype.

General Strategies for Tackling AP Biology FRQs

• Read Carefully: Thoroughly read the entire question before you start writing.
• Plan Your Answer: Take a few minutes to outline your answer before you start writing. This will help you stay focused and organized.
• Use Biological Terminology: Use accurate biological terminology.
• Be Specific: Avoid vague generalizations. Provide specific details and examples.
• Relate to Concepts: Connect your answers to broader biological concepts and principles.
• Address All Parts: Make sure you address all parts of the question.
• Write Clearly: Write in a clear and concise manner.
• Show Your Work: Even if you don't know the exact answer, show your thought process.
• Manage Your Time: Allocate your time wisely. Don't spend too much time on any one question.
• Practice Regularly: The more you practice, the better you will become at answering FRQs.

By understanding the underlying biological principles, practicing regularly, and applying these strategies, you can confidently tackle even the most challenging AP Biology FRQs and achieve success on the exam. Remember to focus on clear communication, logical reasoning, and a strong foundation in core biological concepts. Good luck!