AZSCI Life Science 2 — Questions and Answers
Question 1: A scientist observes that a population of desert tortoises has a wide range of shell thicknesses. After a prolonged drought, the plants that tortoises with thinner shells primarily eat become scarce. The tortoises with thicker shells are better able to consume tougher, more fibrous plants that survived the drought. Which evolutionary concept does this scenario best illustrate?
- Genetic drift
- Gene flow
- Stabilizing selection
- Directional selection (Correct answer)
Correct answer: Directional selection
This scenario is a classic example of directional selection. An environmental pressure (the drought and resulting change in food source) favors an extreme phenotype (thicker shells) over another extreme phenotype (thinner shells). Over time, this will cause the average shell thickness in the population to increase. Stabilizing selection would favor the intermediate phenotype, genetic drift is change due to random chance, and gene flow involves the transfer of genetic material between populations.
Question 2: A cell from a newly discovered aquatic plant is placed in a solution with an unknown solute concentration. The cell rapidly swells and bursts. Which of the following statements provides the most accurate explanation for this observation?
- The solution was hypertonic to the cell, causing water to rush in.
- The solution was isotonic to the cell, but the cell wall failed.
- The cell's contractile vacuole was non-functional, preventing water expulsion.
- The solution was hypotonic to the cell, causing excessive water influx through osmosis. (Correct answer)
Correct answer: The solution was hypotonic to the cell, causing excessive water influx through osmosis.
The cell burst due to a massive influx of water. This process, known as cytolysis, occurs when a cell is placed in a hypotonic solution—a solution with a lower solute concentration than the cell's cytoplasm. Water moves via osmosis from an area of high water concentration (the solution) to an area of lower water concentration (the cell), causing it to swell and burst. A hypertonic solution would cause the cell to shrink. An isotonic solution would result in no net water movement. While a faulty contractile vacuole could be a factor, the primary cause is the osmotic gradient.
Question 3: Which of the following describes a key difference between the lytic cycle and the lysogenic cycle of a bacteriophage?
- The lytic cycle involves the integration of viral DNA into the host chromosome, while the lysogenic cycle does not.
- The lysogenic cycle results in the immediate lysis (bursting) of the host cell, while the lytic cycle does not.
- In the lysogenic cycle, the viral DNA is replicated along with the host DNA during cell division, remaining dormant. (Correct answer)
- Only the lytic cycle requires the host cell's machinery to synthesize new viral particles.
Correct answer: In the lysogenic cycle, the viral DNA is replicated along with the host DNA during cell division, remaining dormant.
A defining characteristic of the lysogenic cycle is the integration of the viral nucleic acid into the host's genome, where it is called a prophage. This prophage is replicated passively along with the host's chromosome as the cell divides. The virus remains dormant until a trigger causes it to enter the lytic cycle. The lytic cycle, in contrast, leads to the rapid replication of the virus and lysis of the host cell. Both cycles utilize the host cell's machinery for replication.
Question 4: A student is modeling the flow of energy in a terrestrial ecosystem. They correctly place producers at the bottom trophic level. If the producers in this ecosystem store 25,000 kcal of energy, approximately how much energy would be available to the tertiary consumers, assuming a typical 10% energy transfer efficiency between trophic levels?
- 2,500 kcal
- 250 kcal
- 25 kcal (Correct answer)
- 2.5 kcal
Correct answer: 25 kcal
Energy transfer between trophic levels is inefficient, with only about 10% of the energy from one level being incorporated into the next. To calculate the energy available to tertiary consumers, you follow the chain: - Primary consumers receive 10% of 25,000 kcal = 2,500 kcal. - Secondary consumers receive 10% of 2,500 kcal = 250 kcal. - Tertiary consumers receive 10% of 250 kcal = 25 kcal.
Question 5: A geneticist is studying a population of flowers where petal color is determined by a single gene with two alleles: red (R) is dominant to white (r). A wildfire drastically reduces the population size, leaving only a few surviving individuals by chance. The new population has a significantly different allele frequency, with the white allele (r) being much more common than in the original population. This is an example of:
- The founder effect
- The bottleneck effect (Correct answer)
- Disruptive selection
- Sympatric speciation
Correct answer: The bottleneck effect
The bottleneck effect is a type of genetic drift that occurs when a population's size is severely reduced by a random event, such as a natural disaster. The surviving individuals may have allele frequencies that differ from the original population simply by chance, leading to a loss of genetic variation. The founder effect is similar but occurs when a small group colonizes a new area. Disruptive selection favors extreme phenotypes, and sympatric speciation is the evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region.
Question 6: In the process of chemiosmosis during cellular respiration, what is the direct source of energy used by ATP synthase to convert ADP and inorganic phosphate (Pi) into ATP?
- The movement of electrons along the electron transport chain.
- The splitting of water molecules to release oxygen.
- The flow of protons (H+ ions) down their electrochemical gradient across the inner mitochondrial membrane. (Correct answer)
- The breakdown of glucose into pyruvate during glycolysis.
Correct answer: The flow of protons (H+ ions) down their electrochemical gradient across the inner mitochondrial membrane.
Chemiosmosis is the process where energy stored in a hydrogen ion gradient is used to drive cellular work, such as the synthesis of ATP. During cellular respiration, the electron transport chain pumps protons (H+ ions) across the inner mitochondrial membrane, creating a high concentration of protons in the intermembrane space. This electrochemical gradient represents potential energy. The protons then flow back into the mitochondrial matrix through the enzyme ATP synthase, and the kinetic energy from this flow powers the synthesis of ATP from ADP and Pi.
AZSCI Life Science 2
A scientist observes that a population of desert tortoises has a wide range of shell thicknesses.
After a prolonged drought, the plants that tortoises with thinner shells primarily eat become scarce.
The tortoises with thicker shells are better able to consume tougher, more fibrous plants that survived the drought.
Which evolutionary concept does this scenario best illustrate?