Apex Mock Exam 2 Actual QUESTIONS WITH
COMPLETE SOLUTIONS – Comprehensive
Practice & Review
Q1:
Scenario: A research team investigates the effect of temperature on metabolic
pathways in spinach chloroplasts. In Treatment 1, isolated chloroplasts are incubated at
35°C with saturating light and CO₂. In Treatment 2, identical chloroplasts are incubated
at 10°C with the same light and CO₂ conditions. Oxygen production in Treatment 2
drops by 80% compared to Treatment 1, despite chlorophyll absorption spectra
remaining unchanged.
Question: Which biological process is most directly responsible for the reduced oxygen
production at lower temperatures?
A. Photolysis of water molecules in Photosystem II becoming energetically unfavorable
B. Reduced kinetic energy limiting Calvin cycle enzyme efficiency, creating a metabolic
bottleneck
C. Decreased fluidity of thylakoid membranes preventing electron transport chain
function
D. Competitive inhibition of rubisco by excess oxygen at low temperatures
Solution:
,Concepts Tested: Photosynthesis (Light Reactions vs. Calvin Cycle), Enzyme Kinetics,
Temperature Effects on Metabolism.
Scenario Deconstruction: The key variables are temperature (independent) and oxygen
production (dependent). Light intensity and CO₂ are saturating (non-limiting). The 80%
drop indicates a process highly sensitive to thermal energy, while unchanged absorption
spectra rule out photochemical disruption (light harvesting remains functional).
Step-by-Step Reasoning:
● Oxygen is produced during photolysis in the light-dependent reactions
(photosystem II), but these reactions are photochemical (driven by photon
energy) and relatively temperature-insensitive.
● The Calvin cycle (light-independent) is enzyme-driven and highly
temperature-dependent. At 10°C, enzyme kinetic energy decreases, slowing
carbon fixation.
● This creates a metabolic bottleneck: NADP⁺ and ADP are not regenerated quickly
enough to accept electrons/H⁺ from the light reactions, causing feedback
inhibition that slows the entire electron transport chain and therefore
photolysis/oxygen production.
● Therefore, the proximate cause is enzyme limitation in the Calvin cycle, not a
direct failure of the light reactions.
Final Answer: B
Why It's Correct / Why Others Are Wrong:
● B is correct: It identifies the enzyme-mediated Calvin cycle as the thermally
sensitive bottleneck that indirectly restricts oxygen production by limiting NADP⁺
regeneration.
● A is incorrect: Photolysis is driven by light energy (photons), not thermal energy; it
proceeds even at low temperatures if light is present.
● C is incorrect: While extreme cold can reduce membrane fluidity, 10°C is not
sufficiently low to prevent electron transport in biological membranes, and the
scenario specifies absorption spectra are unchanged.
● D is incorrect: Photorespiration (rubisco oxygenation) increases at high
temperatures, not low temperatures, and would not explain the oxygen production
decrease.
,Q2:
Scenario: In a controlled pond ecosystem study, researchers introduce a predatory fish
species that selectively feeds on the largest individuals of a native minnow population.
Over 15 generations, the average adult body size of the minnow population decreases
from 12 cm to 7 cm, while sexual maturation occurs at an earlier age. Genetic analysis
confirms these changes are heritable.
Question: This evolutionary pattern best illustrates which mechanism of selection?
A. Disruptive selection favoring both small and large phenotypes simultaneously
B. Stabilizing selection reducing variation around the original mean
C. Directional selection shifting the population mean toward one extreme
D. Genetic drift randomly altering allele frequencies independent of fitness
Solution:
Concepts Tested: Natural Selection (Types of Selection), Population Genetics,
Evolutionary Dynamics.
Scenario Deconstruction: The selective agent is predation (fish) targeting a specific
phenotype (large size). The result is a unidirectional shift in the population mean toward
smaller size and earlier reproduction. Heritability confirms evolutionary change rather
than phenotypic plasticity.
Step-by-Step Reasoning:
● Directional selection occurs when environmental pressures favor one extreme
phenotype over others, causing the population mean to shift toward that
extreme.
, ● Here, "large size" is disadvantageous (higher predation), while smaller size
confers survival advantage.
● The mean body size moved from 12 cm toward 7 cm (directional shift).
● Disruptive selection would favor both small AND large (eliminating medium),
which did not occur.
● Stabilizing selection would favor the intermediate size (12 cm), which is the
opposite of the observed pattern.
Final Answer: C
Why It's Correct / Why Others Are Wrong:
● C is correct: It accurately describes the unidirectional shift in phenotype caused
by selective predation pressure.
● A is incorrect: There is no evidence that large individuals are favored alongside
small ones; large individuals are preferentially removed.
● B is incorrect: Stabilizing selection would maintain or reinforce the original 12 cm
mean by selecting against extreme variants (both very small and very large).
● D is incorrect: The change is non-random and directly correlated with a specific
selective pressure (predation); drift operates stochastically without relationship
to fitness advantages.
Q3:
Scenario: A plant breeder crosses two heterozygous pea plants (RrYy × RrYy), where R
represents round seeds (dominant over wrinkled), and Y represents yellow seeds
(dominant over green). The genes assort independently.
Question: What is the statistical probability that an offspring will display the wrinkled
and green phenotype combination?
A. 1/16
B. 3/16
C. 1/8
D. 9/16
COMPLETE SOLUTIONS – Comprehensive
Practice & Review
Q1:
Scenario: A research team investigates the effect of temperature on metabolic
pathways in spinach chloroplasts. In Treatment 1, isolated chloroplasts are incubated at
35°C with saturating light and CO₂. In Treatment 2, identical chloroplasts are incubated
at 10°C with the same light and CO₂ conditions. Oxygen production in Treatment 2
drops by 80% compared to Treatment 1, despite chlorophyll absorption spectra
remaining unchanged.
Question: Which biological process is most directly responsible for the reduced oxygen
production at lower temperatures?
A. Photolysis of water molecules in Photosystem II becoming energetically unfavorable
B. Reduced kinetic energy limiting Calvin cycle enzyme efficiency, creating a metabolic
bottleneck
C. Decreased fluidity of thylakoid membranes preventing electron transport chain
function
D. Competitive inhibition of rubisco by excess oxygen at low temperatures
Solution:
,Concepts Tested: Photosynthesis (Light Reactions vs. Calvin Cycle), Enzyme Kinetics,
Temperature Effects on Metabolism.
Scenario Deconstruction: The key variables are temperature (independent) and oxygen
production (dependent). Light intensity and CO₂ are saturating (non-limiting). The 80%
drop indicates a process highly sensitive to thermal energy, while unchanged absorption
spectra rule out photochemical disruption (light harvesting remains functional).
Step-by-Step Reasoning:
● Oxygen is produced during photolysis in the light-dependent reactions
(photosystem II), but these reactions are photochemical (driven by photon
energy) and relatively temperature-insensitive.
● The Calvin cycle (light-independent) is enzyme-driven and highly
temperature-dependent. At 10°C, enzyme kinetic energy decreases, slowing
carbon fixation.
● This creates a metabolic bottleneck: NADP⁺ and ADP are not regenerated quickly
enough to accept electrons/H⁺ from the light reactions, causing feedback
inhibition that slows the entire electron transport chain and therefore
photolysis/oxygen production.
● Therefore, the proximate cause is enzyme limitation in the Calvin cycle, not a
direct failure of the light reactions.
Final Answer: B
Why It's Correct / Why Others Are Wrong:
● B is correct: It identifies the enzyme-mediated Calvin cycle as the thermally
sensitive bottleneck that indirectly restricts oxygen production by limiting NADP⁺
regeneration.
● A is incorrect: Photolysis is driven by light energy (photons), not thermal energy; it
proceeds even at low temperatures if light is present.
● C is incorrect: While extreme cold can reduce membrane fluidity, 10°C is not
sufficiently low to prevent electron transport in biological membranes, and the
scenario specifies absorption spectra are unchanged.
● D is incorrect: Photorespiration (rubisco oxygenation) increases at high
temperatures, not low temperatures, and would not explain the oxygen production
decrease.
,Q2:
Scenario: In a controlled pond ecosystem study, researchers introduce a predatory fish
species that selectively feeds on the largest individuals of a native minnow population.
Over 15 generations, the average adult body size of the minnow population decreases
from 12 cm to 7 cm, while sexual maturation occurs at an earlier age. Genetic analysis
confirms these changes are heritable.
Question: This evolutionary pattern best illustrates which mechanism of selection?
A. Disruptive selection favoring both small and large phenotypes simultaneously
B. Stabilizing selection reducing variation around the original mean
C. Directional selection shifting the population mean toward one extreme
D. Genetic drift randomly altering allele frequencies independent of fitness
Solution:
Concepts Tested: Natural Selection (Types of Selection), Population Genetics,
Evolutionary Dynamics.
Scenario Deconstruction: The selective agent is predation (fish) targeting a specific
phenotype (large size). The result is a unidirectional shift in the population mean toward
smaller size and earlier reproduction. Heritability confirms evolutionary change rather
than phenotypic plasticity.
Step-by-Step Reasoning:
● Directional selection occurs when environmental pressures favor one extreme
phenotype over others, causing the population mean to shift toward that
extreme.
, ● Here, "large size" is disadvantageous (higher predation), while smaller size
confers survival advantage.
● The mean body size moved from 12 cm toward 7 cm (directional shift).
● Disruptive selection would favor both small AND large (eliminating medium),
which did not occur.
● Stabilizing selection would favor the intermediate size (12 cm), which is the
opposite of the observed pattern.
Final Answer: C
Why It's Correct / Why Others Are Wrong:
● C is correct: It accurately describes the unidirectional shift in phenotype caused
by selective predation pressure.
● A is incorrect: There is no evidence that large individuals are favored alongside
small ones; large individuals are preferentially removed.
● B is incorrect: Stabilizing selection would maintain or reinforce the original 12 cm
mean by selecting against extreme variants (both very small and very large).
● D is incorrect: The change is non-random and directly correlated with a specific
selective pressure (predation); drift operates stochastically without relationship
to fitness advantages.
Q3:
Scenario: A plant breeder crosses two heterozygous pea plants (RrYy × RrYy), where R
represents round seeds (dominant over wrinkled), and Y represents yellow seeds
(dominant over green). The genes assort independently.
Question: What is the statistical probability that an offspring will display the wrinkled
and green phenotype combination?
A. 1/16
B. 3/16
C. 1/8
D. 9/16
