AP Biology Unit 3 practice questions
Try 10 multiple-choice questions from Unit 3 here. Pick an answer to see why it's right or wrong, then take the timed test above for a full AP-style set.
Question 1
A student measures oxygen consumption rate (nmol O2/min) in isolated mitochondria at five succinate concentrations (mM): 0.5→12, 1.0→22, 2.0→35, 4.0→44, 8.0→47. Which graph choice is most appropriate to present these data and highlight enzyme-saturation behavior in the mitochondria?
Scatter plot: x=succinate (mM), y=O2 rate (nmol/min), smooth curve fit
Histogram: x=O2 rate bins (nmol/min), y=frequency of trials at each bin
Pie chart: slices for each succinate (mM) showing percent of total O2 used
Bar graph: x=succinate (mM) categories, y=mean O2 rate; no trend line needed
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Answer: (A)
Connecting substrate concentration to oxygen consumption requires a continuous x–y display that can show a rapid rise followed by a plateau. A scatter plot with a smooth curve highlights saturation consistent with mitochondrial electron transport reaching a maximum rate.
Question 2
A researcher records ATP production rate (µmol ATP/min) in isolated mitochondria at 20°C, 25°C, 30°C, 35°C, and 40°C with three replicates per temperature and reports mean ± standard deviation. What graph setup most appropriately displays the means while communicating variability and the temperature trend?
Line graph: x=temperature (°C), y=ATP rate (µmol/min), mean points with error bars
Pie chart: each slice is a temperature, sized by mean ATP rate (µmol/min) at that level
Histogram: x=ATP rate bins (µmol/min), y=frequency pooling all temperatures together
Box plot: x=temperature (°C), y=ATP rate (µmol/min) with only medians shown
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Answer: (A)
Temperature is an ordered, continuous variable, so connecting mean values can reveal a trend across conditions. Error bars communicate replicate variability while the line emphasizes how mitochondrial ATP synthesis changes with temperature.
Question 3
A researcher measures oxygen consumption rates in isolated mitochondria at various substrate concentrations and wants to display how the rate changes continuously over increasing NADH concentrations. Which graph type and axis configuration would best represent this relationship?
A line graph with NADH concentration on the x-axis and oxygen consumption rate on the y-axis to show the continuous trend
A bar graph with NADH concentration on the x-axis and oxygen consumption rate on the y-axis to compare discrete categories
A pie chart showing the proportion of oxygen consumed at each NADH concentration level tested in the experimental trials
A histogram with oxygen consumption rate on the x-axis and frequency of observations on the y-axis for distribution analysis
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Answer: (A)
Identifying appropriate graph types for continuous data requires understanding that line graphs display trends across a continuous independent variable. NADH concentration varies continuously, and oxygen consumption represents the dependent response, making this configuration optimal for showing how mitochondrial respiration rate changes with substrate availability.
Question 4
A study tracks citric acid cycle activity in mitochondria by measuring CO₂ production rates at different temperatures: 15°C (8 μmol CO₂/min), 25°C (28 μmol CO₂/min), 35°C (52 μmol CO₂/min), 45°C (31 μmol CO₂/min), and 55°C (9 μmol CO₂/min). To represent how temperature affects the matrix enzymes that oxidize acetyl-CoA, which graphical representation would best reveal the optimal temperature range for mitochondrial function?
Line graph with temperature (°C) on x-axis and CO₂ production rate (μmol/min) on y-axis, with a curved trend line showing peak activity at 35°C
Line graph with CO₂ production rate (μmol/min) on x-axis and temperature (°C) on y-axis, with a curved trend line showing peak activity at 52 μmol/min
Bar graph with temperature (°C) on x-axis and CO₂ production rate (μmol/min) on y-axis, showing discrete temperature categories without connecting trend line
Line graph with temperature (°C) on x-axis and CO₂ production rate (μmol/min) on y-axis, with straight linear trend line connecting all data points
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Answer: (A)
Constructing a line graph with a curved trend line effectively reveals the temperature optimum for matrix enzymes in the citric acid cycle. The mitochondrial matrix contains dehydrogenases and other enzymes that oxidize acetyl-CoA, releasing CO₂ while generating NADH and FADH₂. The curve demonstrates how enzyme activity increases with temperature until denaturation reduces function beyond the optimum.
Question 5
A model of the chloroplast displays thylakoid membranes organized into dense stacks known as grana. Which of the following best explains how this structural arrangement illustrates the biological principle of form following function?
It maximizes the surface area available for photosystems to capture light energy efficiently.
It maximizes the volume available for the stroma to store glucose molecules efficiently.
It minimizes the surface area exposed to light to prevent oxidative damage to pigments.
It maximizes the surface area for the Calvin cycle enzymes to fix carbon dioxide efficiently.
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Answer: (A)
Connecting the folded structure to efficiency reveals that stacking membranes increases the density of photosystems within a given volume. This maximization of surface area allows for greater light absorption and electron transport capacity.
Question 6
Models of the light-dependent reactions depict the accumulation of protons (H+) within the thylakoid space, creating a concentration difference across the membrane. How does this model demonstrate the principle of energy coupling in biological systems?
The electrochemical gradient stores potential energy that powers ATP synthase to produce ATP.
The electrochemical gradient stores potential energy that powers photosystems to split water.
The electrochemical gradient stores potential energy that powers enzymes to fix carbon dioxide.
The electrochemical gradient stores potential energy that powers ribosomes to synthesize proteins.
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Answer: (A)
Linking the proton gradient to work demonstrates how cells store energy in an electrochemical difference. This potential energy is coupled to the mechanical rotation of ATP synthase, driving the endergonic synthesis of ATP.
Question 7
Structural models of the chloroplast identify a double membrane and the presence of circular DNA in the stroma. Which of the following best explains how these features relate to the theory of endosymbiosis?
They suggest chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.
They suggest chloroplasts originated from free-living mitochondria engulfed by ancestral eukaryotic cells.
They suggest chloroplasts originated from infoldings of the plasma membrane in ancestral eukaryotic cells.
They suggest chloroplasts originated from eukaryotic cells that were engulfed by ancestral prokaryotic cells.
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Answer: (A)
Connecting organelle structure to evolutionary history reveals that the double membrane and circular DNA are remnants of a bacterial ancestor. These features support the model that chloroplasts were once independent prokaryotic organisms.
Question 8
A biological model shows that the Calvin cycle occurs in the stroma while light-dependent reactions occur in the thylakoids. How does this spatial separation illustrate the principle of cellular compartmentalization?
It separates the enzymes of carbon fixation from the high proton concentration in the thylakoid space.
It separates the enzymes of carbon fixation from the light-dependent reactions that generate ATP and NADPH.
It separates the enzymes of carbon fixation from the high oxygen concentration in the thylakoid space.
It separates the enzymes of carbon fixation from the high ATP concentration in the thylakoid space.
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Answer: (A)
Connecting spatial organization to chemical environment shows that compartmentalization maintains optimal pH levels. The thylakoid lumen becomes acidic (low pH) during light reactions, while the Calvin cycle enzymes in the stroma require a higher pH to function.
Question 9
The enzyme carbonic anhydrase catalyzes the rapid conversion of carbon dioxide and water into bicarbonate ions and protons in red blood cells. Which statement best explains how this enzyme increases the rate of the reaction?
It lowers the activation energy needed to reach the transition state.
It increases the free energy released during product formation.
It shifts the equilibrium constant toward bicarbonate production.
It provides thermal energy to drive the reaction forward rapidly.
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Answer: (A)
Connecting enzyme function to energy profiles reveals that enzymes speed up reactions by stabilizing the transition state, which lowers the activation energy barrier required for the reaction to proceed.
Question 10
Trypsin is a protease that digests proteins by binding to positively charged lysine or arginine side chains in the substrate. A mutation changes a key aspartic acid residue (negatively charged) in the active site to lysine (positively charged). Which best explains the effect on the reaction rate?
The rate decreases because charge repulsion prevents substrate binding.
The rate decreases because the active site becomes sterically blocked.
The rate increases because the positive charge attracts more substrate.
The rate increases because the mutation lowers the activation energy.
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Answer: (A)
Analyzing the charge interaction reveals that replacing a negative residue with a positive one creates electrostatic repulsion against the positive substrate, preventing the formation of the enzyme-substrate complex.
What's on the AP Biology Unit 3 practice test
The standard version takes about 60 minutes and follows the timing and directions of the AP exam, sized to one unit:
- 20 Multiple Choice Questions (30 minutes)
- 1 FRQ – FRQ 1 – Interpreting and Evaluating Experimental Results (Long) (15 minutes)
- 1 FRQ – FRQ 3 – Scientific Investigation (Short) (15 minutes)
Questions are drawn from 1910 multiple-choice questions and 126 FRQs written for Unit 3, Cellular Energetics.
Practice AP Biology Unit 3 FRQs
Answer the unit's free-response questions one at a time and get a score on every part.
- FRQ 1 – Interpreting and Evaluating Experimental Results (Long) (16 questions)
- FRQ 2 – Interpreting and Evaluating Experimental Results with Graphing (Long) (16 questions)
- FRQ 3 – Scientific Investigation (Short) (27 questions)
- FRQ 4 – Conceptual Analysis (Short) (20 questions)
- FRQ 5 – Analyze Model or Visual Representation (Short) (31 questions)
- FRQ 6 – Analyze Data (Short) (16 questions)
Review Unit 3 before you start
- 3.1 Enzymes
- 3.2 Environmental Impacts on Enzyme Function
- 3.3 Cellular Energy
- 3.4 Photosynthesis
- 3.5 Cellular Respiration
- Cellular Respiration Review
- Fitness and Natural Selection