AP Biology — Practice Exam 3
Format: Section I — 25 multiple-choice questions · Section II — 2 free-response questions Suggested time: 45 min (MCQ) + 25 min (FRQ) Coverage: all 8 CED units. Answer key, reasoning, and FRQ rubrics follow. (Correct answers spread across A–D by design.)
Section I — Multiple Choice
1. The property of water that allows it to dissolve salts and polar molecules is its: A) polarity B) low specific heat C) nonpolarity D) high density
2. Which macromolecule serves as the primary long-term energy storage in animals? A) protein B) nucleic acid C) lipid (fat) D) starch
3. An enzyme that has been denatured has lost its: A) amino acids B) peptide bonds C) carbon D) functional three-dimensional shape
4. In which organelle does the Krebs cycle occur? A) chloroplast B) mitochondrial matrix C) ribosome D) nucleus
5. During the light reactions of photosynthesis, water is split to release: A) glucose B) CO₂ C) oxygen D) ATP only
6. A cell placed in a hypertonic solution will: A) gain water B) lose water and shrink C) stay the same D) burst
7. A signal molecule that binds a receptor to initiate a response is called a: A) ligand B) second messenger C) kinase D) substrate
8. During the cell cycle, DNA is synthesized during: A) G1 B) G2 C) S phase D) mitosis
9. A cross between two organisms heterozygous for one gene (Aa × Aa) gives a phenotypic ratio of: A) 1:1 B) 1:2:1 C) 3:1 D) 9:3:3:1
10. Crossing over during meiosis increases: A) the number of cells B) genetic variation C) chromosome number D) cell size
11. Translation of an mRNA molecule occurs at the: A) nucleus B) ribosome C) Golgi apparatus D) lysosome
12. A mutation that inserts one nucleotide into a gene most likely causes a: A) silent mutation B) point substitution C) frameshift mutation D) no change
13. In a population at Hardy-Weinberg equilibrium, if p = 0.7, then q equals: A) 0.3 B) 0.49 C) 0.7 D) 0.09
14. Which provides the raw material for evolution? A) natural selection B) genetic variation C) carrying capacity D) the niche
15. Approximately what percentage of energy is passed to the next trophic level? A) 90% B) 50% C) 10% D) 1%
16. A relationship in which one organism benefits and the other is harmed is: A) mutualism B) commensalism C) competition D) parasitism
17. Negative feedback maintains homeostasis by: A) amplifying changes B) returning a variable to its set point C) stopping metabolism D) increasing entropy
18. The fluid mosaic model describes the structure of the: A) cell wall B) plasma membrane C) nucleus D) ribosome
19. Facilitated diffusion differs from active transport because it: A) uses ATP B) moves against the gradient C) requires no energy D) occurs only in plants
20. A keystone species is one whose effect on its community is: A) negligible B) proportional to its abundance C) disproportionately large D) limited to plants
21. The enzyme RuBisCO functions in: A) glycolysis B) the Krebs cycle C) carbon fixation (Calvin cycle) D) translation
22. Which best describes the genetic code? A) unique to humans B) nearly universal among organisms C) random D) the same as RNA only
23. Logistic population growth levels off at the: A) biotic potential B) carrying capacity C) trophic level D) niche width
24. A buffer in a biological system functions to: A) raise pH B) lower pH C) resist changes in pH D) add salt
25. Genetic drift has the greatest effect on: A) large populations B) small populations C) stable populations D) growing populations
Section II — Free Response
FRQ 1 (Natural Selection & Evolution — 4 points). A population of beetles lives on tree bark. A new predator hunts by sight; lighter beetles are eaten more often. (a) Predict how the beetle population's coloration will change over generations and name the type of selection. (2 pts) (b) Explain the role of genetic variation in this process. (1 pt) (c) Explain why the allele for lighter color is not immediately eliminated from the population. (1 pt)
FRQ 2 (Cellular Energetics — 4 points). (a) Identify the overall reactants and products of aerobic cellular respiration. (2 pts) (b) Explain why aerobic respiration produces far more ATP than fermentation. (1 pt) (c) Predict the effect on ATP production if a poison blocks the electron transport chain. (1 pt)
Answer key (Section I)
| Q | Ans | Q | Ans | Q | Ans | Q | Ans | Q | Ans |
|---|---|---|---|---|---|---|---|---|---|
| 1 | A | 6 | B | 11 | B | 16 | D | 21 | C |
| 2 | C | 7 | A | 12 | C | 17 | B | 22 | B |
| 3 | D | 8 | C | 13 | A | 18 | B | 23 | B |
| 4 | B | 9 | C | 14 | B | 19 | C | 24 | C |
| 5 | C | 10 | B | 15 | C | 20 | C | 25 | B |
Key distribution: A×4, B×9, C×9, D×3.
Reasoning (Section I)
1. (A) Polarity lets water dissolve polar/ionic substances. 2. (C) Fats are the main long-term energy store in animals. 3. (D) Denaturation = loss of 3-D shape/function. 4. (B) The Krebs cycle runs in the mitochondrial matrix. 5. (C) Photolysis of water releases O₂. 6. (B) Hypertonic surroundings draw water out → shrink. 7. (A) A ligand binds a receptor to start signaling. 8. (C) DNA replicates in S phase. 9. (C) Aa × Aa → 3:1 phenotypic ratio. 10. (B) Crossing over increases genetic variation. 11. (B) Translation occurs at the ribosome. 12. (C) A single insertion shifts the reading frame. 13. (A) q = 1 − p = 0.3. 14. (B) Genetic variation is the raw material for evolution. 15. (C) ~10% energy transfer. 16. (D) Parasitism is +/−. 17. (B) Negative feedback restores the set point. 18. (B) The fluid mosaic model describes the membrane. 19. (C) Facilitated diffusion is passive (no ATP). 20. (C) A keystone species has outsized impact. 21. (C) RuBisCO fixes carbon in the Calvin cycle. 22. (B) The genetic code is nearly universal. 23. (B) Logistic growth plateaus at carrying capacity. 24. (C) Buffers resist pH change. 25. (B) Drift most affects small populations.
FRQ rubrics
FRQ 1 (4 pts). (a) (2) The population will become darker over generations (1); this is directional selection (1). (b) (1) Variation in color must already exist (heritable) for selection to act on. (c) (1) Heterozygous carriers (or surviving light beetles) retain the allele; selection reduces but does not instantly remove a recessive/less-fit allele.
FRQ 2 (4 pts). (a) (2) Reactants: glucose + oxygen (1); products: carbon dioxide + water (+ ATP) (1). (b) (1) Aerobic respiration uses the electron transport chain/oxidative phosphorylation, capturing far more energy than glycolysis/fermentation alone. (c) (1) ATP production drops sharply (only glycolysis-level ATP remains) because the proton gradient cannot form.
Pedagogy: distractors map to specific misconceptions (Q5 confuses the source of O₂; Q13 forgets q = 1 − p; Q19 confuses passive vs active transport). Reviewing why a wrong option attracts is high-yield.