AP Biology — Practice Exam 2

Format: Section I — 25 multiple-choice questions · Section II — 2 free-response questions Suggested time: 45 min (MCQ) + 25 min (FRQ) · calculator permitted for the formula-based items Coverage: all 8 CED units — Chemistry of Life, Cell Structure, Cellular Energetics, Cell Communication, Heredity, Gene Expression, Natural Selection, Ecology. Answer key, worked reasoning, and FRQ rubrics follow. (Correct answers are spread across A–D by design.)

Section I — Multiple Choice

1. Which property of water is most directly responsible for transpirational pull in plants? A) cohesion B) high specific heat C) low density of ice D) ability to act as a solvent

2. A peptide bond forms between the carboxyl group of one amino acid and the ____ of another. A) hydroxyl group B) phosphate group C) amino group D) R-group

3. Which best explains why enzymes are specific to their substrates? A) enzymes are consumed in reactions B) the active site has a complementary shape and chemistry C) all enzymes are identical D) substrates raise activation energy

4. During aerobic respiration, the greatest number of ATP molecules is produced by: A) glycolysis B) the Krebs cycle C) fermentation D) oxidative phosphorylation

5. In photosynthesis, the oxygen released originates from: A) carbon dioxide B) glucose C) water D) ATP

6. A cell placed in a hypotonic solution will most likely: A) gain water and swell B) shrink C) remain unchanged D) lose all solutes

7. Receptor proteins in cell signaling are typically located: A) in the nucleus only B) on or in the plasma membrane C) in the mitochondria D) free in the cytosol exclusively

8. A signal transduction cascade most often results in amplification because: A) each step inactivates the next B) the signal weakens at each step C) it requires no enzymes D) one activated molecule activates many downstream molecules

9. During which phase of the cell cycle is DNA replicated? A) G1 B) G2 C) S D) M

10. A cross between two heterozygous tall plants (Tt × Tt) yields what fraction of short offspring? A) 0 B) 1/4 C) 1/2 D) 3/4

11. Genes located on the same chromosome that are usually inherited together are said to be: A) codominant B) recessive C) linked D) homozygous

12. The flow of genetic information described by the central dogma is: A) DNA → RNA → protein B) protein → RNA → DNA C) RNA → protein → DNA D) DNA → protein → RNA

13. A mutation that changes a single nucleotide and results in a premature stop codon is a: A) silent mutation B) missense mutation C) frameshift mutation D) nonsense mutation

14. In a population at Hardy-Weinberg equilibrium, the frequency of the recessive phenotype is 0.16. The frequency of the recessive allele (q) is: A) 0.04 B) 0.16 C) 0.40 D) 0.84

15. Which condition would most likely increase genetic drift? A) a very large population B) a small, isolated population C) high gene flow D) random mating in a huge population

16. Approximately what percentage of energy is transferred from one trophic level to the next? A) 10% B) 1% C) 50% D) 90%

17. A keystone species is best defined as one that: A) is the most abundant B) is always a top predator C) has an effect on its community disproportionate to its abundance D) is microscopic

18. Negative feedback in homeostasis functions to: A) amplify a change away from the set point B) return a variable toward its set point C) stop all cellular activity D) increase entropy without limit

19. Which molecule carries amino acids to the ribosome during translation? A) mRNA B) rRNA C) DNA polymerase D) tRNA

20. Crossing over during meiosis occurs in: A) prophase I B) metaphase II C) anaphase II D) telophase I

21. The ABO blood group system, with alleles Iᴬ, Iᴮ, and i, illustrates: A) sex linkage B) codominance and multiple alleles C) simple recessive inheritance D) polygenic inheritance only

22. Which of the following is an example of a density-dependent limiting factor? A) a hurricane B) a drought C) competition for food D) a wildfire

23. Smooth endoplasmic reticulum is primarily associated with: A) protein synthesis B) ATP production C) DNA replication D) lipid synthesis and detoxification

24. Two species evolving in response to each other over time is called: A) coevolution B) genetic drift C) bottleneck effect D) founder effect

25. Facilitated diffusion differs from active transport because facilitated diffusion: A) requires ATP B) moves substances against the gradient C) moves substances down the gradient without ATP D) only occurs in plant cells

Section II — Free Response

FRQ 1 (Cellular Energetics — 4 points). A student measures the rate of photosynthesis in an aquatic plant by counting oxygen bubbles released per minute at increasing light intensities. (a) Identify the independent and dependent variables. (1 pt) (b) Predict and explain the shape of the graph as light intensity increases from low to very high. (2 pts) (c) Identify one other factor that could become limiting at high light intensity and explain its effect. (1 pt)

FRQ 2 (Heredity & Gene Expression — 4 points). In fruit flies, body color is controlled by a single gene; gray (G) is dominant to black (g). (a) A heterozygous gray fly is crossed with a black fly. Give the genotypic and phenotypic ratios of the offspring. (2 pts) (b) A mutation in the gene introduces a premature stop codon. Explain how this would affect the protein product. (1 pt) (c) Explain why a recessive allele can persist in a population even when it produces a non-functional protein. (1 pt)

Answer key (Section I)

Key distribution: A×6, B×6, C×8, D×5.

Worked reasoning (Section I)

1. (A) Cohesion (water–water hydrogen bonding) lets a continuous column be pulled up the xylem as water evaporates. 2. (C) Peptide bonds link the carboxyl of one amino acid to the amino group of the next. 3. (B) Specificity comes from the active site's complementary shape and chemistry (induced fit). 4. (D) Oxidative phosphorylation (electron transport + chemiosmosis) yields the most ATP (~26–28). 5. (C) Oxygen is released from the splitting of water in the light reactions. 6. (A) Hypotonic surroundings → net water entry → swelling. 7. (B) Most receptors are membrane proteins (intracellular receptors are the exception). 8. (D) One activated molecule activates many downstream → amplification. 9. (C) DNA is replicated in S phase. 10. (B) Tt × Tt → 1 TT : 2 Tt : 1 tt → 1/4 short (tt). 11. (C) Genes on the same chromosome are linked. 12. (A) DNA → RNA → protein. 13. (D) A premature stop codon is a nonsense mutation. 14. (C) q² = 0.16 → q = 0.40. 15. (B) Drift is strongest in small, isolated populations. 16. (A) ~10% energy transfer between trophic levels. 17. (C) A keystone species' impact is disproportionate to its abundance. 18. (B) Negative feedback restores the set point. 19. (D) tRNA carries amino acids to the ribosome. 20. (A) Crossing over occurs in prophase I. 21. (B) ABO shows codominance (Iᴬ/Iᴮ) and multiple alleles. 22. (C) Competition is density-dependent (weather events are density-independent). 23. (D) Smooth ER makes lipids and detoxifies. 24. (A) Reciprocal evolutionary change between species = coevolution. 25. (C) Facilitated diffusion is passive — down the gradient, no ATP.

FRQ rubrics

FRQ 1 (4 pts). (a) (1) Independent = light intensity; dependent = rate of O₂ production (bubbles/min). (b) (2) Rate increases with light intensity initially (1), then plateaus as another factor becomes limiting / light reactions saturate (1). (c) (1) CO₂ availability (or temperature) becomes limiting; once light is no longer limiting, the rate is capped by CO₂ supply to the Calvin cycle.

FRQ 2 (4 pts). (a) (2) Gg × gg → 1 Gg : 1 gg genotypes (1); 1 gray : 1 black phenotypes (1). (b) (1) A premature stop codon truncates translation, producing a shortened, likely non-functional protein. (c) (1) Recessive alleles persist because heterozygous carriers are unaffected and are not selected against, so the allele is hidden from selection in carriers.

Scoring guidance

• Section I: ~1 point each (25 total).
• Section II: 4 points each (8 total).
• MCQ 21–25 plus both FRQs are the strongest discriminators — review those explanations carefully.

Pedagogy: every distractor maps to a known misconception (e.g. Q5 confuses the source of O₂ with CO₂; Q14 forgets to take the square root for allele frequency; Q25 confuses passive and active transport). Reviewing why a wrong option attracts you is the highest-yield revision move.