AP Biology — Practice Set 1
Cell structure, energetics, gene expression and evolution, plus a scored free-response question — because AP rewards explaining mechanisms, not naming them.
Cell structure, energetics, genetics, gene expression and evolution, plus a free-response question of the kind that carries the essay marks — with the scoring points made explicit.
How to use this: AP Biology rewards explaining mechanisms, not naming them. Most lost points come from answers that identify the right process but never say how it produces the observed result.
Multiple choice
1. Which best explains why the cell membrane is described as a fluid mosaic?
(a) It is made entirely of protein
(b) Phospholipids move laterally while diverse proteins are embedded throughout
(c) It is rigid and impermeable
(d) It consists of a single layer of lipids
Answer: (b)
Fluid refers to lateral movement of phospholipids; mosaic to the variety of embedded proteins.
2. A plant cell is placed in a hypertonic solution. What happens?
(a) It bursts
(b) Water leaves the cell and the membrane pulls away from the wall
(c) It becomes turgid
(d) Nothing, because of the cell wall
Answer: (b) — plasmolysis.
The distinction AP tests: an animal cell in a hypertonic solution shrivels (crenation); a plant cell plasmolyses, with the wall staying put while the membrane pulls inward. Option (d) is the trap for candidates who assume the wall prevents all change.
3. During which stage of cellular respiration is the greatest quantity of ATP produced?
(a) Glycolysis (b) Pyruvate oxidation (c) Krebs cycle (d) Oxidative phosphorylation
Answer: (d)
| Stage | ATP (net, approx.) | Location |
|---|---|---|
| Glycolysis | 2 | Cytoplasm |
| Pyruvate oxidation | 0 | Mitochondrial matrix |
| Krebs cycle | 2 | Matrix |
| Oxidative phosphorylation | ~26–28 | Inner mitochondrial membrane |
What AP actually asks: not the numbers but why — the electron transport chain establishes a proton gradient, and ATP synthase uses it to phosphorylate ADP (chemiosmosis).
4. A mutation changes a codon from GAA to GAG. Both encode glutamate. This is best described as:
(a) A nonsense mutation (b) A frameshift (c) A silent mutation (d) A missense mutation
Answer: (c)
The amino acid is unchanged, which is possible because the genetic code is degenerate — most amino acids have several codons, usually differing at the third base.
Learn the four together:
| Type | Effect |
|---|---|
| Silent | Same amino acid |
| Missense | Different amino acid |
| Nonsense | Premature stop codon |
| Frameshift | Insertion/deletion shifting the reading frame — usually the most severe |
5. In a population at Hardy–Weinberg equilibrium, the frequency of the recessive allele is 0.3. What proportion are heterozygous?
q = 0.3 ⟹ p = 1 - 0.3 = 0.7
Heterozygotes = 2pq = 2(0.7)(0.3) = 0.42 = 42%
Answer: 42%
Both equations are needed: $p + q = 1$ and $p^2 + 2pq + q^2 = 1$. The most common error is reporting $q^2 = 0.09$ (the homozygous recessive proportion) when the question asked for heterozygotes.
6. Which provides the strongest evidence that two species share a recent common ancestor?
(a) They live in the same habitat
(b) They are similar in size
(c) They have highly similar DNA sequences in conserved genes
(d) They eat similar food
Answer: (c)
Molecular similarity in conserved genes is the strongest evidence. Options (a), (b) and (d) can all result from convergent evolution, where unrelated species come to resemble one another under similar selective pressure — the distinction AP is testing.
7. An operon in E. coli is transcribed only when lactose is present. This is an example of:
(a) Constitutive expression (b) Inducible regulation (c) Post-translational modification (d) Alternative splicing
Answer: (b)
The lac operon: lactose (via allolactose) binds the repressor, which then cannot bind the operator, so RNA polymerase transcribes the structural genes.
Contrast with the trp operon, which is repressible — tryptophan activates a repressor and shuts transcription off. AP frequently pairs these two and asks you to distinguish inducible from repressible.
8. Which correctly describes the role of tRNA in translation?
(a) It carries the genetic code from nucleus to cytoplasm
(b) It brings a specific amino acid to the ribosome, matching its anticodon to the mRNA codon
(c) It forms the structural core of the ribosome
(d) It catalyses peptide bond formation
Answer: (b)
(a) describes mRNA; (c) and (d) describe rRNA, which is the actual catalyst of peptide-bond formation — a ribozyme.
9. In an experiment, cells treated with a drug show normal transcription but no protein synthesis. The drug most likely inhibits:
(a) RNA polymerase (b) DNA helicase (c) Ribosome function (d) The spliceosome
Answer: (c)
Transcription is unaffected, so the block is downstream, at translation.
The reasoning pattern: locate the last normal step and the first abnormal one. The target lies between them. This logic answers most AP experimental questions regardless of the pathway involved.
Free response
10. A researcher observes that a population of bacteria initially killed by an antibiotic becomes largely resistant after several months of exposure. (a) Explain how resistance arose. (b) Predict what happens if the antibiotic is withdrawn, and justify. (6 points)
(a) How resistance arose — 4 points
- Pre-existing variation. Random mutation produced a small number of resistant individuals in the population before the antibiotic was applied. (1 point)
- Selective pressure. The antibiotic killed susceptible bacteria while resistant ones survived. (1 point)
- Differential reproduction. Survivors reproduced, passing the resistance allele to offspring. (1 point)
- Change in allele frequency. Over generations the resistance allele rose in frequency until most of the population carried it. (1 point)
(b) Prediction — 2 points
If the antibiotic is withdrawn, the proportion of resistant bacteria will likely decline gradually, because resistance often carries a fitness cost — resources spent on resistance mechanisms (efflux pumps, modified enzymes) are unavailable for growth. Without the antibiotic, susceptible bacteria reproduce faster and regain ground. (1 point for the prediction, 1 for the justification.)
The error that costs the most points here: writing that bacteria "developed resistance because they needed it" or "adapted to the antibiotic." That is Lamarckian and is scored as incorrect, not merely imprecise. Mutations arise randomly and beforehand; the antibiotic selects, it does not instruct.
What AP Biology rewards
- Mechanism, not vocabulary. "Natural selection occurred" earns little. Describing variation, selection, reproduction and the change in frequency earns the points.
- Never describe evolution as purposeful. Organisms do not evolve in order to. Readers are instructed to penalise it.
- Justify predictions. Part (b) style questions always carry a separate point for the reasoning, and answers frequently give the prediction alone.
- Use the data given. When an experiment is described, refer to its actual results rather than to general principles.
Where to go next
- AP Calculus AB Practice Set 1 — if you are taking both
- ACT Science Practice Set 1 — experimental-design reasoning, faster and lighter
- A-Level Biology Practice Paper 1 — the same content at UK depth
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