AP Biology connects major life science themes with data analysis, experimental reasoning, and clear written explanations. This cheat sheet helps students organize the four AP Biology Big Ideas and the skills needed for multiple-choice questions and free-response questions. It is useful for review before unit tests, labs, and the AP exam because it links concepts to evidence and practice habits.
Students need this reference to move beyond memorizing terms and toward explaining biological patterns with mechanisms.
Key Facts
- The four AP Biology Big Ideas are evolution, energetics, information storage and transmission, and systems interactions.
- Natural selection requires genetic variation, heritability, differential survival or reproduction, and changes in allele frequencies over generations.
- The Hardy-Weinberg equations are p + q = 1 and p^2 + 2pq + q^2 = 1 for a two-allele population with no evolution occurring.
- Free energy change is described by delta G = delta H - T delta S, and a negative delta G means a process is spontaneous under those conditions.
- Water potential is calculated as psi = psi_s + psi_p, where solute potential and pressure potential determine net water movement.
- Chi-square is calculated as chi-square = sum of (observed - expected)^2 / expected and tests whether differences from expected results are likely due to chance.
- In experimental design, the independent variable is changed, the dependent variable is measured, and controlled variables are kept constant.
- Strong AP Biology explanations use claim, evidence, and reasoning, with evidence from data and reasoning that connects the evidence to a biological mechanism.
Vocabulary
- Evolution
- Evolution is the change in allele frequencies in a population over generations.
- Homeostasis
- Homeostasis is the maintenance of stable internal conditions through feedback mechanisms.
- Free energy
- Free energy is the energy available to do work in a biological or chemical system.
- Independent variable
- The independent variable is the factor deliberately changed by the investigator in an experiment.
- Null hypothesis
- A null hypothesis states that there is no significant difference or relationship beyond what random chance can explain.
- Claim-evidence-reasoning
- Claim-evidence-reasoning is a response structure that states an answer, supports it with data, and explains the biological logic.
Common Mistakes to Avoid
- Writing vague FRQ answers, because AP Biology scoring rewards specific biological mechanisms and evidence rather than general statements.
- Confusing correlation with causation, because two variables changing together does not prove that one variable directly caused the other.
- Forgetting units and axis labels on graphs, because unlabeled data displays make trends difficult to interpret and may lose points.
- Using observed values as expected values in chi-square calculations, because expected values must come from a hypothesis, ratio, or model.
- Describing individuals as evolving, because evolution happens in populations through changes in allele frequencies across generations.
Practice Questions
- 1 In a population, the frequency of allele q is 0.30. Using Hardy-Weinberg equilibrium, calculate p, q^2, 2pq, and p^2.
- 2 A student counts 80 purple-flowered plants and 20 white-flowered plants from a cross expected to produce a 3:1 ratio. Calculate the chi-square value.
- 3 A potato core has solute potential -0.40 MPa and pressure potential 0.10 MPa. Calculate its water potential and predict whether water enters if the surrounding solution is -0.20 MPa.
- 4 Explain how one AP Biology Big Idea can connect cellular respiration, population ecology, and feedback regulation without relying only on memorized definitions.