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.
Understanding AP Bio Big Ideas and Practice Reference
The Big Ideas are most useful when you treat them as connected lenses rather than separate chapters. A change in DNA can alter a protein, which can change a trait, affect survival, and shift a population over many generations. That single chain links information, evolution, energy use, and system behavior.
In exam questions, start by locating the level of biology involved. It may concern a molecule, cell, organism, population, or ecosystem. Then trace cause and effect across levels.
Natural selection acts on differences in reproductive success among individuals, but evolution is the resulting change in a population's genetic makeup. Students often lose points by saying an individual evolves because it needs a trait.
Individuals can survive or reproduce differently. Populations change over time.
Energetics questions require careful attention to what is being transferred and what is being transformed. Cells do not create energy. They capture usable energy from food or light, convert some into ATP, then release much of it as heat.
Enzymes speed reactions by lowering activation energy. They do not change the overall energy change of a reaction or get used up during normal activity. When a prompt mentions temperature, pH, substrate concentration, or an inhibitor, connect that condition to enzyme shape, collision frequency, or active site function.
Water movement is another common application. Water moves toward the side with lower free water availability.
A membrane, dissolved solutes, and physical pressure all affect that movement. Plant cell questions often depend on recognizing that a cell wall can resist expansion and create pressure within the cell.
Data analysis is not separate from biology. A graph is evidence about a biological process. Read the axes before reading the pattern.
Identify units, scale, sample size, error bars, and whether the graph shows individual trials or averages. A rising line can show correlation without proving one factor caused the other. Experimental design helps determine causation because it compares groups that differ in one planned condition.
Replication matters because living systems vary naturally. A chi-square test compares observed counts with counts predicted by a model. A large difference is not automatically meaningful.
The test estimates whether random sampling could reasonably explain the difference. The result must be interpreted using the stated threshold for significance.
Free-response answers earn credit when each sentence does a job. State a precise claim that answers the task. Select numerical or descriptive evidence directly from the figure, table, or experiment.
Then explain the biological mechanism that makes the evidence support the claim. Avoid copying data without interpretation. Avoid vague statements such as the results prove the hypothesis.
Instead, name the measured trend and connect it to a process such as diffusion, cell signaling, transcription, selection, or feedback regulation. If asked to predict a result, extend the pattern using the mechanism, not a guess.
Practice underlining task verbs such as describe, calculate, justify, and predict. Each verb asks for a different kind of response, and answering the wrong task can cost points even when the biology is correct.