AP Biology free-response questions: experiments, data and models

The six free-response questions

Source: AP Central, AP Biology exam page, checked 7 October 2026. The section lasts 1 hour 30 minutes and is 50% of the score. Calculators are permitted.

Task verbs decide the points

AP Biology questions are built from task verbs, and each verb wants a specific kind of answer. 'Identify' wants a short, precise answer with no explanation. 'Describe' wants the relevant characteristics. 'Explain' wants how or why, usually a cause and its effect linked by 'because'. 'Justify' wants evidence or reasoning that supports a claim. 'Predict' wants an outcome, often followed by 'justify your prediction'.

Experimental questions almost always ask for the independent and dependent variables, a control, a null hypothesis, and how a change would affect results. Name variables precisely: 'temperature of the enzyme solution (°C)', not 'heat'. A null hypothesis states no effect: 'temperature has no effect on the rate of oxygen production'.

Worked example 1: an enzyme experiment

A student measures the volume of oxygen released in 5 minutes when catalase extract is added to hydrogen peroxide at different temperatures: 10 °C, 4 mL; 20 °C, 8 mL; 30 °C, 13 mL; 40 °C, 9 mL; 50 °C, 2 mL. (a) Identify the independent and dependent variables. (b) State the null hypothesis. (c) Calculate the rate at 30 °C. (d) Explain the result at 50 °C.

1. (a) Independent variable: temperature of the reaction mixture (°C). Dependent variable: volume of oxygen released in 5 minutes (mL).
2. (b) Null hypothesis: temperature has no effect on the volume of oxygen released by catalase.
3. (c) Rate = 13 mL ÷ 5 min = 2.6 mL per minute. Show the division and give units.
4. (d) At 50 °C the enzyme has denatured: high temperature disrupts the hydrogen bonds and other interactions that maintain its three-dimensional shape, so the active site changes shape and substrate can no longer bind, and the rate falls. Note that 'the enzyme died' earns nothing; enzymes are not alive.
5. A typical extension asks for a control: an identical tube with boiled extract (or water instead of extract) at each temperature, to show that oxygen comes from the enzyme-catalyzed reaction.

Worked example 2: chi-square on a genetic cross

Two heterozygous plants are crossed. Purple flowers are dominant. Of 400 offspring, 290 are purple and 110 are white. Is the result consistent with a 3:1 ratio at p = 0.05?

1. Expected: 3/4 of 400 = 300 purple and 1/4 of 400 = 100 white.
2. Chi-square = sum of (observed - expected)^2 / expected = (290 - 300)^2/300 + (110 - 100)^2/100 = 100/300 + 100/100 = 0.333 + 1 = 1.33.
3. Degrees of freedom = number of categories - 1 = 1. The critical value from the formula sheet at p = 0.05 is 3.84.
4. Because 1.33 < 3.84, we fail to reject the null hypothesis: the difference from 3:1 can be explained by chance.
5. Wording matters: write 'fail to reject the null hypothesis', not 'accept' or 'prove'.

Common mistakes

• Describing a trend when the question says explain. Add the biological reason with 'because'.
• Vague variables ('the plants', 'growth') instead of measurable ones with units.
• Graphing errors: no title, unlabeled axes or units, uneven scales, wrong graph type (a bar graph for continuous data), or missing error bars when data give means with standard error.
• Writing that results 'prove' a hypothesis. Data support or fail to support it.
• Answering beyond the question. Extra wrong statements can contradict a correct one.
• Running out of time on the long questions. With 90 minutes for 34 points, a long question deserves roughly 20 to 25 minutes.

How one-to-one lessons help with AP Biology FRQs

Biology students often know the content but lose points on the wording. In lessons the tutor sets one long and two short questions under time, then goes through each part against a point-by-point list and rewrites the weak sentences with the student on the shared whiteboard. Graphing is practiced on paper because answers are handwritten. Over a few weeks the student builds a set of model sentences for each task verb and a checklist for experimental design.

Released free-response questions with scoring guidelines are on AP Central, and they are the best practice material. The tutor also fills content gaps found along the way, most often in cellular energetics and gene expression.

Self-check

• Can you state a null hypothesis for an experiment on light intensity and photosynthesis?
• Can you explain the difference between 'describe' and 'explain' with an example?
• Can you choose between a line graph and a bar graph for a given data set?
• Can you calculate a chi-square value and find the degrees of freedom?
• Can you explain denaturation in terms of bonds and the active site?