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AP Biology · Unit 2 · Free Response

The FRQ: Six Questions, Six Shapes

Half the exam is six handwritten questions, and every one of them is built from the same short list of parts. Learn the shapes and the verbs and the free response stops being a writing test. It becomes a checklist.

What the free response section looks like

Six questions in 90 minutes, worth half your score. Two long questions at 9 points each, four short questions at 4 points each. You write by hand in a booklet. A formula sheet is provided. Every point is one task. A 4 point question is four things to say, not four paragraphs. Nobody earns a fifth point for writing more.

The six question numbers are six fixed types, and the parts inside each one follow a ladder that barely changes from year to year. Part (a) is nearly always a content question you could answer without reading the experiment. The last part is nearly always "justify" or "support," and that is where the average score falls apart. On recent exams, short questions averaged under 2.5 out of 4 and long questions averaged under 5 out of 9. The describe points get earned. The justify points do not.

This page gives you the six ladders, a translation table for the task verbs, the eight archetypes that make up almost every part, the graphing checklist, and two practice FRQs on Unit 2 content that you score yourself one point at a time.

Zoom out

The Six Ladders

This is the structure the College Board publishes. Memorize the shape of each one and you will know what part (d) is going to ask before you read part (a).

Question	Type	The ladder, part by part
1 9 pts	Interpret and evaluate experimental results	(a) Describe a concept behind the experiment. (b) Identify design elements: the control, the independent and dependent variables, and justify why they were chosen. (c) Analyze the data: describe a trend, compare groups. (d) Predict the effect of a change and justify it.
2 9 pts	Same, with graphing	(a) Describe a concept. (b) Construct a graph from a table, usually with error bars. This is 4 of the 9 points. (c) Analyze the data, often using the error bars to say what is significant. (d) Predict and justify.
3 4 pts	Scientific investigation	(a) Describe a concept. (b) Identify a control or a procedure, or state the null hypothesis. (c) Predict the result. (d) Justify the prediction.
4 4 pts	Conceptual analysis	(a) Describe. (b) Explain a mechanism. (c) Predict the effect of a disruption. (d) Justify.
5 4 pts	Analyze a model or visual	(a) Describe one part of the model. (b) Explain a relationship between two parts. (c) Trace, mark, or identify something on the model. (d) Explain what the model shows about a bigger idea.
6 4 pts	Analyze data	(a) Describe the data (a value, a trend). (b) Describe a second feature of the data (often the variation, not the mean). (c) Use the data to support or refute a hypothesis. (d) Explain how the result connects to a biological principle.

The move Before you read the stimulus, read every part's verb and write the ladder in the margin: D, I, P, J. Now you know what each part costs and what shape of sentence earns it. Then read the experiment hunting for the things the ladder needs: the control, the variable, the trend.

Zoom in

The Verb Is the Rubric

Same move as the multiple choice: find the verb first. On the FRQ the verb is not a hint. It is the literal scoring criterion. Readers are trained on these definitions, and the Chief Reader reports list "confusion between identify and describe" and "restating the stimulus instead of using biology" as point losses every single year.

The verb says	The point requires	What loses it
Identify	Name it. One phrase. "The control is the tube with substrate and no enzyme."	Writing three sentences. Naming a held constant condition when asked for the control group.
Describe	Give characteristics of the thing. What it is, what it does. No "why" required.	Vagueness ("ribosomes make amino acids"). Describing the wrong step of a process.
Explain	Give the mechanism that connects A to B. Cause, then effect, with the biology in between.	Restating that A and B are related without the step in between. Restating the stimulus.
Predict	State a specific outcome with a direction. "The rate will decrease." "The cell will gain mass."	"It will be affected." "More" or "fewer" when a specific value is possible (zero copies, not "fewer").
Justify	Give the reasoning that supports a claim or prediction you already made. Evidence, then mechanism.	A second prediction instead of a reason. Reasoning with no reference to the data or the scenario.
Support a claim	Point to specific data ("group 2 produced 18.6 mg/g vs 4.1") and say why the data mean what you say they mean.	Evidence with no reasoning. Reasoning with no numbers. Using the wrong figure when the question names one.
Calculate	Show the setup, the arithmetic, the unit, and the sign. Box it.	No units. Wrong sign on ψ_S. Celsius instead of kelvin. Dividing nucleotides by 2 to get amino acids.
Construct a graph	Four separate points: labeled axes with units, a scale that fits the data, correct plotting, error bars or a key. See the checklist below.	Missing units. A y axis that cuts off the top bar. Asymmetric error bars.
State the null hypothesis	"There is no difference" or "X has no effect on Y."	Writing the experimental hypothesis ("X will increase Y").
Determine / Evaluate	Reach a conclusion and say what in the data got you there.	A conclusion with no data behind it.

The single most important habit

Underline the verb in every part before you write. Then write one sentence per point that does exactly what the verb says. If the part is worth 1 point and asks you to "identify," your answer is a phrase. If it asks you to "justify," your answer is two sentences: the evidence, then the why. Match the length to the verb, not to how much you know.

The thought process

The FRQ Chain: From Reading the Question to Writing the Point

Same idea as the causal chains on the strategy pages. On a multiple choice question you run the chain from the clue to the concept to the answer. On an FRQ you run it from the verb to the tool to the sentence. If you cannot say each link out loud, you are not ready to write.

Read the parts first, not the stimulus. Underline the verb in (a), (b), (c), (d). Write the ladder in the margin: D, I, P, J. Note the points. Now you know what each part costs.therefore
Pick the tool for each part. Describe means archetype 1. Identify a control means archetype 2. Construct means the graph checklist. Predict and justify means archetypes 6 and 7, two sentences, direction then mechanism. You have decided the shape of every answer before reading a word of the experiment.therefore
Now read the stimulus, hunting. You are looking for exactly what the ladder needs: the control group, the variable they changed, the thing they measured, the trend in the table, the constraint the passage volunteers ("permeable to water but not sucrose"). Sort as you go: amber for what this question invented, blue for what you already knew.therefore
Conjure the class content. Ask which of the 15 concepts this is secretly about. A proton pump is active transport (Unit 2, Concept 12). A pH shift is R group charges and active site shape (Unit 2, Concept 2, which is Unit 1, Concept 13 in a lab coat). Say the concept's one line definition to yourself. That line is going to be the mechanism in your justify.therefore
Build the causal chain for the scenario. Change → first effect → next effect → outcome. Drug blocks pump → H⁺ stops entering → lysosomal pH rises → R group charges change → active site changes shape → substrate cannot bind → activity drops. The predict point is the last link. The justify point is the links in between.therefore
Write one sentence per point. The sentence does what the verb says. Direction word if it is a predict. A number from the table if it is a support. The mechanism from step 4 if it is an explain or justify. Then stop.therefore
Check, then leave. Does each sentence match its verb? Is there a direction word where one belongs? A unit on every number? Did you use the figure the question named? Yes to all, move to the next part. Do not add a sentence "just in case." It cannot earn a point and it can cost you time.

Where the chain breaks for most students Between step 3 and step 4. They read the stimulus, feel like they understand it, and start writing about the stimulus. The reader calls that "restating the prompt" and gives zero. Step 4 is the one that turns the scenario into biology. If you skip it, every justify point is gone.

The chain, run on Practice A below

Link	What you actually think
1. Verbs	(a) Describe, 1. (b) Explain, 1. (c) Predict, 1. (d) Justify, 1. Four sentences total, two of them with a mechanism.
2. Tools	(a) giveaway describe. (b) explain a mechanism. (c) predict the disruption, needs a direction. (d) justify, evidence then mechanism.
3. Hunt	Amber: enzymes best at pH 5, cytosol 7.2, pumps move H⁺ in, a drug blocks the pumps. Blue: lysosome, hydrolytic enzymes, proton pump, active site.
4. Concepts	Pump moving H⁺ from low to high concentration is active transport (U2 C12). Enzyme shape and pH is U2 C2. Specificity is the active site's shape (U2 C1).
5. Chain	Block pump → no H⁺ in → pH rises toward 7.2 → R group charges shift → active site shape changes → substrate does not bind → activity falls.
6. Sentences	(a) Shape complementary to substrate. (b) Against the gradient, using ATP. (c) Activity decreases. (d) pH rises, R groups, active site, no binding.
7. Check	Direction in (c)? Yes, "decreases." Mechanism in (b) and (d)? Yes. Done. Four points, about six sentences.

The patterns

Eight Archetypes That Make Up Almost Every Part

Across the released exams, the parts repeat. Here are the eight shapes with a Unit 2 example of each, a model answer that earns the point, and the specific way students lose it.

1. The giveaway describe

Part (a) of almost every question A one point content question on a basic idea, attached to an experiment that does not matter yet.
Example: "Describe how the structure of the phospholipid bilayer makes the membrane selectively permeable."
Earns it: "The nonpolar fatty acid tails form a hydrophobic interior that small nonpolar molecules cross freely, while ions and large polar molecules cannot pass without a transport protein."
Loses it: "The membrane only lets some things through" with no mention of the hydrophobic interior. Describing the heads and tails without connecting them to what crosses.

2. Name the control, then say why

Two points that travel together Example: "Identify the control group in the catalase experiment and justify its use."
Earns it: "The control is the tube with hydrogen peroxide and no catalase. It shows how much oxygen forms without the enzyme, so any extra oxygen in the other tubes can be attributed to catalase."
Loses it: Naming a controlled variable ("temperature was kept constant") instead of the control group. Justifying with "baseline" or "to compare" without saying what the control isolates.

3. Variables and the null hypothesis

One phrase each Example: "Identify the independent and dependent variables. State the null hypothesis."
Earns it: "IV: sucrose concentration. DV: percent change in mass. Null hypothesis: sucrose concentration has no effect on the change in mass of the potato cores."
Loses it: Writing the experimental hypothesis for the null. Swapping IV and DV. Listing the thing measured as the IV.

4. Build the graph

Four points, four separate checks On Question 2 the graph is worth as much as the rest of the question combined. Readers score four things independently: axes labeled with units, a scale that fits all the data, correct plotting, and error bars drawn correctly or a key where needed. You can get 3 of 4 with a perfect graph that forgot "°C."
Loses it: No units. A y axis that tops out below the tallest bar. Error bars that are uneven above and below the mean. Bar graph for a continuous variable or a line graph for categories.

5. Describe the data, then use the error bars

Trend first, significance second Example: "Describe the effect of sucrose concentration on mass change. Determine whether the 0.6 M and 0.8 M results differ significantly."
Earns it: "As sucrose concentration increases, percent change in mass decreases, from +18 at 0.0 M to −20 at 1.0 M. The ±2 SE bars for 0.6 and 0.8 M overlap, so the difference is not statistically significant."
Loses it: Saying "significant" because one bar is taller. Ignoring the error bars the stem told you about. Reading bar height when the x axis is the variable of interest. Using the wrong figure.

6. Predict the disruption

A mutation, a drug, a knockout, a removal Example: "Predict the effect on lysosomal enzyme activity if a drug blocks the proton pumps in the lysosomal membrane."
Earns it: "Lysosomal enzyme activity will decrease."
Loses it: "The enzymes will be affected." No direction. Predicting the mechanism instead of the outcome (that is the justify point, not this one).

7. Justify or support, in two sentences

Evidence, then mechanism The template: "The data show ___ . This happens because ___ ." The first blank is a number or an observation from the scenario. The second blank is biology from your notes.
Example: "Justify your prediction about the proton pump."
Earns it: "Without the pumps, H⁺ is no longer moved into the lysosome, so its pH rises toward the cytosol's 7.2. The change in H⁺ concentration alters the charges on R groups in the enzymes' active sites, changing their shape so substrate no longer binds."
Loses it: Restating the scenario. Reasoning with no link to the active site. A second prediction instead of a reason.

8. Calculate, and read the model

Show work, keep the sign, read the arrows Calculation example: "Calculate the solute potential of a 0.4 M sucrose solution at 25°C." Earns it: "ψ_S = −iCRT = −(1)(0.4 mol/L)(0.0831 L·bar/mol·K)(298 K) = −9.9 bars." Loses it: positive sign, 25 instead of 298, no units.
Model example: A pathway diagram where enzyme 1 is inhibited by the final product. Earns it: reading the T bar as inhibition and the arrow as conversion, then naming it negative feedback. Loses it: treating every line as "leads to," confusing the allosteric site with the active site, calling it positive feedback.

Worth four points on Question 2

The Graph Checklist

What a four point graph looks like

Check these four, in this order, before you move on

Axes labeled with units. Independent variable on x. Dependent on y. "Mean change in mass (g)" not "mass." "Temperature (°C)" not "temp."
Scale fits the data. Look at the biggest number in the table, add its error bar, and make sure the axis goes past it. Even intervals.
Plot correctly. Bar graph for categories or treatment groups. Line graph for a continuous variable like time or concentration. Points where the table says.
Error bars or key. If the table gives SE or SD, draw bars the same distance above and below each mean. If the stem says ±2 SE, double it. More than one series, add a key.

The trap Students draw a beautiful graph and lose two points for "mass" with no unit and a y axis that stops at 3 when the top error bar reaches 3.6. The graph points are the easiest points on the exam to get all of, and the easiest to drop one of.

What to watch for

Six Ways to Lose a Point You Knew

✗Answering the wrong verb. Explaining when asked to identify. Predicting a mechanism when asked to predict an outcome. Describing the stimulus when asked to justify.

✗Restating the stimulus. "Buffelgrass increased after the fire because the fire created more space for buffelgrass." That is the scenario, repeated. The point wanted competition, resource availability, and a reason.

✗No direction. "The rate changes." "The cell is affected." Every predict point needs an arrow: up, down, in, out, faster, slower.

✗Evidence without reasoning, or reasoning without evidence. A support point is two halves. Cite the number, then say why it means what you claim.

✗Significance by eyeball. A taller bar is not a significant difference. Overlapping error bars means you cannot claim one. Say "the error bars do not overlap" out loud.

✗Imprecise words. "Hydrogen bond" for the bond hydrolysis breaks. "Adaptation" for every evolutionary process. "Opposite polarities attract." "Baseline." Readers are trained to see these as not knowing.

✓The point: one sentence that does exactly what the verb says, with a direction word where one belongs, a number where one is available, and the mechanism named.

Practice FRQ A · Short · Conceptual analysis · 4 points

The Lysosome and the Pump

Run the chain first: verbs, tools, hunt, concepts, chain, then sentences. Write your answer to each part in the box. Then reveal the rubric for that part and check the box only if your answer actually does what the rubric says. Be a hard grader. The exam will be.

Lysosomes are membrane bound organelles that contain hydrolytic enzymes. These enzymes function best at a pH of about 5. The cytosol surrounding the lysosome has a pH of about 7.2. Proton pumps embedded in the lysosomal membrane move H⁺ ions from the cytosol into the lysosome.

(a) Describe how the structure of an enzyme's active site accounts for the enzyme's specificity for its substrate. 1 point

1 point: The active site has a specific three dimensional shape (and/or chemical properties from its R groups) that fits or binds only a substrate with a complementary shape.

Not enough: "The active site is where the substrate binds." That describes location, not specificity. Trap: explaining what the enzyme does to the substrate. The verb was describe, the target was specificity.

(b) Explain how the proton pump maintains the difference in pH between the lysosome and the cytosol. 1 point

1 point: The pump moves H⁺ against its concentration gradient (from lower concentration in the cytosol to higher in the lysosome) using energy from ATP, keeping the H⁺ concentration inside high and the pH low.

Not enough: "The pump moves protons into the lysosome." That restates the stimulus. The point needs against the gradient and uses energy, because that is the mechanism that explains why the difference persists.

(c) A drug blocks the proton pumps in the lysosomal membrane. Predict the effect of the drug on the activity of the lysosomal enzymes. 1 point

1 point: Enzyme activity will decrease (or the enzymes will stop working).

Not enough: "The enzymes will be affected." No direction, no point. Also not the point: a paragraph on R groups. That is part (d). Save it.

(d) Justify your prediction. 1 point

1 point: Without the pumps, H⁺ is no longer concentrated in the lysosome, so lysosomal pH rises toward 7.2. The changed H⁺ concentration alters charges on R groups in the active site, changing its shape so the substrate no longer fits or binds.

Not enough: "The pH will change so the enzymes denature." Readers want the link from pH to the active site to substrate binding. The 2025 Chief Reader report specifically names "explaining denaturation without connecting to substrate binding failure" as a point loss.

Your score on A0 / 4

Practice FRQ B · Long · Experimental results with graphing · 9 points

Dialysis Bags in Distilled Water

This one has a graph in it. Draw it on paper, then grade it against the four checks. The rest of the parts work like Practice A.

Students filled five dialysis tubing bags with 25 mL of sucrose solution at different concentrations, tied them off, and recorded the mass of each bag. Dialysis tubing is permeable to water but not to sucrose. Each bag was placed in a beaker of distilled water for 30 minutes, then removed, blotted, and weighed again. Each treatment was repeated five times. Mean change in mass and standard error (SE) are shown in Table 1.

Table 1. Change in mass of dialysis bags after 30 minutes in distilled water

Sucrose concentration in bag (M)	Mean change in mass (g)	SE (g)	n
0.0	0.0	0.1	5
0.2	1.1	0.2	5
0.4	2.0	0.2	5
0.6	2.9	0.3	5
0.8	3.6	0.3	5

(a) Describe the role of the dialysis tubing in this experiment. 1 point

1 point: It acts as a selectively permeable membrane: water can cross it but sucrose cannot, so water moves by osmosis while the sucrose stays inside the bag.

Not enough: "It holds the sucrose." The point is the selective permeability, and the passage handed you the word.

(b) Construct an appropriately labeled graph of the data in Table 1, including error bars of ±2 SE. 4 points

Draw it on paper. Then check each of the four graph points against your drawing.

1 point, axes: x axis labeled "Sucrose concentration (M)" and y axis labeled "Mean change in mass (g)," both with units. 1 point, scale: y axis reaches at least 4.2 g (the 0.8 M mean plus 2 SE) with even intervals; x axis spaced evenly. 1 point, plotting: five bars or points at 0.0, 1.1, 2.0, 2.9, 3.6 in the correct positions. 1 point, error bars: drawn ±2 SE, which is ±0.2, ±0.4, ±0.4, ±0.6, ±0.6, the same distance above and below each mean.

Model graph

Error bars are ±2 SE. The 0.0 M bar has zero height; its error bar still gets drawn.

(c) (i) Describe the relationship between sucrose concentration and change in mass. (ii) Using the error bars, determine whether the 0.6 M and 0.8 M treatments differ significantly, and explain your reasoning. 2 points

1 point (i): As sucrose concentration in the bag increases, the mass gained increases (from 0.0 g at 0.0 M to 3.6 g at 0.8 M). 1 point (ii): The ±2 SE error bars for 0.6 M (2.3 to 3.5 g) and 0.8 M (3.0 to 4.2 g) overlap, so the difference is not statistically significant.

Not enough for (ii): "0.8 gained more, so it is significant." A bigger mean is not significance. You have to do the overlap test and say the result.

(d) A sixth bag containing 0.4 M sucrose is placed in a beaker of 0.4 M sucrose solution instead of distilled water. (i) Predict the change in mass of this bag after 30 minutes. (ii) Justify your prediction using water potential. 2 points

1 point (i): No change in mass (approximately 0 g). 1 point (ii): The solutions inside and outside the bag have the same solute concentration, so they have the same (solute and total) water potential. With no water potential gradient there is no net movement of water by osmosis.

Not enough for (ii): "They are isotonic so nothing happens." The part said using water potential. Say the words: equal water potential, no gradient, no net osmosis.

Your score on B0 / 9

The golden rule

One point, one sentence, one verb. Underline the verb, write the sentence that does what it says, include the direction and the number if there is one, name the mechanism, and stop. Then go to the next part.

Back to the content

Every part (a) on this page came straight from the 15 concepts. If one of them was fuzzy, that is where to go.

Unit 2 strategy Unit 2 practice quiz