AP Biology Unit 4 Practice Test: Cell Communication & Cell Cycle MCQs (Free PDF)

Here's a free AP Biology Unit 4 practice test: 20 AP-style MCQs on cell communication and the cell cycle, seven data-based stimulus sets, and a full rationale for every answer, all aligned to the 2025 CED.

Below, three mistakes I see students make on Unit 4 and how to avoid them, plus one worked example so you can see the reasoning.

Unit 4 is 10-15% of the exam (see the full 8-unit breakdown for the rest) and sits at a crossroads for a lot of the other units: signal transduction feeds into gene expression (Unit 6), and cell-cycle regulation ties directly to cancer, mitosis and later to meiosis in Unit 5. The MCQs are heavy on dose-response tables, receptor-and-second-messenger comparison tables and cell-cycle phase distributions.

The AP Bio MCQ section is 60 questions in 90 minutes, and most items anchor to a stimulus (see the MCQ strategy guide for the general playbook). Unit 4 leans on:

• Dose-response tables (hormone concentration vs cellular response)
• Receptor-and-second-messenger comparison tables across cell lines
• 'Mutate this component, predict the effect' tables that give you a wild-type baseline plus several mutations with and without ligand
• Cell-cycle phase-distribution tables from flow cytometry (%G1 / %S / %G2 / %M) under different treatments
• Cyclin and CDK level tables tracked across the cell cycle

The practice pack pulls that mix in: 16 of the 20 questions sit on data-table stimuli (most stimuli support two or three questions in a row, matching the real paper), and every question is tagged to a Unit 4 topic and a named Science Practice.

Every year I see students walk into Unit 4 MCQs making the same three moves. The signal-amplification one in particular was flagged explicitly in the 2023 Chief Reader Report. Spot them before the exam does.

This is a repeat Chief Reader complaint on Unit 4 (see 2023 CR Q1). Students see a signal transduction pathway and write 'the signal is amplified through a phosphorylation cascade'; that phrasing does not earn the amplification point.

The 2023 AP Biology Chief Reader Report Q1 spelled out what examiners actually want: each activated enzyme in the cascade catalyzes activation of MANY copies of the next protein, so one ligand-receptor event turns into thousands or millions of activated end-molecules. The correct sentence looks like 'phosphorylating a protein in step 1 activates 10 proteins in step 2, each of which activates 10 more in step 3'.

MCQ distractors will offer 'the signal molecule multiplies' (wrong; the ligand doesn't replicate) or the label 'phosphorylation cascade' with no mechanism (unearned).

Textbook diagrams show membrane-bound receptors so often that students conclude every signal molecule binds a surface receptor. Hydrophobic ligands (steroid hormones like testosterone, estrogen, cortisol) cross the plasma membrane directly and bind intracellular receptors, which then act as transcription factors. This is one of the top Unit 4 traps.

Any MCQ mentioning a hormone that 'enters the cell' or 'binds a receptor in the cytoplasm' is testing whether you can identify a hydrophobic ligand. The rule: hydrophilic (peptide) ligands need surface receptors; hydrophobic (steroid, thyroid) ligands cross the membrane.

Two related cell-cycle mistakes cluster here.

First, students describe checkpoints (G1, G2, M/spindle) as if they're physical barriers. Checkpoints are regulatory systems built from proteins (cyclins, CDKs, p53); nothing 'opens' or 'closes'. A p53 mutation doesn't 'break the gate', it removes the STOP signal that would normally halt a damaged cell.

Second, students assume any tumor is metastatic. A tumor is just an abnormal cell mass produced by uncontrolled division; malignancy and metastasis require additional properties (invasion, spread through the bloodstream). Benign tumors do not metastasize, and that distinction shows up in MCQ distractors regularly.

Here's a question straight from the Unit 4 pack. The stimulus: human cells were treated with different agents that affect the cell cycle. The percentage of cells in each phase was measured after 24 hours by flow cytometry combining DNA content with a mitotic marker (phospho-histone H3, used to separate G2 from M).

The stem: 'Which conclusion about the G1 checkpoint is best supported by Table 5?'

• (A) The G1 checkpoint responds to DNA damage by halting cells before they enter S phase.
• (B) The G1 checkpoint responds only to problems with spindle microtubule attachment to kinetochores during mitosis.
• (C) UV radiation triggers a specific arrest at the M checkpoint.
• (D) The G1 checkpoint cannot detect any type of cellular stress.

Answer: A. UV radiation causes DNA damage, and Table 5 shows a large shift into G1 (55% in the control to 78% under UV) with a drop in later phases. This matches the role of the G1 checkpoint: prevent entry into S phase (DNA replication) until damage is repaired. B describes the M-phase checkpoint. C is contradicted by the very low M count in the UV row (3%). D contradicts the specific G1 accumulation.

Signal transduction MCQs draw pathways with a mix of arrows (activation) and T-bars (inhibition). Students who scan the diagram and pick 'output goes up' without tracing lose points routinely.

Put your finger on the ligand, walk step by step to the output, and count the sign flips. Removing an inhibitor removes a MINUS, which is a net PLUS on the output. If the stem asks about mutating a phosphatase (which normally removes phosphate groups and shuts pathways off), losing that phosphatase keeps the pathway ON longer.

The 2025 AP Biology Chief Reader Report Q2 flagged predicting the effects of a disruption in a signaling pathway (Skill 6.E) as a Unit 4 gap that year.

A common Unit 4 MCQ shows a histogram of DNA content per cell (from flow cytometry) with a big peak at 2N or 4N. If cells are piling up at 2N, that means they can't pass G1; they're STUCK, not moving fast. If they pile up at 4N, they can't pass the G2 or the spindle checkpoint. Reading accumulation as speed is a top trap. When you see a phase distribution shift, ask 'what gate isn't opening?' before you pick.

The FRQ version of the MCQ trap in Tip 1 is picking 'the protein gets phosphorylated' when the question is asking what the cell actually does. Signal transduction always leads to one of four outcomes:

• Change in gene expression
• Change in enzyme activity or metabolism
• Change in cell shape or motility
• Apoptosis

If the answer choice doesn't name one of those four, it's almost certainly not the full answer.

Three-pass method.

• First, an untimed pass: no clock, about 90 seconds per question. The goal is diagnostic, to see which stimulus types (mutation tables, cell-cycle phase tables, cyclin-CDK level tables) trip you up.
• Second, a timed pass one week later: 30 minutes for 20 questions, matching the AP exam pace of about 1.5 minutes per MCQ.
• Third, a review pass: check the answer key at the back of the PDF, revise the weak topics on Cognito's AP Biology notes, then retry the missed questions a week after that.

Our FRQ guide is worth pairing with the pack once you're comfortable with the MCQ format. The gap between passes is where the retention comes from.