AP Biology Unit 4: Cell Communication and Cell Cycle MCQs – 30 Practice Questions with Answers (College Board CED)
AP-level practice for Unit 4 of the College Board CED (Fall 2025): cell communication, introduction to signal transduction, signal transduction pathways, feedback, the cell cycle, and regulation of the cell cycle. Questions emphasise pathway logic, experimental evidence, and control of division.
Cell Communication
Q1. A signalling molecule is released by one cell and acts only on neighbouring cells close to the release site. What type of signalling is this?
- (A) Paracrine signalling, because the signal acts locally on nearby target cells
- (B) Endocrine signalling, because the signal travels through the bloodstream
- (C) Autocrine signalling, because the signal acts on a distant organ
- (D) Synaptic signalling, because the signal crosses a synaptic cleft between neurons
Show Answer
Answer: A – Paracrine signals diffuse over short distances to nearby cells, unlike endocrine signals that travel in blood or autocrine signals that act on the releasing cell itself.
Q2. A hormone released into the bloodstream affects cells in a distant organ, but has no effect on most other tissues. Why is its action selective?
- (A) The hormone is destroyed before it reaches other tissues, breaking down in the bloodstream on its journey
- (B) Only the target organ receives blood
- (C) Only target cells carry the specific receptor that binds the hormone
- (D) The hormone changes its structure as it travels
Show Answer
Answer: C – A signal affects only cells with a matching receptor, so a widely distributed hormone can still act selectively.
Q3. A cell responds to a signal, while a neighbouring cell exposed to the same signal does not. The signal reaches both cells equally. What is the most likely explanation?
- (A) The responding cell is larger than its neighbour
- (B) The responding cell expresses the receptor for the signal, while its neighbour does not
- (C) The signal is blocked by the plasma membrane of every cell, so it cannot reach any receptor inside
- (D) The neighbouring cell has already divided
Show Answer
Answer: B – Reception depends on receptor expression, so two cells in the same environment can respond differently to the same ligand.
Q4. A signalling molecule is hydrophobic and passes through the plasma membrane to bind a receptor inside the cell. Which kind of receptor is most likely involved?
- (A) A channel receptor that opens a pore in the membrane
- (B) A receptor on the outer membrane surface that cannot be crossed by any signalling molecule
- (C) A receptor that pumps the molecule out of the cell
- (D) An intracellular receptor, such as one in the cytoplasm or nucleus
Show Answer
Answer: D – Hydrophobic ligands such as steroid hormones cross the lipid bilayer and bind intracellular receptors, while hydrophilic ligands generally bind surface receptors.
Q5. At a synapse, a neuron releases a chemical that acts across a tiny gap on the next cell within milliseconds. What is this form of signalling?
- (A) Synaptic signalling, in which a neurotransmitter crosses the synaptic cleft to a specific postsynaptic cell
- (B) Endocrine signalling, in which a hormone circulates for hours and reaches tissues throughout the body, acting on distant target organs
- (C) Paracrine signalling over a whole tissue region
- (D) Direct contact signalling through plasmodesmata
Show Answer
Answer: A – Synaptic signalling is fast and targeted: neurotransmitter released at a synapse acts on the adjacent postsynaptic cell.
Q6. Two cells are connected by gap junctions, and a small signalling molecule passes directly from one cytoplasm to the other. What type of communication is this?
- (A) Endocrine signalling through the blood
- (B) Direct cell-to-cell signalling through channels linking the cytoplasms
- (C) Synaptic signalling across an extracellular gap between two neighbouring nerve cells
- (D) Long-distance signalling by a hormone
Show Answer
Answer: B – Gap junctions form channels between animal cells that let small molecules and ions move directly from cell to cell.
Q7. A researcher removes the receptor for a growth factor from a cell line. The cells no longer divide in response to the growth factor, but they still divide in response to a different signal. What does this show?
- (A) Growth factors act without receptors
- (B) All signals share the same receptor
- (C) Reception is specific: losing one receptor blocks only the pathway that depends on it
- (D) The cells have lost the ability to divide entirely, so no signal of any kind can make them grow
Show Answer
Answer: C – Because the cells still respond to another signal, the defect is specific to reception of the growth factor, demonstrating receptor specificity.
Signal Transduction Pathways
Q8. A ligand binds a G protein-coupled receptor, and the cell produces many molecules of cyclic AMP. Which step in transduction does the cyclic AMP represent?
- (A) Reception, because cyclic AMP binds the ligand outside the cell and starts the response there
- (B) Response, because cyclic AMP is the final cellular outcome
- (C) Termination, because cyclic AMP destroys the receptor
- (D) A second messenger relay that spreads and amplifies the signal inside the cell
Show Answer
Answer: D – Cyclic AMP is a second messenger: it is produced inside the cell after reception and relays the signal to multiple downstream targets.
Q9. A single activated receptor leads to the activation of hundreds of target enzymes. What feature of signal transduction explains this multiplication?
- (A) Amplification, because each step in a cascade can activate many molecules at the next step
- (B) Termination, because the signal is switched off quickly and the pathway returns to its resting state
- (C) Reception, because many ligands bind one receptor
- (D) Diffusion, because the ligand spreads evenly
Show Answer
Answer: A – Cascades amplify signals: one activated molecule can activate many at the next step, and the effect compounds down the pathway.
Q10. In a phosphorylation cascade, each kinase adds a phosphate group to the next kinase in the chain. What does this chain achieve?
- (A) It destroys the original signal immediately
- (B) It converts the signal into DNA directly
- (C) It relays and amplifies the signal while allowing regulation at multiple steps
- (D) It prevents any cellular response from occurring, blocking the pathway at every step along the way
Show Answer
Answer: C – Phosphorylation cascades transmit the signal inward, amplify it, and offer several points where the pathway can be controlled or switched off.
Q11. A cell’s response to a signal ends shortly after the signal is removed, even though the receptor is still present. Which mechanism best explains the rapid shut-off?
- (A) The ligand permanently fuses with the receptor
- (B) Enzymes such as phosphodiesterases or phosphatases reverse the transduction steps, terminating the signal
- (C) The cell divides and dilutes the signal
- (D) The signal is stored in the nucleus for later use, remaining available to trigger a response at any time in the future
Show Answer
Answer: B – Termination is active: enzymes break down second messengers or remove phosphate groups, resetting the pathway for future signals.
Q12. Calcium ions are released from the endoplasmic reticulum into the cytoplasm after a signal arrives, triggering a response. What role are the calcium ions playing?
- (A) They act as the extracellular ligand
- (B) They act as the receptor protein
- (C) They act as DNA that encodes the response, carrying the genetic instructions for the reaction
- (D) They act as a second messenger relaying the signal within the cell
Show Answer
Answer: D – A rise in cytoplasmic calcium is a classic second-messenger event, carrying the signal from the membrane to intracellular targets.
Q13. A signalling pathway in one cell type causes cell division, while the same pathway in another cell type causes the cell to differentiate. What does this show about signal transduction?
- (A) The response depends on the cell’s own proteins and context, not only on the signal itself
- (B) The signal changes its chemical identity in different cells, becoming a different molecule in each tissue
- (C) Receptors are identical in every cell type
- (D) Second messengers are never involved in differentiation
Show Answer
Answer: A – The same signal can produce different responses because cells differ in their relay proteins, transcription factors, and gene expression programmes.
Q14. A drug blocks the enzyme that produces cyclic AMP. A cell exposed to a signal that normally works through cyclic AMP shows a much weaker response. What does this experiment confirm?
- (A) Cyclic AMP is the extracellular ligand for the signal, binding receptors on the cell surface
- (B) Cyclic AMP is a necessary intermediate in this transduction pathway
- (C) The receptor has been destroyed by the drug
- (D) The cell no longer has a plasma membrane
Show Answer
Answer: B – Removing a pathway intermediate and losing the response shows that the intermediate is required for transduction in this pathway.
Q15. A signal activates a transcription factor that enters the nucleus and switches on a set of genes. What stage of cell signalling does this gene activation represent?
- (A) Reception at the membrane
- (B) Production of a second messenger, such as cyclic AMP, inside the responding cell
- (C) The cellular response to the transduced signal
- (D) Termination of the pathway
Show Answer
Answer: C – Changing gene expression is a response: it is the outcome produced after reception and transduction have carried the signal inward.
Feedback Mechanisms
Q16. Blood glucose rises after a meal, insulin is released, cells take up glucose, and blood glucose falls back toward normal. What type of feedback is this?
- (A) Positive feedback, because insulin increases glucose further and drives levels even higher, with every cycle amplifying the rise
- (B) No feedback, because insulin acts independently
- (C) Feedforward control, because the meal predicts the response
- (D) Negative feedback, because the response counteracts the original change and restores the set point
Show Answer
Answer: D – Negative feedback reverses the stimulus: rising glucose triggers a response that lowers glucose back toward its normal range.
Q17. During childbirth, contractions stimulate the release of oxytocin, which strengthens further contractions until delivery. What type of feedback is this?
- (A) Positive feedback, because the response amplifies the original stimulus
- (B) Negative feedback, because contractions are suppressed
- (C) No feedback, because oxytocin acts on a different organ
- (D) Homeostatic feedback, because the system returns to rest immediately after each contraction ends
Show Answer
Answer: A – Positive feedback amplifies a change rather than reversing it, driving the process toward completion, here delivery.
Q18. A thermostat switches heating on when a room cools below a set temperature and off when it warms back up. A student uses this as an analogy for blood glucose control. What is the key similarity?
- (A) Both systems amplify every deviation without limit
- (B) Both use negative feedback to hold a variable near a set point
- (C) Both rely on positive feedback to reach a new state, amplifying each change until it completes
- (D) Both work without any sensor or control centre
Show Answer
Answer: B – Both are negative feedback loops: a sensor detects deviation from a set point and triggers a response that restores it.
Q19. When blood glucose falls, glucagon is released and the liver releases glucose into the blood. Together, insulin and glucagon illustrate which principle?
- (A) A single hormone controls glucose in both directions, raising and lowering levels as needed
- (B) Positive feedback always overrides negative feedback
- (C) Antagonistic hormones push a variable in opposite directions to maintain balance
- (D) Hormones act only when blood glucose is high
Show Answer
Answer: C – Insulin lowers blood glucose while glucagon raises it, so the pair provides opposing control that keeps glucose within a narrow range.
Q20. A patient’s blood glucose remains high despite high insulin levels. Which explanation is most consistent with disrupted feedback?
- (A) Insulin production has stopped completely and no insulin is present in the bloodstream at all, so glucose cannot be cleared
- (B) Glucose has stopped entering the blood from food
- (C) The liver has stopped storing glucose permanently
- (D) Target cells respond poorly to insulin, so the negative feedback loop cannot restore normal glucose
Show Answer
Answer: D – If target cells are insulin-resistant, the feedback signal is present but ineffective, so glucose stays elevated despite high insulin.
The Cell Cycle
Q21. A cell spends most of its life in interphase, with DNA replication occurring in one specific phase. Which phase is it?
- (A) S phase, during which the DNA is replicated before division
- (B) G1 phase, during which the cell divides
- (C) M phase, during which the cell grows only
- (D) G2 phase, during which chromosomes separate and move to opposite poles of the cell
Show Answer
Answer: A – S phase (synthesis phase) is when DNA replication occurs, between the G1 and G2 growth phases of interphase.
Q22. During mitosis, sister chromatids separate and move to opposite poles. At which stage does this happen?
- (A) Prophase, when chromosomes condense
- (B) Metaphase, when chromosomes align at the equator, attached to spindle fibres from both poles
- (C) Anaphase, when sister chromatids are pulled apart
- (D) Telophase, when nuclear envelopes reform
Show Answer
Answer: C – Anaphase is defined by the separation of sister chromatids and their movement toward opposite poles of the cell.
Q23. A cell is observed with chromosomes aligned along the middle of the spindle. Which stage is it in, and what happens next?
- (A) Prophase, followed by chromosome condensation and breakdown of the nuclear envelope
- (B) Metaphase, followed by separation of sister chromatids in anaphase
- (C) Telophase, followed by DNA replication
- (D) Cytokinesis, followed by nuclear division
Show Answer
Answer: B – Alignment at the metaphase plate identifies metaphase, and the next stage is anaphase, when chromatids separate.
Q24. Cytokinesis in an animal cell differs from cytokinesis in a plant cell. What is the key difference?
- (A) Animal cells form a cell plate, while plant cells form a cleavage furrow that pinches the cell in two from the outside inward, dividing the cytoplasm
- (B) Plant cells divide their nuclei, while animal cells do not
- (C) Animal cells replicate DNA during cytokinesis, while plant cells do not
- (D) Animal cells divide by a cleavage furrow pinching inward, while plant cells build a cell plate across the middle
Show Answer
Answer: D – The rigid wall prevents furrowing in plant cells, so they construct a cell plate instead, while animal cells pinch apart with a cleavage furrow.
Q25. A researcher counts chromosomes in a cell before S phase and after mitosis. Both counts are the same. What does this confirm about mitosis?
- (A) Mitosis produces genetically identical daughter cells with the same chromosome number as the parent
- (B) Mitosis halves the chromosome number for reproduction
- (C) Mitosis doubles the chromosome number in each generation, so daughter cells have twice as many chromosomes
- (D) Mitosis changes the DNA sequence of every chromosome
Show Answer
Answer: A – DNA is replicated in S phase and then distributed equally, so mitosis preserves chromosome number and genetic identity in the daughters.
Regulation of the Cell Cycle
Q26. A cell with damaged DNA is stopped before it copies its DNA. Which checkpoint has acted?
- (A) The M checkpoint, which checks spindle attachment
- (B) The G2 checkpoint, which checks for completed replication
- (C) The G1 checkpoint, which checks cell size, nutrients, and DNA integrity before S phase
- (D) The cytokinesis checkpoint, which checks the cleavage furrow and the distribution of organelles
Show Answer
Answer: C – The G1 checkpoint is the main decision point before DNA synthesis, halting the cycle if DNA is damaged or conditions are unsuitable.
Q27. Cyclin levels rise and fall during the cell cycle, while the kinase partner stays roughly constant. What does this pattern imply?
- (A) Cyclins are destroyed after every division and never remade, so the cycle can run only once
- (B) Cyclin abundance controls when the cyclin-dependent kinase complex is active
- (C) Kinases act alone without any regulatory partner
- (D) Cyclin levels are irrelevant to cell cycle progression
Show Answer
Answer: B – The kinase is present throughout, but it is active only when bound to its cyclin partner, so rhythmic cyclin synthesis and destruction times the cycle.
Q28. Normal cells in a dish stop dividing when they touch each other and form a single layer. Cancer cells from the same tissue keep dividing and pile up. What control has the cancer cells lost?
- (A) The ability to replicate DNA
- (B) The ability to form a plasma membrane, so cells can no longer enclose their contents at all
- (C) The ability to carry out glycolysis
- (D) Density-dependent inhibition of cell division
Show Answer
Answer: D – Density-dependent inhibition normally halts division at confluence; loss of this control is a hallmark of cancerous growth.
Q29. A proto-oncogene mutates into an oncogene that constantly signals the cell to divide. What has changed at the molecular level?
- (A) A normal growth-promoting gene has become overactive, driving uncontrolled division
- (B) A DNA repair gene has started repairing damage faster, fixing every error before division begins
- (C) A tumour suppressor gene has become more active
- (D) The cell has lost all of its cyclins
Show Answer
Answer: A – Proto-oncogenes normally promote controlled division; mutation to an oncogene leaves that growth signal permanently on.
Q30. A tumour suppressor gene is inactivated by mutation in a cell line. What is the expected effect on cell cycle control?
- (A) The cell cycle speeds up only during S phase
- (B) The cell gains a new organelle for division
- (C) A brake on the cell cycle is lost, so damaged or unready cells may divide when they should have stopped
- (D) The cell can no longer replicate its DNA at all, so it cannot enter S phase or divide again under any conditions
Show Answer
Answer: C – Tumour suppressors restrain division and enforce checkpoints, so losing one removes a brake and permits inappropriate division.
Revisit any questions you missed, then check the CED topic list for Unit 4 to confirm which areas need more revision before moving on to Unit 5.