AP Biology / Unit 4
AP Biology Unit 4: Cell Communication and Cell Cycle
Unit 4 is where AP Biology gets mechanistic at a systems level. Cells receive signals, relay them through molecular cascades, and decide whether to grow, divide, or self-destruct. It's 10–15% of the exam and one of the most FRQ-heavy units.
Aligned to the 2025–26 College Board CED (Topics 4.1–4.6).
Types of cell signaling
| Type | Mechanism | Example |
|---|---|---|
| Direct contact | Cells physically touch; surface molecules bind to neighbor | Gap junctions; plasmodesmata |
| Autocrine | Cell signals itself | Some immune cells |
| Paracrine | Local diffusion to nearby cells | Neurotransmitters across a synapse |
| Endocrine | Long-distance via hormones through bloodstream | Insulin from pancreas to liver and muscle |
Only cells with the correct receptor respond to a given signal. Specificity is set by the receptor, not the ligand.
Signal transduction: reception → transduction → response
- Reception. Ligand binds a receptor and changes its shape. Cell-surface receptors handle large/polar ligands; intracellular receptors handle small nonpolar ones like steroid hormones.
- Transduction. Relayed and amplified through phosphorylation cascades, second messengers (cAMP, Ca²⁺), or G-protein coupled receptors.
- Response. Altered gene expression, enzyme activity, or behavior (movement, secretion, division, apoptosis).
Why so many steps? Amplification. One ligand can trigger thousands of downstream events — and every step is a regulation point.
Feedback mechanisms
| Type | What happens | Example |
|---|---|---|
| Negative feedback | Product inhibits upstream step → restores homeostasis | Insulin/glucagon, thermoregulation |
| Positive feedback | Product amplifies upstream step → drives to completion | Childbirth (oxytocin), clotting cascade, LH surge |
Critical: “positive” vs “negative” refers to whether the response amplifies or dampens the stimulus — not whether the outcome is good or bad.
The cell cycle
| Phase | What happens | Checkpoint |
|---|---|---|
| G₁ (Gap 1) | Cell growth, protein synthesis | G₁: DNA damage, cell size, nutrients |
| S (Synthesis) | DNA replication; each chromosome → 2 sister chromatids | — |
| G₂ (Gap 2) | Growth, prep for division | G₂: DNA replication errors |
| M (Mitosis) | Nuclear division (PMAT) + cytokinesis | M / spindle: chromosomes attached to spindle |
| G₀ | Non-dividing resting state | — |
Interphase = G₁ + S + G₂, ~90% of the cycle.
Mitosis vs. meiosis at a glance
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Gamete production |
| Divisions | 1 | 2 |
| Daughter cells | 2 diploid, identical | 4 haploid, unique |
| Crossing over | No | Yes (Prophase I) |
| Independent assortment | No | Yes (Metaphase I) |
Meiosis I separates homologous chromosomes. Meiosis II separates sister chromatids. Mixing those up is the most common Unit 4 FRQ mistake.
Cell cycle regulation, cancer, and apoptosis
Cyclins + CDKs: cyclin rises → binds CDK → activates CDK → CDK phosphorylates targets → cell advances through checkpoint. Cyclin is then degraded so the cell can't advance prematurely.
- Oncogenes: mutated proto-oncogenes — accelerator stuck on.
- Tumor suppressors (e.g., p53): lost or mutated — brakes gone.
- Apoptosis: programmed cell death; intrinsic (mitochondria) or extrinsic (death receptor) pathway.
Why students lose points here
- Confusing mitosis (identical daughters) with meiosis (haploid, variable).
- Skipping “amplification” when explaining why signaling has multiple steps.
- Calling feedback “positive” because the outcome is beneficial — wrong framing.
- Forgetting cytokinesis when describing cell division.
- Treating G₀ cells as dead — they're alive and metabolically active.
FRQ patterns
- Explain how a signal produces a cellular response — name a specific transduction mechanism (phosphorylation cascade, cAMP).
- Describe the cellular consequence of a checkpoint, tumor suppressor, or CDK mutation — then the organism-level outcome.
- Identify the feedback type and explain the mechanism in terms of stimulus and response.
How Unit 4 connects
- Unit 5: meiosis goes deeper; Unit 4 sets up the cell-cycle logic that makes it intelligible.
- Unit 6: signal transduction ultimately alters gene expression via activated transcription factors.
- Unit 7: mutations in proto-oncogenes and tumor suppressors connect cell biology to evolution at the population level.
FAQ
What is a second messenger?
A second messenger is a small intracellular signaling molecule (like cAMP, Ca²⁺, or IP₃) that relays and amplifies a signal from the cell surface to targets inside the cell. They're called 'second' because the first messenger is the extracellular ligand.
Why can steroid hormones cross the plasma membrane but protein hormones cannot?
Steroid hormones are nonpolar/lipid-soluble — they pass through the hydrophobic core of the membrane to reach intracellular receptors. Protein hormones are large and polar; they bind to cell-surface receptors instead.
What is the difference between an oncogene and a tumor suppressor?
An oncogene is a mutated proto-oncogene that's constantly 'on' — the accelerator stuck down. A tumor suppressor (like p53) normally brakes division; when lost or mutated, the brakes are gone. Cancer typically requires mutations in both.
What are cyclins and why do they matter?
Cyclins are proteins whose concentrations rise and fall during the cell cycle. They activate cyclin-dependent kinases (CDKs), which phosphorylate target proteins to push the cell through a checkpoint. When cyclin is degraded, the CDK goes inactive, preventing premature advance.
What is apoptosis and why does it matter?
Apoptosis is programmed cell death — an ordered, energy-requiring process that eliminates damaged or unneeded cells without harming neighbors. It's essential for development, immune function, and tumor suppression. Failure of apoptosis contributes to cancer.
Keep going
- AP Biology guide
- Unit 2: Cell Structure and Function
- Unit 5: Heredity
- Unit 6: Gene Expression and Regulation
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