Unit 4: Cell Communication and Cell Cycle bridges how cells send and receive information with how they divide and grow. It makes up about 10–15% of the AP Biology Exam and connects directly to genetics, cancer biology, and homeostasis.

In this RevisionDojo Unit 4 Review, we’ll cover:

• Cell signaling types and pathways
• Signal transduction and cellular responses
• Feedback mechanisms in biology
• Cell cycle stages and regulation
• Common AP exam question formats
• Study strategies for maximum points

Unit 4 Overview

The College Board breaks Unit 4 into two major themes:

1. Cell Communication – How cells detect and respond to signals
2. Cell Cycle – How cells grow, replicate DNA, and divide

1. Cell Communication

Types of Cell Signaling

• Autocrine: Cell signals itself (e.g., immune cell self-activation)
• Paracrine: Local signaling to nearby cells (e.g., neurotransmitters)
• Endocrine: Long-distance signaling via hormones in bloodstream
• Juxtacrine: Direct contact between cells via membrane-bound molecules

2. Signal Transduction Pathways

Three Stages:

1. Reception: Signal molecule (ligand) binds to receptor
2. Transduction: Relay of signal through secondary messengers and protein activation
3. Response: Cellular change (e.g., gene expression, enzyme activation)

Common Examples:

• G protein-coupled receptors (GPCRs)
• Tyrosine kinase receptors
• Ligand-gated ion channels

3. Feedback Mechanisms

• Negative feedback: Maintains homeostasis by reducing initial stimulus (e.g., blood glucose regulation via insulin)
• Positive feedback: Amplifies a process (e.g., oxytocin during childbirth)

AP Tip: Be able to predict the effect of disrupting a feedback loop.

4. The Cell Cycle

Stages:

• Interphase:
  • G1: Cell growth
  • S: DNA replication
  • G2: Preparation for mitosis
• Mitotic (M) Phase:
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase & Cytokinesis

5. Regulation of the Cell Cycle

• Controlled by cyclins and cyclin-dependent kinases (Cdks)
• Checkpoints:
  • G1 checkpoint – commitment to divide
  • G2 checkpoint – DNA replication completion
  • M checkpoint – spindle fiber attachment
• Disruption can lead to uncontrolled growth (cancer)

6. Connections to Other Units

• Unit 3: Signal transduction often triggers metabolic changes
• Unit 5: Cell cycle regulation is critical for proper heredity
• Unit 6: Signaling pathways influence gene expression

Common AP Biology Exam Questions for Unit 4

Multiple Choice Examples:

• Identify the type of signaling involved in neurotransmitter release
• Predict the effect of a mutated receptor on cell function

FRQ Examples:

• Describe how a signal transduction pathway results in gene expression changes
• Explain how a cell cycle checkpoint prevents damaged DNA from being passed on

Study Tips for Unit 4

1. Diagram Pathways: Include signal, receptor, transduction, and response.
2. Memorize Key Examples: Hormones, neurotransmitters, and immune signals.
3. Practice Predictive Questions: What happens if a protein in the pathway is missing?
4. Link to Cancer Biology: Understand how checkpoint failures lead to tumor growth.

Frequently Asked Questions

1. How much of the exam is Unit 4?
Roughly 10–15% of the total score.

2. Do I need to know every step in a pathway?
No—focus on general flow and examples of each stage.

3. Is cancer biology tested?
Yes, often in the context of disrupted checkpoints.

4. Are positive and negative feedback equally tested?
Negative feedback is more common, but positive appears in reproductive and immune examples.

Call to Action

Unit 4 is about control and coordination—two skills that AP Biology loves to test. Master it, and you’ll unlock points in both MCQs and FRQs.

👉 Use RevisionDojo’s AP Bio practice questions and pathway diagrams to fully prepare for Unit 4 and stay on track for a 5.