The unit where cells coordinate. How cells communicate (cell signaling and signal transduction), how they maintain stability (feedback), and how they decide when to divide (the cell cycle and its regulation). Many key concepts in Unit 4 set up the rest of the course.
Seven topics from the College Board CED, in order.
Topic 4.1
Cell Communication
The four ways cells send signals — direct contact, paracrine, synaptic, and endocrine — and what each is used for.
Topic 4.2
Introduction to Signal Transduction
Why signals need to be converted (transduced) into an internal cellular response, and the three-stage framework.
Topic 4.3
Signal Transduction
Reception, transduction, and response. Receptor types, second messengers, phosphorylation cascades, and signal amplification.
Topic 4.4
Changes in Signal Transduction Pathways
How mutations, drugs, or environmental factors can alter signaling — and how those changes cause disease, including cancer.
Topic 4.5
Feedback
Negative feedback maintains homeostasis (set point); positive feedback drives processes to completion. Real-world examples of each.
Topic 4.6
Cell Cycle
The four phases of interphase plus mitosis (PMAT) and cytokinesis. Sister chromatids and chromosome behavior throughout.
Topic 4.7
Regulation of Cell Cycle
Checkpoints (G₁, G₂, M), cyclins and CDKs, p53 — and what happens when regulation fails: cancer.
About Unit 4
Unit 4 is the unit on how cells coordinate. After Unit 3's metabolism, this unit covers two related questions: how do cells communicate with each other (cell signaling and signal transduction), and how do cells decide when to divide (the cell cycle and its regulation)? The themes overlap — external signals like growth factors often regulate the cell cycle.
This unit is 10–15% of the AP Bio exam and takes about 11–13 class periods. The most-tested concepts are the three stages of signal transduction (reception–transduction–response), positive vs. negative feedback, the phases of mitosis, and how regulatory failures (like p53 mutations) lead to cancer.
The College Board ties Unit 4 to three of the four Big Ideas:
Big Idea 2
Energetics — feedback loops maintain homeostasis
Big Idea 3
Information — signals are received, transduced, and responded to
Big Idea 4
Systems — signaling and the cell cycle integrate cellular behavior