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Unit 4 · Cell Communication & Cell Cycle Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Biology Unit 4 Visual Review

A topic-by-topic visual walkthrough of Cell Communication and the Cell Cycle — signaling, signal transduction pathways, feedback, mitosis, and cell-cycle checkpoints.

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TOPIC 4.1 Cell Communication FOUR WAYS CELLS SIGNAL — BY DISTANCE Direct contact Cells touch via gap junctions or surface molecules. shortest range Paracrine Local signals to nearby cells (e.g., neurotransmitters). short range Endocrine Hormones travel far through the blood- stream. long range Autocrine A cell signals itself (or its own cell type). self-signaling Why signaling matters Cells must coordinate with each other to keep a multicellular organism working as a unit — growth, immune response, and homeostasis all depend on cells "talking." Signaling is remarkably ancient and conserved: the same core mechanisms appear across yeast, plants, and animals — evidence of common ancestry. Every signal follows three stages: reception (signal binds receptor) → transduction (relay inside the cell) → response (a cellular action). Details on the next slides. Cells communicate over different distances — but every signal goes reception → transduction → response. The Review Hub · AP Biology Unit 4 TOPIC 4.2 Introduction to Signal Transduction 1 · Reception A ligand (signal) binds a specific receptor protein. → 2 · Transduction A relay of molecules passes the signal along inside. → 3 · Response The cell acts — e.g., turns a gene on or activates an enzyme. Membrane receptors For large or hydrophilic ligands that CAN'T cross the membrane. The receptor spans the membrane and relays the signal inward (e.g., G protein-coupled receptors, receptor tyrosine kinases, ligand-gated ion channels). Most water-soluble signals use these. Intracellular receptors For small or hydrophobic ligands that CAN cross the membrane — such as steroid hormones. The receptor sits inside the cytoplasm or nucleus, and the complex often acts directly as a transcription factor. Lipid-soluble signals slip right through the bilayer. A ligand binds a specific receptor — like a key in a lock — starting the transduction cascade. The Review Hub · AP Biology Unit 4 TOPIC 4.3 Signal Transduction Pathways signal → relay 1 → relay 2 → relay 3 → response Each step can amplify the signal — a "cascade." How the relay works Proteins pass the signal by adding or removing phosphate groups — phosphorylation cascades. Kinases turn proteins ON; phosphatases turn them OFF. Second messengers spread the signal fast Small molecules like cyclic AMP (cAMP) and calcium ions (Ca²⁺) diffuse quickly to relay the message. One receptor can activate many messengers. Amplification & response Because each activated protein can activate many of the next, a single signal molecule can trigger a huge cellular response — the signal is amplified at every step. The final response may be: changing gene expression, activating an enzyme, opening a channel, or altering the cell's shape or movement. A mutation anywhere in the pathway can disrupt the whole response. A phosphorylation cascade amplifies the signal — one ligand can produce a massive response. The Review Hub · AP Biology Unit 4 TOPIC 4.4 Feedback Mechanisms Negative feedback output REVERSES the change → stability The response shuts the original stimulus OFF, keeping conditions near a set point. This maintains homeostasis. Examples: • Blood glucose control (insulin / glucagon) • Body temperature regulation (sweating/shivering) • Enzyme end-product inhibition Positive feedback output AMPLIFIES the change → completion The response INCREASES the original stimulus, pushing the system further from its start. Drives a process to completion. Examples: • Childbirth — oxytocin intensifies contractions • Blood clotting cascade • Ripening fruit releasing ethylene The key distinction Negative feedback is the more common form — it counteracts change to hold a variable steady (homeostasis). Positive feedback is less common — it reinforces change to reach an end state quickly. Negative feedback restores balance; positive feedback amplifies toward an end point. The Review Hub · AP Biology Unit 4 TOPIC 4.5 The Cell Cycle G₁ S G₂ M Cell Cycle Interphase (G₁ → S → G₂) — most of the cell's life G₁: cell grows & does its job · S: DNA is REPLICATED (chromosomes copied) · G₂: grows more & makes proteins to prepare for division. The cell is NOT dividing during interphase — it's preparing. M phase — mitosis + cytokinesis Mitosis divides the nucleus in four stages: Prophase → Metaphase → Anaphase → Telophase Chromosomes condense, line up at the middle, then sister chromatids separate to opposite poles. Cytokinesis splits the cytoplasm. Result: two genetically identical diploid daughter cells Mitosis produces cells with the SAME chromosome number as the parent — used for growth, repair, and asexual reproduction. Interphase (grow, copy DNA, grow) then M phase (mitosis) → two identical daughter cells. The Review Hub · AP Biology Unit 4 TOPIC 4.6 Regulation of the Cell Cycle THREE MAIN CHECKPOINTS — QUALITY CONTROL G₁ checkpoint the "restriction point" Is the cell big enough, with enough nutrients and undamaged DNA? If not → G₀ resting state. G₂ checkpoint before mitosis Was all the DNA copied correctly and without damage? If not → pause & repair. M (spindle) checkpoint during metaphase Are all chromosomes attached to the spindle and lined up correctly? If not → delay anaphase. The molecular drivers Cyclins and cyclin-dependent kinases (CDKs) form complexes that push the cell past each checkpoint. Cyclin levels rise and fall in a repeating pattern — their timing drives the whole cycle forward. Growth factors from other cells can trigger division. When regulation fails: cancer If checkpoint controls break down, cells divide uncontrollably, forming tumors. • Proto-oncogenes (normally "go") can mutate into oncogenes stuck ON. • Tumor-suppressor genes (like p53) normally "stop." Checkpoints (run by cyclins & CDKs) verify each step — losing this control leads to cancer. The Review Hub · AP Biology Unit 4
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How to use the visual review

Spend 30 seconds per slide before clicking next. Look at the diagram, then ask yourself: "Could I draw this from memory and explain it?"

Use the fullscreen button () on desktop for the best experience. Use arrow keys to navigate. Tap "Show all slides" to jump around.

This is great for review the night before the exam — fast, visual, and covers everything you need to remember about Unit 4's signaling and cell cycle content.