Rucete ✏ AP Biology In a Nutshell
8. Cell Communication and Signaling
This chapter explains how cells communicate through chemical signals, how signal transduction pathways process those signals, and how feedback mechanisms maintain homeostasis or amplify biological processes.
Types of Cell Signaling
• Cells communicate using ligands—chemical signals that bind to receptors and trigger responses.
• Four types of cell signaling:
– Autocrine: Cell signals itself by releasing ligands that bind to its own receptors (e.g., cancer cell self-stimulation).
– Juxtacrine: Direct contact between cells; ligands pass through gap junctions or cell surfaces (e.g., immune cells, plasmodesmata).
– Paracrine: Ligands travel short distances to nearby cells (e.g., neurotransmitters across a synapse).
– Endocrine: Hormones travel long distances via the bloodstream (e.g., insulin, pheromones).
Signal Transduction
• Signal transduction connects external signals to internal responses, regulating gene expression, growth, secretion, and more.
• Ligands may be:
– Hydrophilic: Bind to receptors on the plasma membrane; cannot cross bilayer.
– Hydrophobic: Pass through membrane and bind to receptors inside the cell, often affecting DNA.
Three Stages of Signal Transduction
1. Reception: – Ligand binds to a receptor protein (membrane-bound or intracellular). – Binding causes a conformational change in receptor shape. – Examples: G-protein-coupled receptors, receptor tyrosine kinases.
2. Transduction: – A series of intracellular reactions triggered by the receptor. – Includes: • Signaling cascades that amplify the signal. • Kinases (add phosphate groups to activate molecules). • Phosphatases (remove phosphates to deactivate molecules). • Secondary messengers (e.g., cAMP, created by adenylyl cyclase).
3. Response: – The final effect on the cell (e.g., gene activation, secretion, ion channel opening, apoptosis).
Disruptions in Signal Transduction
• Disrupting any stage—reception, transduction, or response—can alter or block cellular function.
• Examples:
– Inhibiting adenylyl cyclase → no cAMP produced → reduced signal amplification.
– Mutations in G-protein receptors → faulty signaling → diseases like cholera or cancer.
– Receptor tyrosine kinase overactivation → uncontrolled cell division → tumors.
– Autoimmune disorders may involve faulty self-signaling.
Feedback Mechanisms
• Feedback helps regulate responses and maintain homeostasis.
Negative Feedback
• Counters a change to return to baseline; stabilizes the system.
• Example: – High blood glucose → insulin released → glucose uptake → blood glucose drops → insulin stops.
• Other examples: thermoregulation, osmoregulation.
Positive Feedback
• Amplifies a response until a specific outcome is reached; destabilizes temporarily.
• Examples: – Oxytocin during childbirth → more uterine contractions → more oxytocin until birth. – Blood clotting → platelet signaling accelerates clot formation.
In a Nutshell
Cells use chemical signals to coordinate growth, development, and responses to stimuli. Signal transduction pathways translate these external signals into cellular actions, with feedback mechanisms ensuring regulation. Disruptions can lead to disease, while feedback loops maintain balance or drive rapid changes when needed.