Cell Communication and Homeostasis
Explain feedback loops, receptors, signaling molecules, and response pathways. Use examples from blood glucose, temperature, and water balance.
A lesson of Biology, in Homeostasis, Body Systems, and Ecology, about 10 minutes.
By the end of this lesson, you will be able to explain signal transduction pathways, describe how cells communicate using chemical signals, define homeostasis, and explain feedback mechanisms.
Cell Communication
Cells must communicate to coordinate growth, development, immunity, and metabolism. Communication uses chemical signals (hormones, neurotransmitters, growth factors) that bind to receptors.
Signal Transduction Pathway
- Reception: a signal molecule (ligand) binds to a specific receptor (usually a protein)
- Transduction: the signal is passed through the cell via a cascade of molecular changes (second messengers, protein phosphorylation)
- Response: the cell changes its behavior — gene expression, enzyme activity, cell division, etc.
Local signaling:
- Paracrine: signal affects nearby cells (growth factors, cytokines)
- Synaptic: neurotransmitter across a synapse
Long-distance signaling:
- Endocrine: hormones travel through the bloodstream to target cells throughout the body
Receptor Types
Membrane receptors (for hydrophilic signals that can't cross the membrane):
- G-protein coupled receptors: bind signal → activate G-protein → activate enzyme → produce second messenger (cAMP)
- Receptor tyrosine kinases: dimerize, auto-phosphorylate → activate downstream proteins
Intracellular receptors (for lipophilic signals, e.g., steroid hormones that cross the membrane):
- Signal-receptor complex enters nucleus → acts as transcription factor → changes gene expression
Homeostasis
Homeostasis: the maintenance of a relatively stable internal environment despite external changes.
Examples: body temperature (37°C), blood pH (7.35–7.45), blood glucose (70–110 mg/dL).
Feedback Mechanisms
Negative feedback: the response reduces the initial stimulus → restores equilibrium.
Example: thermostat. Body temperature rises → sweat response → temperature falls → sweat response stops.
Example: blood glucose. Glucose rises → pancreas secretes insulin → cells take up glucose → glucose falls → insulin secretion decreases.