Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 11.1 External Signals Are Converted to Responses Within the Cell
Cells communicate through chemical signals, converting external messages into internal cellular responses through a process called signal transduction.
Evolution of Cell Signaling
Cell signaling likely evolved in ancient prokaryotes and unicellular eukaryotes
Bacteria use quorum sensing to coordinate behavior based on population density
Example: biofilm formation and toxin secretion by bacteria like S. aureus
Yeast cells use chemical mating signals to identify and fuse with cells of the opposite type
Signal Transduction Pathway
Signaling involves three stages:
Reception: Signal molecule binds to a specific receptor
Transduction: Receptor changes shape, initiating a pathway inside the cell
Response: The cell carries out an activity, like gene expression or enzyme activation
Local and Long-Distance Signaling
Direct contact: Through cell junctions (animals and plants) or surface molecules
Local signaling:
Paracrine signaling: signaling molecules act on nearby cells
Synaptic signaling: neurotransmitters cross synapses in nerve cells
Long-distance signaling:
Endocrine signaling: hormones travel via bloodstream to target cells
Only cells with the right receptors respond to specific hormones
Sutherland’s Discovery
Epinephrine cannot activate enzymes directly in vitro; requires intact cells
Concluded that epinephrine acts through a cell surface receptor and internal signaling steps
Led to the model of signal reception → transduction → response
In a Nutshell
Cells detect external signals and convert them into specific internal responses using a multi-step signaling pathway. This process is fundamental to both unicellular and multicellular life and ensures proper coordination of cellular activities.