01Ways cells talk
Direct contact: gap junctions (animals) and plasmodesmata (plants) let signals pass between adjacent cells; immune cells check surface molecules by touch.
Local signaling: paracrine signals affect nearby cells (growth factors); synaptic signals are neurotransmitters across a synapse.
Long-distance: endocrine hormones travel in the blood (insulin, adrenaline). Autocrine signals act back on the cell that released them.
02Quorum sensing
Bacteria release signal molecules; when enough cells are present, the concentration crosses a threshold and they switch on genes together (biofilms, bioluminescence). It shows signaling evolved early.
03Notebook box
Distance ladder: contact < paracrine/synaptic < endocrine.
Worked example: A signal acts within milliseconds on one target cell next door. Type? → Synaptic (neurotransmitter).