Day 1 Cell Communication Summary

three stages of cell signaling

reception, transduction, response

reception

ligand binds a specific receptor

transduction

signal is relayed/amplified through a pathway

response

specific cellular outcome: gene expression, metabolic change, or altered membrane potential

signaling range

local signaling vs long distance signaling

local signaling

paracrine — diffusing short distances to nearby cells; synaptic — across a synapse via neurotransmitters

long distance signaling

endocrine/hormonal — hormones travel through the circulatory system to distant target cells

water soluble signals (peptide, hormones, epinephrine) can’t…

… cross the membrane, so they bind surface receptors (GPCRs, RTKs, ligand-gated ion channels)

lipid soluble/small signals (steroid/thyroid hormones) can…

…cross the membrane directly and bind intracellular receptors, often acting as transcription factors

four receptor types

GPCR, ligand gated ion channels, intracellular receptors

GPCR

use a G protein + GTP to relay the signal

RTK

dimerize and autophosphorylate, can trigger multiple pathways at once

ligand gated ion channels

open/close to let specific ions flow

intracellular receptors

bind lipid-soluble signals, directly regulate gene expression

signal transduction

relies heavily on phosphorylation cascades: kinases add a phosphate from ATP (usually activating a protein); phosphatases remove a phosphate (usually inactivating it) — this is a widespread, reversible on/off switch mechanism

second messengers (like cAMP, made from ATP by adenylyl cyclase)

rapidly spread a signal throughout the cell. Amplification happens because each activated molecule can activate many more molecules at the next step, producing an exponential (geometric) increase — a single signal molecule can ultimately affect hundreds of millions of downstream molecules

specificity

comes from each cell type's unique collection of receptors and relay proteins — the same signal (e.g., epinephrine) can produce different, even opposite, effects in different target cells

signal termination is essential because

falling ligand concentration lets receptors revert to inactive; phosphatases remove activating phosphates; phosphodiesterase breaks down cAMP into AMP, shutting off that part of the cascade