Cell Signalling and Transduction
The signaling molecules called steroid hormones

are made in one location of the body but have their effects some distance away
are hydrophilic and so cannot penetrate the plasma membrane
bind to cell surface receptors to trigger chemical cascades
never enter the blood of humans

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Cell Signalling and Transduction
A signal cascade induced by adrenaline or thyroxine

must begin with receipt of the signal molecule by a surface receptor
involves the activation of a G protein
All of these
results in the activation of a sequence of enzymes needed for the cell effect

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