Cell Signalling and Transduction
The signaling molecules called steroid hormones

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

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

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

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