Cell Signalling and Transduction
The signaling molecules called steroid hormones

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

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Cell Signalling and Transduction
In the signal transduction mechanism known as protein phosphorylation

phosphorylated proteins act with enzymes to trigger the signal cascade
receptor kinases play a key role in triggering the signal cascade
the signaling molecule binds to a surface receptor
All of these

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Cell Signalling and Transduction
The binding of ligands to many G-proteins linked receptors leads to shortlived

decrease in the concentration of certain intracellular signaling molecules called second messenger
decrease in the concentration of certain extracellular signaling molecules called first messenger
increase in the concentration of certain intracellular signaling molecules called second messenger
increase in the concentration of certain extracellular signaling molecules called first messenger

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