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
The binding of ligands to many G-proteins linked receptors leads to shortlived

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

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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
results in the activation of a sequence of enzymes needed for the cell effect
All of these
involves the activation of a G protein

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Cell Signalling and Transduction
Nitroglycerin has long been administered to human patients suffering from chronic chest pain (angina). This medication works because it

is broken down into hormones that affect the heart
mimics the action of signal receptors
interferes with chemical cascades that trigger contraction of heart muscle
breaks down into nitric oxide, which increases blood flow to the heart

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