Cell Signalling and Transduction
Two key organizing principles for large multicellular organisms are

prokaryotic cell structure and cell specialization
cell specialization and communication between cells
simple nerve reflexes and cell specialization
communication between cells and simple nerve reflexes

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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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