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

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

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Cell Signalling and Transduction
The signaling molecules called steroid hormones

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

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