Protein signalling reactions take milliseconds since they are already present in the cell, but gene expression reactions take hours. This occurs because both the protein and the modifying enzyme needed to change the protein are already present in the cell.
The proteins known as receptors in cells attach to signalling molecules to start a physiological reaction. Different chemicals are unique to certain receptors. Nerve growth factor receptors bind nerve growth factor, insulin receptors bind insulin, dopamine receptors bind dopamine, and so on.
The intracellular receptor is the same in many different types of cells, but the collection of genes that the receptor controls varies depending on the cell type.
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