MoBio Overview of Cell Signaling Chapter 6
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Figure 6-A-1. Major pathways for signals to enter a cell.

A cell needs to communicate with its environment so that it can make appropriate responses. The external signal may enter a cell via four major pathways:

  1. Hydrophobic molecules.
  2. Ion channels.
  3. G-protein-coupled receptors.
  4. Enzymes.

They are discussed in Sections B, C, D and E.

In many cases, the signal continues to propagate within the cell and often reaches nuclear DNA to express proteins. The intracellular signaling is discussed in Section F. Section G deals with the mechanism and signaling in apoptosis (programmed cell death).