Despite the protective membrane that separates a cell from the environment, cells need the ability to detect and respond to environmental changes. Additionally, cells often need to communicate with one another. Unicellular and multicellular organisms use a variety of cell signaling mechanisms to communicate to respond to the environment.
Cells respond to many types of information, often through receptor proteins positioned on the membrane. For example, skin cells respond to and transmit touch information, while photoreceptors in the retina can detect light. Most cells, however, have evolved to respond to chemical signals, including hormones, neurotransmitters, and many other types of signaling molecules. Cells can even coordinate different responses elicited by the same signaling molecule.
Typically, cell signaling involves three steps: (1) reception of the signal, (2) signal transduction, and (3) a response. In most signal reception, a membrane-impermeable molecule, or ligand, causes a change in a membrane receptor; however, some signaling molecules, such as hormones, can traverse the membrane to reach their internal receptors. The membrane receptor can then send this signal to intracellular messengers, which transduces the message into a cellular response. This intracellular response may include a change transcription, translation, protein activation, or many others.
Unicellular organisms such as bacteria can use a type of cell signaling called quorum sensing to detect their concentration in a colony and generate coordinated responses. Eukaryotic cells can release ligands that target the same cell that produced the signal (autocrine signaling) or neighboring cells (paracrine signaling). Signals can even be sent over long distances, as in the case of some hormones, and produce responses in distant cells, called endocrine signaling. Contact-dependent signaling describes physical pathways created between neighboring cells through which cytoplasmic signals can rapidly pass. Nervous system cells can generate rapid responses through a specialization of cell signaling called synaptic signaling.
