Living system whether that be as simple as a unicellular organism or as complex as a human, all of us require communication and are constantly aware of surrounding around. No cell lives in isolation. Unicellular organism sense the availability of nutrients, toxicants around. Communicate with other for e.g. for reproduction purpose. Multicellular organisms, the cells need to communicate to bring about coordinated response that is beneficial to the organism as a whole. Cellular communication is accomplished via a process called cell signaling i.e. by generating, transmitting and receiving signals.

Cell signaling/Signal transduction

Transduction- which literally means conversion from one form to another (“leading through or across”). Cell signaling/Signal transduction refers to all biochemical processes by which cells translate extracellular signals originating from beyond the plasma membrane into changes that occur inside the cells and which all together bring about specific cellular responses. Signal transduction involves a series of molecular events that detect, amplify, and integrate diverse external signals to generate responses. 

Principle of cell signaling

Signal-transduction mediates through receiving the signal, processing/amplifying it for the targeted control. They detect, amplify, and integrate diverse external signals to generate responses such as changes in enzyme activity, gene expression, or ion-channel activity.

Thus they can be divided into three stages; reception, transduction, and response.


Signal transduction is started with chemical, physical, or electrical signal. When a signal molecule binds to a receptor protein, it causes to change its shape (conformational/structural changes). It can therefore, interact with other intracellular molecules. The receptors can be;

[I] Membrane receptors 

The membrane function by binding the signal molecule (ligand) that occurs as a stimulus. As most of the body processes starts with the chemical reaction received from the environment or food, these chemical signals are called ligands. In the response of membrane receptors with ligands, the production of a second signal occurs known as a second messenger.

[II] Intracellular receptors 

They are found inside the cell, either in the cytoplasm or in the nucleus of the target cell (the cell receiving the signal). The chemical messengers which are hydrophobic or very small (steroid hormones for example) can pass through the plasma membrane so that they bind intracellular receptors. After the binding they are activated by the signal molecule. Thus the activated receptor can initiate a cellular response. For example, a change in expression of a gene.


Transduction refers to passing of the signals. This controls the sequence of molecular interactions signals from receptors. The amplification of the signal occurs by the network of the enzyme. These enzyme act in specific ways to generate precise physiological response. Generally, a signaling pathway is the phosphorylation of a few specific target-cell proteins, which changes their activities and thus the activities of the cell. Thus, the activated receptor interacts with cellular machinery, producing a second signal or a change in the activity of a cellular protein; the metabolic activity of the target cell undergoes a change.


Generally, response involves removal of the signal, which often terminates the cellular response. However, a signal transduction can also include controlling the behavior of proteins in the cascade, which in turn regulate them (on or off like a switch). Signaling pathways thus triggers the signal for regulation of cell via cell division or apoptosis (programmed cell death).

Intracelluar signaling pathway activated by an extracellular signal molecule

[Source of Figure 1:  Alberts, Bruce, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter. Molecular Biology of the Cell]

Types of pathway of Signal Transduction

Universal mechanisms of cellular regulation have been discovered by biologists involving the same small set of cell-signaling mechanisms. This provides additional proof for establishing the evolutionary relationship of all life. Depending on the distance that the signaling molecule has to travel, there are different types of signaling; intercellular and extracellular.

However, cells interact with each other directly, requiring cell–cell contact, or indirectly, via molecules secreted by one cell, which are then carried away to target cells. The t details of several representative signal-transduction systems, classified according to the type of receptor. Though the stimulation and response of each signal is different, the overall mechanisms or general feature of signal transduction are common to all. To illustrate these general features of signaling systems, of six basic receptor types.

  1. G protein–coupled receptors

G protein coupled receptors indirectly activate enzymes that generate intracellular second messengers. They combine through GTP-binding proteins, or G proteins inorder to do so. Example-adrenergic receptor system that detects epinephrine (adrenaline).

2. Receptor tyrosine kinases receptors

Tyrosine kinase are plasma membrane receptors that are also enzymes.These is activated by its extracellular ligand, so that it catalyzes the phosphorylation of several cytosolic or plasma membrane proteins. For example; insulin based receptor-the receptor for epidermal growth factor (EGFR) is another.

3. Guanylyl cyclases receptors

Guanylyl cyclases receptorsare also plasma membrane receptors having an enzymatic cytoplasmic domain. They combine with the intracellular second messenger cyclic guanosine monophosphate (cGMP). cGMP is a cytosolic protein kinase that phosphorylates cellular proteins and thereby changes their activities.


After the binding of chemical ligands or changes in transmembrane potential , gated ion channels of the plasma membrane control opening and closing  can be therefore referred as gate channels. These are the simplest signal transducers. For example, the acetylcholine receptor ion channel in neurotransmission.

5.  Adhesion receptors (integrins) Integrin Signaling : A short Review

The adhesion receptorsinteract with macromolecular components of the extracellular matrix for example; collagen and gives instructions to the cytoskeletal system. As a result cell migration or adherence to the matrix occurs.

6. Nuclear receptors that bind specific ligands (such as the hormone estrogen) and alter the rate at which specific genes are transcribed and translated into cellular proteins.

Six general types of signal transduers

[Source of Figure 2: Lehninger, Albert L., Cox, Michael M. Nelson, David L. Lehninger Principles of Biochemistry]


  1. JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Chapter 15, Signal-Transduction Pathways: An Introduction to Information Metabolism. 
  2. Lehninger, Albert L., Cox, Michael M. Nelson, David L. Lehninger Principles Of Biochemistry. New York : W.H. Freeman, 2008 pp433-484
  3. Alberts, Bruce, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter. Molecular Biology of the Cell. New York: Garland Science, 2002.pp 813-831
  4. Lodish, Harvey F. Molecular Cell Biology. New York: W.H. Freeman and Co, 5th edition. Pp 533-540

Aishwarya Thapa

I am a graduate student in Biotechnology. Believing in the notion of sharing the knowledge, my passion to write has driven me to be a content writer on different topics of bio-science.

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