14.1What is Signal Transduction?

All living organisms respond to physical and chemical stimuli in the external environment and accordingly demonstrate various adaptive behaviors. For instance, cells of multicellular organisms communicate with each other in various ways, as shown in Figure 14-1A, so that the entire organism can make an integrated and purposeful response and maintain its internal environment. Such communication between cells is called intercellular signal transduction. Signal transduction also occurs within a cell as shown in Figure 14-1B, where signals are exchanged between organelles and molecules. This is called intracellular signal transduction.

Figure 14-1 Basis of signal transduction

A) Types of intercellular signal transduction. a) Autocrine: Self-activation by signal molecules that act on the same cell as the cell which secreted them; b) Paracrine: Activation of cells in the vicinity; c) Endocrine: Activation of distant cells as a result of signal molecules flowing through blood vessels, etc.; d) Cell adhesion: Membrane proteins of cells adhere to each other and gets activated; e) Nervous system: Part of a cell extends and forms a synapse.

B) Basic mechanism of intracellular signal transduction. in the figure represents the activated state of proteins such as receptors.

The molecule that receives intercellular signals is called a receptor. Cells have countless receptor proteins on their membrane surface, which bind specifically with different substances to transduce signals into the cells from the extracellular environment. Molecules that bind specifically with receptors are called ligands; in the context of signal transduction, they are often referred to as signal molecules. Each type of receptor can only bind with a specific signal molecule; this property is known as specificity. This is why each signal molecule exerts a specific action. Signal molecules mediating signal transduction between cells are called first messengers, whereas those that are produced and mobilized inside cells triggered by the signals received from the first messengers are called second messengers.

During intracellular signal transduction, when a signal molecule from outside a cell bind to a receptor on the cell membrane, the receptor molecule transforms and starts to stimulate other molecules (activation). These activated receptors in turn activate other substances in the cell, and through this chain of events, information from outside the cell is transmitted to the nucleus to adjust gene expression, or to intracellular proteins or organelles involved in cellular functions to prompt appropriate response to stimuli from the external environment (Figure 14-1B).

It is thus no exaggeration to say that all the reactions exhibited by living organisms toward stimuli are the result of signal transduction. For instance, sugar tastes sweet due to signal transduction that starts with the binding of sugar receptors in tongue cells with sugar (signals from the sugar enter the cell). Therefore, in order to understand the various phenomena seen in living organisms from a molecular biology perspective, it is extremely important to understand signal transduction.

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